We will continue our course series e-science tools in atmospheric science also in 2025! The next course "Data Analysis and Model Evaluation Tools in Environmental and Climate Science" will be May 5-16, 2025 in Kristineberg, Sweden. More info and registration can be found here.

I will also teach at the Aerosol Chemistry and Physics course in spring 2025.

Study of microphysical and chemical properties of aerosols and clouds using a broad range of measurement techniques (i.e., in-situ and remote-sensing techniques on ground-based and airborne platforms within dedicated field campaigns and laboratory studies), modeling of aerosol optical properties, aerosol hygroscopicity, aerosol-cloud interactions, study of ambient aerosols and clouds in the Arctic.

Some updates:

• I was the co-chief scientist of the ARTofMELT expedition 2023. More infos and impressions can be found at SU's and SPRS's project websites.
• We always offer interesting topics for master or bachelor work. Just contact me if you are interested to work with atmospheric aerosols or clouds!

Participants of the ARTofMELT2023 expedition on board the I/B Oden.

 

Supervision

Main supervisor:

• Linn Karlsson (Thesis: Aerosol–cloud interactions in a warming Arctic, defended June 2022)
• Gabriel Freitas (Thesis: Bioaerosols and their importance for low-level Arctic clouds, defended December 2023)
• Current: Almuth Neuberger, Julia Asplund, Lea Haberstock

Co-advisor:

• Emelie Graham (Thesis: Insights into key processes governing atmospheric aerosol loadings and their interactions with clouds, defended April 2022)
• Julika Zinke (Thesis: Factors influencing emission fluxes and bacterial enrichment in sea spray aerosols: Insights from laboratory and field studies, defended October 2023)
• Yvette Gramlich (Thesis: Chemical composition of Arctic aerosols and their link to clouds, defended September 2023)
• Current: Hari Nair, Fredrik Mattsson, Sneha Aggarwal, Rawan Fayad

A selection from Stockholm University publication database

• Contribution of primary biological aerosol particles to low-level Arctic cloud condensation nuclei

  Gabriel Pereira Freitas (et al.).

  Mixed-phase clouds (MPC) are key players in the Arctic climate system due to their role in modulating solar and terrestrial radiation. Such radiative interactions critically rely on the ice content of MPC which, in turn, partly depends on the availability of ice nucleating particles (INP). INP sources and concentrations are poorly understood in the Arctic. Recently, INP active at high temperatures were linked to be primary biological aerosol particles (PBAP). Here, we investigated for a full year the PBAP abundance and variability within cloud residuals, directly sampled by a multiparameter bioaerosol spectrometer coupled to a ground-based counterflow virtual impactor inlet at the Zeppelin Observatory (475 m asl), Ny-Ålesund, Svalbard. PBAP concentrations (10⁻³–10⁻²L⁻¹) and contributions to coarse-mode aerosol (1 in every 10³–10⁴) within cloud residuals were found to be close to those expected for concentrations of high-temperature INP. Transmission electron microscopy also confirmed the presence of PBAP, most likely bacteria, within the cloud residual samples. Seasonally, our results reveal an elevated presence of PBAP within cloud residuals during the summer. Parallel water vapor isotope measurements points towards a link between summer clouds and regionally sourced air masses. Low-level MPC were predominantly observed at the beginning and end of summer, and one explanation for their presence is the existence of high-temperature INP. In this study, we present observational evidence that PBAP might play a role in determining the phase of low-level Arctic clouds, with potential implications for the Arctic climate given ongoing changes in the hydrological and biogeochemical cycles that influence the PBAP flux in and towards the Arctic.

  Read more about Contribution of primary biological aerosol particles to low-level Arctic cloud condensation nuclei

• In-situ molecular characterization of Arctic cloud residuals using FIGAERO-CIMS behind a ground-based counterflow virtual impactor

  Yvette Gramlich (et al.).

  The role organic aerosols play in Arctic cloud formation is still poorly understood. In this study we address this issue by in-situ observations of cloud residuals at the Zeppelin Observatory in Ny-Ålesund, Svalbard (approx. 480 m a. s. l.) The measurements were part of the one-year long Ny-Ålesund Aerosol and Cloud Experiment 2019-2020 (NASCENT). We deployed a Filter Inlet for Gases and AEROsols coupled to a Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) behind a ground-based counterflow virtual impactor (GCVI) to obtain the chemical composition of cloud residuals at molecular level. Between December 2019 and December 2020, we observed in total 14 cloud events. The compositions of the cloud residuals show a clear signal of methanesulfonic acid in spring, summer and autumn, but not in the winter, suggesting a marine contribution to the aerosol population activating into cloud droplets. In addition, we observe organic compounds in the cloud residuals throughout the entire year, with elevated fractions in summer. The biomass burning tracer levoglucosan was found in the cloud residuals as well, with highest contributions to the cloud residual mass at the end of summer (end of June until mid-September). Inorganic compounds (sulfuric acid and nitric acid) were also detected in the cloud residuals. Sulfuric acid concentrations were especially elevated in two of the cloud residuals (May 21 and September 12, 2020), but followed the overall pattern of the levels of MSA in the cloud residuals during the rest of the year. Overall, the results show the contribution of a marine contribution to the aerosol population able to form clouds in the Arctic environment during the summer months.

  Read more about In-situ molecular characterization of Arctic cloud residuals using FIGAERO-CIMS behind a ground-based counterflow virtual impactor

• Measurement report: Year-long observation of Hydroperoxymethyl Thioformate (HPMTF) at the Zeppelin Observatory, Svalbard – seasonal behaviour and relationship to other oxidation products from dimethyl sulfide

  Karolina Siegel (et al.).

  Dimethyl sulfide (DMS), a gas produced by phytoplankton blooms, is the largest source of atmospheric sulfur over marine areas. DMS undergoes photochemical oxidation in the atmosphere to form a range of oxidation products, out of which methanesulfonic acid (MSA, CH₃SO₃H) and sulfuric acid (SA, H₂SO₄) are well-known for participating in the formation and growth of atmospheric aerosol particles, which has implications for cloud formation over oceanic and coastal regions. Recently, a new oxidation product of DMS, hydroperoxymethyl thioformate (HPMTF, C₂H₃OSO₂H) was discovered and later also measured in the atmosphere. Little is still known about the fate of this compound in the atmosphere and its potential to partition to the particle phase. In this study, we present a full year of concurrent gas- and particle phase observations of HPMTF, MSA, SA and other DMS oxidation products at the Zeppelin Observatory, Ny-Ålesund, Svalbard. This is the first time HPMTF has been measured in Svalbard and has been attempted to be observed in the particle phase in the real atmosphere. The results show that HPMTF production largely follows the same pattern as MSA during the sunlit months (April-September), indicating that they are formed via the same oxidation pathway. HPMTF was however not observed in significant amounts in the particle phase, despite high gas-phase levels. Particulate MSA and SA were observed during the sunlit months, although the highest median levels of particulate SA were measured in February, coinciding with the highest gaseous SA levels. We further show that gas- and particle phase MSA and SA are linked in May-September, whereas HPMTF lies outside of this correlation. These results provide more information about the relationship between HPMTF and other DMS oxidation products in a new part of the world, and about HPMTF’s ability to contribute to particle growth and cloud formation.

  Read more about Measurement report: Year-long observation of Hydroperoxymethyl Thioformate (HPMTF) at the Zeppelin Observatory, Svalbard – seasonal behaviour and relationship to other oxidation products from dimethyl sulfide

• Arctic observations of hydroperoxymethyl thioformate (HPMTF) – seasonal behavior and relationship to other oxidation products of dimethyl sulfide at the Zeppelin Observatory, Svalbard

  2023. Karolina Siegel (et al.). Atmospheric Chemistry And Physics 23 (13), 7569-7587

  Article

  Dimethyl sulfide (DMS), a gas produced by phytoplankton, is the largest source of atmospheric sulfur over marine areas. DMS undergoes oxidation in the atmosphere to form a range of oxidation products, out of which sulfuric acid (SA) is well known for participating in the formation and growth of atmospheric aerosol particles, and the same is also presumed for methanesulfonic acid (MSA). Recently, a new oxidation product of DMS, hydroperoxymethyl thioformate (HPMTF), was discovered and later also measured in the atmosphere. Little is still known about the fate of this compound and its potential to partition into the particle phase. In this study, we present a full year (2020) of concurrent gas- and particle-phase observations of HPMTF, MSA, SA and other DMS oxidation products at the Zeppelin Observatory (Ny-Ålesund, Svalbard) located in the Arctic. This is the first time HPMTF has been measured in Svalbard and attempted to be observed in atmospheric particles. The results show that gas-phase HPMTF concentrations largely follow the same pattern as MSA during the sunlit months (April–September), indicating production of HPMTF around Svalbard. However, HPMTF was not observed in significant amounts in the particle phase, despite high gas-phase levels. Particulate MSA and SA were observed during the sunlit months, although the highest median levels of particulate SA were measured in February, coinciding with the highest gaseous SA levels with assumed anthropogenic origin. We further show that gas- and particle-phase MSA and SA are coupled in May–July, whereas HPMTF lies outside of this correlation due to the low particulate concentrations. These results provide more information about the relationship between HPMTF and other DMS oxidation products, in a part of the world where these have not been explored yet, and about HPMTF's ability to contribute to particle growth and cloud formation.

  Read more about Arctic observations of hydroperoxymethyl thioformate (HPMTF) – seasonal behavior and relationship to other oxidation products of dimethyl sulfide at the Zeppelin Observatory, Svalbard

• Black carbon scavenging by low-level Arctic clouds

  2023. Paul Zieger (et al.). Nature Communications 14 (1)

  Article

  Black carbon (BC) from anthropogenic and natural sources has a pronounced climatic effect on the polar environment. The interaction of BC with low-level Arctic clouds, important for understanding BC deposition from the atmosphere, is studied using the first long-term observational data set of equivalent black carbon (eBC) inside and outside of clouds observed at Zeppelin Observatory, Svalbard. We show that the measured cloud residual eBC concentrations have a clear seasonal cycle with a maximum in early spring, due to the Arctic haze phenomenon, followed by cleaner summer months with very low concentrations. The scavenged fraction of eBC was positively correlated with the cloud water content and showed lower scavenged fractions at low temperatures, which may be due to mixed-phase cloud processes. A trajectory analysis revealed potential sources of eBC and the need to ensure that aerosol-cloud measurements are collocated, given the differences in air mass origin of cloudy and non-cloudy periods. Black carbon in the Arctic has pronounced climatic effects, whilst residing in the atmosphere or after being deposited. Here long-term observations of black carbon inside Arctic clouds are used to study their seasonality, sources and links to other meteorological parameters.

  Read more about Black carbon scavenging by low-level Arctic clouds

• Late summer transition from a free-tropospheric to boundary layer source of Aitken mode aerosol in the high Arctic

  2023. Ruth Price (et al.). Atmospheric Chemistry And Physics 23 (5), 2927-2961

  Article

  In the Arctic, the aerosol budget plays a particular role in determining the behaviour of clouds, which are important for the surface energy balance and thus for the region's climate. A key question is the extent to which cloud condensation nuclei in the high Arctic summertime boundary layer are controlled by local emission and formation processes as opposed to transport from outside. Each of these sources is likely to respond differently to future changes in ice cover. Here we use a global model and observations from ship and aircraft field campaigns to understand the source of high Arctic aerosol in late summer. We find that particles formed remotely, i.e. at latitudes outside the Arctic, are the dominant source of boundary layer Aitken mode particles during the sea ice melt period up to the end of August. Particles from such remote sources, entrained into the boundary layer from the free troposphere, account for nucleation and Aitken mode particle concentrations that are otherwise underestimated by the model. This source from outside the high Arctic declines as photochemical rates decrease towards the end of summer and is largely replaced by local new particle formation driven by iodic acid created during freeze-up. Such a local source increases the simulated Aitken mode particle concentrations by 2 orders of magnitude during sea ice freeze-up and is consistent with strong fluctuations in nucleation mode concentrations that occur in September. Our results suggest a high-Arctic aerosol regime shift in late summer, and only after this shift do cloud condensation nuclei become sensitive to local aerosol processes.

  Read more about Late summer transition from a free-tropospheric to boundary layer source of Aitken mode aerosol in the high Arctic

• Regionally sourced bioaerosols drive high-temperature ice nucleating particles in the Arctic

  2023. Gabriel Pereira Freitas (et al.). Nature Communications 14

  Article

  Primary biological aerosol particles (PBAP) play an important role in the climate system, facilitating the formation of ice within clouds, consequently PBAP may be important in understanding the rapidly changing Arctic. Within this work, we use single-particle fluorescence spectroscopy to identify and quantify PBAP at an Arctic mountain site, with transmission electronic microscopy analysis supporting the presence of PBAP. We find that PBAP concentrations range between 10⁻³–10⁻¹ L⁻¹ and peak in summer. Evidences suggest that the terrestrial Arctic biosphere is an important regional source of PBAP, given the high correlation to air temperature, surface albedo, surface vegetation and PBAP tracers. PBAP clearly correlate with high-temperature ice nucleating particles (INP) (>-15 °C), of which a high a fraction (>90%) are proteinaceous in summer, implying biological origin. These findings will contribute to an improved understanding of sources and characteristics of Arctic PBAP and their links to INP.

  Read more about Regionally sourced bioaerosols drive high-temperature ice nucleating particles in the Arctic

• Revealing the chemical characteristics of Arctic low-level cloud residuals – in situ observations from a mountain site

  2023. Yvette Gramlich (et al.). Atmospheric Chemistry And Physics 23 (12), 6813-6834

  Article

  The role aerosol chemical composition plays in Arctic low-level cloud formation is still poorly understood. In this study we address this issue by combining in situ observations of the chemical characteristics of cloud residuals (dried liquid cloud droplets or ice crystals) and aerosol particles from the Zeppelin Observatory in Ny-Ålesund, Svalbard (approx. 480 m a.s.l.). These measurements were part of the 1-year-long Ny-Ålesund Aerosol and Cloud Experiment 2019–2020 (NASCENT). To obtain the chemical composition of cloud residuals at molecular level, we deployed a Filter Inlet for Gases and AEROsols coupled to a Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) with iodide as the reagent ion behind a ground-based counterflow virtual impactor (GCVI). The station was enshrouded in clouds roughly 15 % of the time during NASCENT, out of which we analyzed 14 cloud events between December 2019 and December 2020. During the entire year, the composition of the cloud residuals shows contributions from oxygenated organic compounds, including organonitrates, and traces of the biomass burning tracer levoglucosan. In summer, methanesulfonic acid (MSA), an oxidation product of dimethyl sulfide (DMS), shows large contributions to the sampled mass, indicating marine natural sources of cloud condensation nuclei (CCN) and ice nucleating particle (INP) mass during the sunlit part of the year. In addition, we also find contributions of the inorganic acids nitric acid and sulfuric acid, with outstanding high absolute signals of sulfuric acid in one cloud residual sample in spring and one in late summer (21 May and 12 September 2020), probably caused by high anthropogenic sulfur emissions near the Barents Sea and Kara Sea. During one particular cloud event, on 18 May 2020, the air mass origin did not change before, during, or after the cloud. We therefore chose it as a case study to investigate cloud impact on aerosol physicochemical properties. We show that the overall chemical composition of the organic aerosol particles was similar before, during, and after the cloud, indicating that the particles had already undergone one or several cycles of cloud processing before being measured as residuals at the Zeppelin Observatory and/or that, on the timescales of the observed cloud event, cloud processing of the organic fraction can be neglected. Meanwhile, there were on average fewer particles but relatively more in the accumulation mode after the cloud. Comparing the signals of sulfur-containing compounds of cloud residuals with aerosols during cloud-free conditions, we find that sulfuric acid had a higher relative contribution to the cloud residuals than to aerosols during cloud-free conditions, but we did not observe an increase in particulate MSA due to the cloud. Overall, the chemical composition, especially of the organic fraction of the Arctic cloud residuals, reflected the overall composition of the general aerosol population well. Our results thus suggest that most aerosols can serve as seeds for low-level clouds in the Arctic.

  Read more about Revealing the chemical characteristics of Arctic low-level cloud residuals – in situ observations from a mountain site

• Sea spray emissions from the Baltic Sea: Comparison of aerosol eddy covariance fluxes and chamber-simulated sea spray emissions

  2023. Julika Zinke (et al.). Atmospheric Chemistry And Physics

  Article

   To bridge the gap between in situ and laboratory estimates of sea spray aerosol (SSA) production fluxes, we conducted two research campaigns in the vicinity of an eddy covariance (EC) flux tower on the island of Östergarnsholm in the Baltic Sea during May and August 2021. To accomplish this, we performed EC flux measurements simultaneously with laboratory measurements using a plunging jet sea spray simulation chamber containing local seawater sampled close to the footprint of the flux tower. We observed a log-linear relationship between wind speed and EC-derived SSA emission fluxes, a power-law relationship between significant wave height and EC-derived SSA emission fluxes, and a linear relationship between wave Reynolds number and EC-derived SSA emission fluxes, all of which are consistent with earlier studies. Although we observed a weak negative relationship between particle production in the sea spray simulation chamber and seawater chlorophyll-α concentration and a weak positive relationship with the concentration of fluorescent dissolved organic matter in seawater, we did not observe any significant impact of dissolved oxygen on particle production in the chamber.

  To obtain an estimate of the size-resolved emission spectrum for particles with dry diameters between 0.015 and 10 μm, we combined the estimates of SSA particle production fluxes obtained using the EC measurements and the chamber measurements in three different ways: 1) using the traditional continuous whitecap method, 2) using air entrainment measurements, and 3) simply scaling the chamber data to the EC fluxes. In doing so, we observed that the magnitude of the EC-derived emission fluxes compared relatively well to the magnitude of the fluxes obtained using the chamber air entrainment method, as well as the previous flux measurements of Nilsson et al. (2021) and the parameterisations of Mårtensson et al. (2003) and Salter et al. (2015). As a result of these measurements, we have derived a wind speed-dependent and wave state-dependent SSA parameterization for particles with dry diameters between 0.015 and 10 μm for low-salinity waters such as the Baltic Sea, thus providing a more accurate estimation of SSA production fluxes.

  Read more about Sea spray emissions from the Baltic Sea

• The Representation of Sea Salt Aerosols and Their Role in Polar Climate Within CMIP6

  2023. Rémy Lapere (et al.). Journal of Geophysical Research - Atmospheres 128 (6)

  Article

  Natural aerosols and their interactions with clouds remain an important uncertainty within climate models, especially at the poles. Here, we study the behavior of sea salt aerosols (SSaer) in the Arctic and Antarctic within 12 climate models from CMIP6. We investigate the driving factors that control SSaer abundances and show large differences based on the choice of the source function, and the representation of aerosol processes in the atmosphere. Close to the poles, the CMIP6 models do not match observed seasonal cycles of surface concentrations, likely due to the absence of wintertime SSaer sources such as blowing snow. Further away from the poles, simulated concentrations have the correct seasonality, but have a positive mean bias of up to one order of magnitude. SSaer optical depth is derived from the MODIS data and compared to modeled values, revealing good agreement, except for winter months. Better agreement for aerosol optical depth than surface concentration may indicate a need for improving the vertical distribution, the size distribution and/or hygroscopicity of modeled polar SSaer. Source functions used in CMIP6 emit very different numbers of small SSaer, potentially exacerbating cloud-aerosol interaction uncertainties in these remote regions. For future climate scenarios SSP126 and SSP585, we show that SSaer concentrations increase at both poles at the end of the 21st century, with more than two times mid-20th century values in the Arctic. The pre-industrial climate CMIP6 experiments suggest there is a large uncertainty in the polar radiative budget due to SSaer.Plain Language Summary Aerosols emitted from the ocean, such as sea salt particles (aerosols), are critical for the climate of polar regions. However, there is still uncertainty in their representation in climate models. The purpose of this work is to evaluate the representation of sea salt aerosols (SSaer) in the Arctic and Antarctic in a recent model inter-comparison initiative, and to assess the consequences for our understanding of present-day and future polar climate. We find that the models disagree between them and with observations from ground stations and from space. This suggests that the formulation of sea salt emissions in global models is not adapted for polar regions. With sea ice retreat, SSaer will most likely increase in the future, which makes addressing the current uncertainty an important next step for the scientific community.

  Read more about The Representation of Sea Salt Aerosols and Their Role in Polar Climate Within CMIP6

• Atmospheric composition in the European Arctic and 30 years of the Zeppelin Observatory, Ny-Ålesund

  2022. Stephen M. Platt (et al.). Atmospheric Chemistry And Physics 22 (5), 3321-3369

  Article

  The Zeppelin Observatory (78.90^∘ N, 11.88^∘ E) is located on Zeppelin Mountain at 472 m a.s.l. on Spitsbergen, the largest island of the Svalbard archipelago. Established in 1989, the observatory is part of Ny-Ålesund Research Station and an important atmospheric measurement site, one of only a few in the high Arctic, and a part of several European and global monitoring programmes and research infrastructures, notably the European Monitoring and Evaluation Programme (EMEP); the Arctic Monitoring and Assessment Programme (AMAP); the Global Atmosphere Watch (GAW); the Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS); the Advanced Global Atmospheric Gases Experiment (AGAGE) network; and the Integrated Carbon Observation System (ICOS). The observatory is jointly operated by the Norwegian Polar Institute (NPI), Stockholm University, and the Norwegian Institute for Air Research (NILU). Here we detail the establishment of the Zeppelin Observatory including historical measurements of atmospheric composition in the European Arctic leading to its construction. We present a history of the measurements at the observatory and review the current state of the European Arctic atmosphere, including results from trends in greenhouse gases, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), other traces gases, persistent organic pollutants (POPs) and heavy metals, aerosols and Arctic haze, and atmospheric transport phenomena, and provide an outline of future research directions.

  Read more about Atmospheric composition in the European Arctic and 30 years of the Zeppelin Observatory, Ny-Ålesund

• Composition and mixing state of Arctic aerosol and cloud residual particles from long-term sinale-particle observations at Zeppelin Observatory, Svalbard

  2022. Kouji Adachi (et al.). Atmospheric Chemistry And Physics 22 (21), 14421-14439

  Article

  The Arctic region is sensitive to climate change and is warming faster than the global average. Aerosol particles change cloud properties by acting as cloud condensation nuclei and ice-nucleating particles, thus influencing the Arctic climate system. Therefore, understanding the aerosol particle properties in the Arctic is needed to interpret and simulate their influences on climate. In this study, we collected ambient aerosol particles using whole-air and PM₁₀ inlets and residual particles of cloud droplets and ice crystals from Arctic low-level clouds (typically, all-liquid or mixed-phase clouds) using a counterflow virtual impactor inlet at the Zeppelin Observatory near Ny-Ålesund, Svalbard, within a time frame of 4 years. We measured the composition and mixing state of individual fine-mode particles in 239 samples using transmission electron microscopy. On the basis of their composition, the aerosol and cloud residual particles were classified as mineral dust, sea salt, K-bearing, sulfate, and carbonaceous particles. The number fraction of aerosol particles showed seasonal changes, with sulfate dominating in summer and sea salt increasing in winter. There was no measurable difference in the fractions between ambient aerosol and cloud residual particles collected at ambient temperatures above 0 ^∘C. On the other hand, cloud residual samples collected at ambient temperatures below 0 ^∘C had several times more sea salt and mineral dust particles and fewer sulfates than ambient aerosol samples, suggesting that sea spray and mineral dust particles may influence the formation of cloud particles in Arctic mixed-phase clouds. We also found that 43 % of mineral dust particles from cloud residual samples were mixed with sea salt, whereas only 18 % of mineral dust particles in ambient aerosol samples were mixed with sea salt. This study highlights the variety in aerosol compositions and mixing states that influence or are influenced by aerosol–cloud interactions in Arctic low-level clouds.

  Read more about Composition and mixing state of Arctic aerosol and cloud residual particles from long-term sinale-particle observations at Zeppelin Observatory, Svalbard

• Emission of primary bioaerosol particles from Baltic seawater

  2022. Gabriel Pereira Freitas (et al.). Environmental Science 2 (5), 1170-1182

  Article

  Bioaerosols are particles of biological origin with various important atmospheric implications, for example, within cloud formation where bioaerosols can act as cloud condensation or ice nuclei. Their sources and properties, however, are poorly understood. We conducted a controlled sea spray experiment to determine the properties and emission of primary biological aerosol particles (PBAP) originating from Baltic seawater. Using a single-particle fluorescence and light-scattering instrument, the Multiparameter Bioaerosol Spectrometer (MBS), we differentiated PBAP within sea spray aerosol (SSA). Overall, approximately 1 in 10⁴ particles larger than 0.8 μm in diameter were classified as PBAP. The optically-determined morphology of the nascent and fluorescent SSA particles showed a clear transition in symmetry and elongation most likely due to changes in the biogeochemical properties of the surface water. These shifts were also reflected in a clear change of the bacterial community composition of the aerosol and seawater as determined by 16S rRNA-gene analysis, which were significantly distinct from each other, suggesting a preferential emission of specific bacteria to the atmosphere. Our results demonstrate the capability of the MBS to identify and count PBAP within SSA on a single-particle basis and will help to better constrain the emission of marine PBAP and their dependence on the seawater's biogeochemical properties.

  Read more about Emission of primary bioaerosol particles from Baltic seawater

• Highly Active Ice-Nucleating Particles at the Summer North Pole

  2022. Grace C. E. Porter (et al.). Journal of Geophysical Research - Atmospheres 127 (6)

  Article

  The amount of ice versus supercooled water in clouds is important for their radiative properties and role in climate feedbacks. Hence, knowledge of the concentration of ice-nucleating particles (INPs) is needed. Generally, the concentrations of INPs are found to be very low in remote marine locations allowing cloud water to persist in a supercooled state. We had expected the concentrations of INPs at the North Pole to be very low given the distance from open ocean and terrestrial sources coupled with effective wet scavenging processes. Here we show that during summer 2018 (August and September) high concentrations of biological INPs (active at >−20°C) were sporadically present at the North Pole. In fact, INP concentrations were sometimes as high as those recorded at mid-latitude locations strongly impacted by highly active biological INPs, in strong contrast to the Southern Ocean. Furthermore, using a balloon borne sampler we demonstrated that INP concentrations were often different at the surface versus higher in the boundary layer where clouds form. Back trajectory analysis suggests strong sources of INPs near the Russian coast, possibly associated with wind-driven sea spray production, whereas the pack ice, open leads, and the marginal ice zone were not sources of highly active INPs. These findings suggest that primary ice production, and therefore Arctic climate, is sensitive to transport from locations such as the Russian coast that are already experiencing marked climate change.

  Read more about Highly Active Ice-Nucleating Particles at the Summer North Pole

• Physical and Chemical Properties of Cloud Droplet Residuals and Aerosol Particles During the Arctic Ocean 2018 Expedition

  2022. Linn Karlsson (et al.). Journal of Geophysical Research - Atmospheres 127 (11)

  Article

  Detailed knowledge of the physical and chemical properties and sources of particles that form clouds is especially important in pristine areas like the Arctic, where particle concentrations are often low and observations are sparse. Here, we present in situ cloud and aerosol measurements from the central Arctic Ocean in August–September 2018 combined with air parcel source analysis. We provide direct experimental evidence that Aitken mode particles (particles with diameters ≲70 nm) significantly contribute to cloud condensation nuclei (CCN) or cloud droplet residuals, especially after the freeze-up of the sea ice in the transition toward fall. These Aitken mode particles were associated with air that spent more time over the pack ice, while size distributions dominated by accumulation mode particles (particles with diameters ≳70 nm) showed a stronger contribution of oceanic air and slightly different source regions. This was accompanied by changes in the average chemical composition of the accumulation mode aerosol with an increased relative contribution of organic material toward fall. Addition of aerosol mass due to aqueous-phase chemistry during in-cloud processing was probably small over the pack ice given the fact that we observed very similar particle size distributions in both the whole-air and cloud droplet residual data. These aerosol–cloud interaction observations provide valuable insight into the origin and physical and chemical properties of CCN over the pristine central Arctic Ocean.

  Read more about Physical and Chemical Properties of Cloud Droplet Residuals and Aerosol Particles During the Arctic Ocean 2018 Expedition

• Surface composition of size-selected sea salt particles under the influence of organic acids studied in situ using synchrotron radiation X-ray photoelectron spectroscopy

  2022. Minna Patanen (et al.). Environmental Science 2 (5), 1032-1040

  Article

  Sea spray aerosols play a key role in the climate system by scattering solar radiation and by serving as cloud condensation nuclei. Despite their importance, the impact of sea spray aerosols on global climate remains highly uncertain. One of the key knowledge gaps in our understanding of sea spray aerosol is the chemical composition of the particle surface, important for various atmospheric chemical processes, as a function of size and bulk composition. Here, we have applied X-ray photoelectron spectroscopy (XPS) to determine the surface composition of both pure inorganic sea salt aerosols and sea salt aerosols spiked with an amino acid (phenylalanine) and a straight chain fatty acid (octanoic acid). Importantly, the use of a differential mobility analyser allowed size-selection of 150, 250 and 350 nm monodisperse aerosol particles for comparison to polydisperse aerosol particles. We observed enrichment of magnesium at the particle surfaces relative to chloride in all aerosols tested, across all particle sizes. Interestingly, the magnitude of this enrichment was dependent on the type of organic present in the solution as well as the particle size. Our results suggest that the observed enrichment in magnesium is an inorganic effect which can be either enhanced or diminished by the addition of organic substances.

  Read more about Surface composition of size-selected sea salt particles under the influence of organic acids studied in situ using synchrotron radiation X-ray photoelectron spectroscopy

• The Effect of Seawater Salinity and Seawater Temperature on Sea Salt Aerosol Production

  2022. Julika Zinke (et al.). Journal of Geophysical Research - Atmospheres 127 (16)

  Article

  To improve our understanding of the impact of sea salt aerosols (SSA) on the Earth's climate, it is critical to understand the physical mechanisms which determine the size-resolved SSA production flux. Of the factors affecting SSA emissions, seawater salinity has perhaps received the least attention in the literature and previous studies have produced conflicting results. Here, we present a series of laboratory experiments designed to investigate the role of salinity on aerosol production from artificial seawater using a continuous plunging jet. During these experiments, the aerosol and surface bubble size distributions were monitored while the salinity was decreased from 35 to 0 g kg(-1). Three distinct salinity regimes were identified: (a) A high salinity regime, 10-35 g kg(-1), where lower salinity resulted in only minor reductions in particle number flux but notable reductions in particle volume flux; (b) an intermediate salinity regime, 5-10 g kg(-1), with a local maximum in particle number flux; (c) a low salinity regime, <5 g kg(-1), characterized by a rapid decrease in particle number flux at lower salinities and dominated by small particles and larger bubbles. We discuss the implications of our results through comparison of the size-resolved aerosol flux and the surface bubble population at different salinities. Finally, by varying the seawater temperature at three specific salinities we have also developed a simple parameterization of the particle production flux as a function of seawater temperature and salinity. The range of seawater salinity and temperature studied is representative of the global oceans and lower salinity water bodies such as the Baltic Sea.

  Read more about The Effect of Seawater Salinity and Seawater Temperature on Sea Salt Aerosol Production

• The Ny-Ålesund Aerosol Cloud Experiment (NASCENT): Overview and First Results

  2022. J. T. Pasquier (et al.). Bulletin of The American Meteorological Society - (BAMS) 103 (11), e2533-E2558

  Article

  The Arctic is warming at more than twice the rate of the global average. This warming is influenced by clouds, which modulate the solar and terrestrial radiative fluxes and, thus, determine the surface energy budget. However, the interactions among clouds, aerosols, and radiative fluxes in the Arctic are still poorly understood. To address these uncertainties, the Ny-Ålesund Aerosol Cloud Experiment (NASCENT) study was conducted from September 2019 to August 2020 in Ny-Ålesund, Svalbard. The campaign’s primary goal was to elucidate the life cycle of aerosols in the Arctic and to determine how they modulate cloud properties throughout the year. In situ and remote sensing observations were taken on the ground at sea level, at a mountaintop station, and with a tethered balloon system. An overview of the meteorological and the main aerosol seasonality encountered during the NASCENT year is introduced, followed by a presentation of first scientific highlights. In particular, we present new findings on aerosol physicochemical and molecular properties. Further, the role of cloud droplet activation and ice crystal nucleation in the formation and persistence of mixed-phase clouds, and the occurrence of secondary ice processes, are discussed and compared to the representation of cloud processes within the regional Weather Research and Forecasting Model. The paper concludes with research questions that are to be addressed in upcoming NASCENT publications.  

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• Tropical and Boreal Forest – Atmosphere Interactions: A Review

  2022. Paulo Artaxo (et al.). Tellus. Series B, Chemical and physical meteorology 74 (1), 24-163

  Article

  This review presents how the boreal and the tropical forests affect the atmosphere, its chemical composition, its function, and further how that affects the climate and, in return, the ecosystems through feedback processes. Observations from key tower sites standing out due to their long-term comprehensive observations: The Amazon Tall Tower Observatory in Central Amazonia, the Zotino Tall Tower Observatory in Siberia, and the Station to Measure Ecosystem-Atmosphere Relations at Hyytiäla in Finland. The review is complemented by short-term observations from networks and large experiments.

  The review discusses atmospheric chemistry observations, aerosol formation and processing, physiochemical aerosol, and cloud condensation nuclei properties and finds surprising similarities and important differences in the two ecosystems. The aerosol concentrations and chemistry are similar, particularly concerning the main chemical components, both dominated by an organic fraction, while the boreal ecosystem has generally higher concentrations of inorganics, due to higher influence of long-range transported air pollution. The emissions of biogenic volatile organic compounds are dominated by isoprene and monoterpene in the tropical and boreal regions, respectively, being the main precursors of the organic aerosol fraction.

  Observations and modeling studies show that climate change and deforestation affect the ecosystems such that the carbon and hydrological cycles in Amazonia are changing to carbon neutrality and affect precipitation downwind. In Africa, the tropical forests are so far maintaining their carbon sink.

  It is urgent to better understand the interaction between these major ecosystems, the atmosphere, and climate, which calls for more observation sites, providing long-term data on water, carbon, and other biogeochemical cycles. This is essential in finding a sustainable balance between forest preservation and reforestation versus a potential increase in food production and biofuels, which are critical in maintaining ecosystem services and global climate stability. Reducing global warming and deforestation is vital for tropical forests.

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• Using Novel Molecular-Level Chemical Composition Observations of High Arctic Organic Aerosol for Predictions of Cloud Condensation Nuclei

  2022. Karolina Siegel (et al.). Environmental Science and Technology 56 (19), 13888-13899

  Article

  Predictions of cloud droplet activation in the late summertime (September) central Arctic Ocean are made using κ-Köhler theory with novel observations of the aerosol chemical composition from a high-resolution time-of-flight chemical ionization mass spectrometer with a filter inlet for gases and aerosols (FIGAERO-CIMS) and an aerosol mass spectrometer (AMS), deployed during the Arctic Ocean 2018 expedition onboard the Swedish icebreaker Oden. We find that the hygroscopicity parameter κ of the total aerosol is 0.39 ± 0.19 (mean ± std). The predicted activation diameter of ∼25 to 130 nm particles is overestimated by 5%, leading to an underestimation of the cloud condensation nuclei (CCN) number concentration by 4–8%. From this, we conclude that the aerosol in the High Arctic late summer is acidic and therefore highly cloud active, with a substantial CCN contribution from Aitken mode particles. Variability in the predicted activation diameter is addressed mainly as a result of uncertainties in the aerosol size distribution measurements. The organic κ was on average 0.13, close to the commonly assumed κ of 0.1, and therefore did not significantly influence the predictions. These conclusions are supported by laboratory experiments of the activation potential of seven organic compounds selected as representative of the measured aerosol. 

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• A global study of hygroscopicity-driven light-scattering enhancement in the context of other in situ aerosol optical properties

  2021. Gloria Titos (et al.). Atmospheric Chemistry And Physics 21 (17), 13031-13050

  Article

  The scattering and backscattering enhancement factors (f(RH) and f_b(RH)) describe how aerosol particle light scattering and backscattering, respectively, change with relative humidity (RH). They are important parameters in estimating direct aerosol radiative forcing (DARF). In this study we use the dataset presented in Burgos et al. (2019) that compiles f(RH) and f_b(RH) measurements at three wavelengths (i.e., 450, 550 and 700 nm) performed with tandem nephelometer systems at multiple sites around the world. We present an overview of f(RH) and f_b(RH) based on both long-term and campaign observations from 23 sites representing a range of aerosol types. The scattering enhancement shows a strong variability from site to site, with no clear pattern with respect to the total scattering coefficient. In general, higher f(RH) is observed at Arctic and marine sites, while lower values are found at urban and desert sites, although a consistent pattern as a function of site type is not observed. The backscattering enhancement f_b(RH) is consistently lower than f(RH) at all sites, with the difference between f(RH) and f_b(RH) increasing for aerosol with higher f(RH). This is consistent with Mie theory, which predicts higher enhancement of the light scattering in the forward than in the backward direction as the particle takes up water. Our results show that the scattering enhancement is higher for PM₁ than PM₁₀ at most sites, which is also supported by theory due to the change in scattering efficiency with the size parameter that relates particle size and the wavelength of incident light. At marine-influenced sites this difference is enhanced when coarse particles (likely sea salt) predominate. For most sites, f(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of f(RH) is found to be low or even exhibit the opposite pattern. The impact of RH on aerosol properties used to calculate radiative forcing (e.g., single-scattering albedo, ω₀, and backscattered fraction, b) is evaluated. The single-scattering albedo generally increases with RH, while b decreases. The net effect of aerosol hygroscopicity on radiative forcing efficiency (RFE) is an increase in the absolute forcing effect (negative sign) by a factor of up to 4 at RH = 90 % compared to dry conditions (RH < 40 %). Because of the scarcity of scattering enhancement measurements, an attempt was made to use other more commonly available aerosol parameters (i.e., ω₀ and scattering Ångström exponent, α_sp) to parameterize f(RH). The majority of sites (75 %) showed a consistent trend with ω₀ (higher f(RH = 85 %) for higher ω₀), while no clear pattern was observed between f(RH = 85 %) and α_sp. This suggests that aerosol ω₀ is more promising than α_sp as a surrogate for the scattering enhancement factor, although neither parameter is ideal. Nonetheless, the qualitative relationship observed between ω₀ and f(RH) could serve as a constraint on global model simulations.

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• A long-term study of cloud residuals from low-level Arctic clouds

  2021. Linn Karlsson (et al.). Atmospheric Chemistry And Physics 21 (11), 8933-8959

  Article

  To constrain uncertainties in radiative forcings associated with aerosol-cloud interactions, improved understanding of Arctic cloud formation is required, yet long-term measurements of the relevant cloud and aerosol properties remain sparse. We present the first long-term study of cloud residuals, i.e. particles that were involved in cloud formation and cloud processes, in Arctic low-level clouds measured at Zeppelin Observatory, Svalbard. To continuously sample cloud droplets and ice crystals and separate them from non-activated aerosol, a ground-based counter-flow virtual impactor inlet system (GCVI) was used. A detailed evaluation of the GCVI measurements, using concurrent cloud particle size distributions, meteorological parameters, and aerosol measurements, is presented for both warm and cold clouds, and the potential contribution of sampling artefacts is discussed in detail. We find an excellent agreement of the GCVI sampling efficiency of liquid clouds using two independent approaches. The 2-year data set of cloud residual size distributions and number concentrations reveals that the cloud residuals follow the typical seasonal cycle of Arctic aerosol, with a maximum concentration in spring and summer and a minimum concentration in the late autumn and winter months. We observed average activation diameters in the range of 58-78 nm for updraught velocities below 1 m s(-1). A cluster analysis also revealed cloud residual size distributions that were dominated by Aitken mode particles down to around 20-30 nm. During the winter months, some of these small particles may be the result of ice, snow, or ice crystal shattering artefacts in the GCVI inlet; however, cloud residuals down to 20 nm in size were also observed during conditions when artefacts are less likely.

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• Aerosols in current and future Arctic climate

  2021. Julia Schmale, Paul Zieger, Annica M. L. Ekman. Nature Climate Change 11 (2), 95-105

  Article

  Aerosol-climate interactions are important in the Arctic, but they exhibit large spatiotemporal variability. This Perspective argues for community-driven model and observational improvement, emphasizing the need to understand natural aerosol processes and quantify how their baseline is changing. Mechanisms of Arctic amplification and Arctic climate change are difficult to pinpoint, and current climate models do not represent the complex local processes and feedbacks at play, in particular for aerosol-climate interactions. This Perspective highlights the role of aerosols in contemporary Arctic climate change and stresses that the Arctic natural aerosol baseline is changing fast and its regional characteristics are very diverse. We argue that to improve understanding of present day and future Arctic, more detailed knowledge is needed on natural Arctic aerosol emissions, their evolution and transport, and the effects on cloud microphysics. In particular, observation and modelling work should focus on the sensitivity of aerosol-climate interactions to the rapidly evolving base state of the Arctic.

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• Differing Mechanisms of New Particle Formation at Two Arctic Sites

  2021. Lisa J. Beck (et al.). Geophysical Research Letters 48 (4)

  Article

  New particle formation in the Arctic atmosphere is an important source of aerosol particles. Understanding the processes of Arctic secondary aerosol formation is crucial due to their significant impact on cloud properties and therefore Arctic amplification. We observed the molecular formation of new particles from low-volatility vapors at two Arctic sites with differing surroundings. In Svalbard, sulfuric acid (SA) and methane sulfonic acid (MSA) contribute to the formation of secondary aerosol and to some extent to cloud condensation nuclei (CCN). This occurs via ion-induced nucleation of SA and NH3 and subsequent growth by mainly SA and MSA condensation during springtime and highly oxygenated organic molecules during summertime. By contrast, in an ice-covered region around Villum, we observed new particle formation driven by iodic acid but its concentration was insufficient to grow nucleated particles to CCN sizes. Our results provide new insight about sources and precursors of Arctic secondary aerosol particles.

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• Estimates of mass absorption cross sections of black carbon for filter-based absorption photometers in the Arctic

  2021. Sho Ohata (et al.). Atmospheric Measurement Techniques 14 (10), 6723-6748

  Article

  Long-term measurements of atmospheric mass concentrations of black carbon (BC) are needed to investigate changes in its emission, transport, and deposition. However, depending on instrumentation, parameters related to BC such as aerosol absorption coefficient (b_abs) have been measured instead. Most ground-based measurements of b_abs in the Arctic have been made by filter-based absorption photometers, including particle soot absorption photometers (PSAPs), continuous light absorption photometers (CLAPs), Aethalometers, and multi-angle absorption photometers (MAAPs). The measured b_abs can be converted to mass concentrations of BC (M_BC) by assuming the value of the mass absorption cross section (MAC; M_BC= babs/ MAC). However, the accuracy of conversion of b_abs to M_BC has not been adequately assessed. Here, we introduce a systematic method for deriving MAC values from b_abs measured by these instruments and independently measured M_BC. In this method, M_BC was measured with a filter-based absorption photometer with a heated inlet (COSMOS). COSMOS-derived M_BC (M_BC (COSMOS)) is traceable to a rigorously calibrated single particle soot photometer (SP2), and the absolute accuracy of M_BC (COSMOS) has been demonstrated previously to be about 15 % in Asia and the Arctic. The necessary conditions for application of this method are a high correlation of the measured b_abs with independently measured M_BC and long-term stability of the regression slope, which is denoted as MAC_cor (MAC derived from the correlation). In general, b_abs–M_BC (COSMOS) correlations were high (r²= 0.76–0.95 for hourly data) at Alert in Canada, Ny-Ålesund in Svalbard, Barrow (NOAA Barrow Observatory) in Alaska, Pallastunturi in Finland, and Fukue in Japan and stable for up to 10 years. We successfully estimated MAC_cor values (10.8–15.1 m² g⁻¹ at a wavelength of 550 nm for hourly data) for these instruments, and these MAC_cor values can be used to obtain error-constrained estimates of M_BC from b_abs measured at these sites even in the past, when COSMOS measurements were not made. Because the absolute values of M_BC at these Arctic sites estimated by this method are consistent with each other, they are applicable to the study of spatial and temporal variation in M_BC in the Arctic and to evaluation of the performance of numerical model calculations.

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• Insights into the molecular composition of semi-volatile aerosols in the summertime central Arctic Ocean using FIGAERO-CIMS

  2021. Karolina Siegel (et al.). Environmental Science 1 (4), 161-175

  Article

  The remote central Arctic during summertime has a pristine atmosphere with very low aerosol particle concentrations. As the region becomes increasingly ice-free during summer, enhanced ocean-atmosphere fluxes of aerosol particles and precursor gases may therefore have impacts on the climate. However, large knowledge gaps remain regarding the sources and physicochemical properties of aerosols in this region. Here, we present insights into the molecular composition of semi-volatile aerosol components collected in September 2018 during the MOCCHA (Microbiology-Ocean-Cloud-Coupling in the High Arctic) campaign as part of the Arctic Ocean 2018 expedition with the Swedish Icebreaker Oden. Analysis was performed offline in the laboratory using an iodide High Resolution Time-of-Flight Chemical Ionization Mass Spectrometer with a Filter Inlet for Gases and AEROsols (FIGAERO-HRToF-CIMS). Our analysis revealed significant signal from organic and sulfur-containing compounds, indicative of marine aerosol sources, with a wide range of carbon numbers and O : C ratios. Several of the sulfur-containing compounds are oxidation products of dimethyl sulfide (DMS), a gas released by phytoplankton and ice algae. Comparison of the time series of particulate and gas-phase DMS oxidation products did not reveal a significant correlation, indicative of the different lifetimes of precursor and oxidation products in the different phases. This is the first time the FIGAERO-HRToF-CIMS was used to investigate the composition of aerosols in the central Arctic. The detailed information on the molecular composition of Arctic aerosols presented here can be used for the assessment of aerosol solubility and volatility, which is relevant for understanding aerosol-cloud interactions.

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• New Insights Into the Composition and Origins of Ultrafine Aerosol in the Summertime High Arctic

  2021. M. J. Lawler (et al.). Geophysical Research Letters 48 (21)

  Article

  The summertime high Arctic atmosphere is characterized by extremely low aerosol abundance, such that small natural aerosol inputs have a strong influence on cloud formation and surface temperature. The physical sources and the mechanisms responsible for aerosol formation and development in this climate-critical and changing region are still uncertain. We report time-resolved measurements of high Arctic Aitken mode (∼20–60 nm diameter) aerosol composition during August–September 2018. During a significant Aitken mode formation event, the particles were composed of a combination of primary and secondary materials. These results highlight the importance of primary aerosol sources for high Arctic cloud formation, and they imply the action of a poorly understood atmospheric mechanism separating larger particles into multiple sub-particles.

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• Sea Spray Aerosol Chamber Study on Selective Transfer and Enrichment of Free and Combined Amino Acids

  2021. Nadja Triesch (et al.). ACS Earth and Space Chemistry 5 (6), 1564-1574

  Article

  Free (FAAs) and combined amino acids (CAAs) were investigated on size-resolved samples of nascent sea spray aerosol (SSA) particles generated during controlled laboratory experiments. Compared to seawater, the amino acids were strongly enriched on the SSA particles. The enrichment factors (EFaer) on submicron SSA particles (EFaer Sigma(FAA): 2.5 x 10(6) and EFaer Sigma CAA: 7.9 x 10(5)) were 1-2 orders of magnitude higher than on supermicron ones (EFaerSFAA: 1.0 x 105 and EFaer Sigma(CAA): 7.3 x 10(4)) and continuously increased toward smaller SSA particles. Molecular-level analysis showed that the more polar the FAAs, the more they are enriched on the SSA particles (especially FAAs with polar acid side chains, e.g., aspartic acid: EFaer of 5.8 x 10(6)). Comparison of the amino acids present on nascent SSA with those present on ambient marine aerosol particles revealed a higher complexity of the amino acids of the nascent SSA, suggesting that atmospheric processes likely reduce the amino acid diversity. In addition, our results highlight that although almost all the amino acids studied are transferred to the atmosphere via bubble bursting under controlled conditions, two amino acids, gamma-aminobutyric acid (GABA) and glycine likely have additional sources to the atmosphere. GABA is likely formed on ambient marine submicron aerosol particles to a large extent (35-47% of Sigma FAA). Glycine likely originates from long-range transport processes or photochemical reactions, as discussed in the literature; however, our results highlight the potential for a direct oceanic source via bubble bursting (similar to 20% of Sigma FAA). Overall, bubble-bursting-derived total amino acids made up 11-18% of the mass of dissolved organic carbon on the submicron SSA particles.

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• The development of a miniaturised balloon-borne cloud water sampler and its first deployment in the high Arctic

  2021. Julika Zinke (et al.). Tellus. Series B, Chemical and physical meteorology 73 (1), 1-12

  Article

  The chemical composition of cloud water can be used to infer the sources of particles upon which cloud droplets and ice crystals have formed. In order to obtain cloud water for analysis of chemical composition for elevated clouds in the pristine high Arctic, balloon-borne active cloud water sampling systems are the optimal approach. However, such systems have not been feasible to deploy previously due to their weight and the challenging environmental conditions. We have taken advantage of recent developments in battery technology to develop a miniaturised cloud water sampler for balloon-borne collection of cloud water. Our sampler is a bulk sampler with a cloud drop cutoff diameter of approximately 8 mu m and an estimated collection efficiency of 70%. The sampler was heated to prevent excessive ice accumulation and was able to operate for several hours under the extreme conditions encountered in the high Arctic. We have tested and deployed the new sampler on a tethered balloon during the Microbiology-Ocean-Cloud-Coupling in the High Arctic (MOCCHA) campaign in August and September 2018 close to the North pole. The sampler was able to successfully retrieve cloud water samples that were analysed to determine their chemical composition as well as their ice-nucleating activity. Given the pristine conditions found in the high Arctic we have placed significant emphasis on the development of a suitable cleaning procedure to minimise background contamination by the sampler itself.

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• Using correlations between observed equivalent black carbon and aerosol size distribution to derive size resolved BC mass concentration: a method applied on long-term observations performed at Zeppelin station, Ny-Ålesund, Svalbard

  2021. Peter Tunved (et al.). Tellus. Series B, Chemical and physical meteorology 73 (1), 1-17

  Article

  The aim of this study was to explore particle size dependent properties by combining long-term observations of equivalent black carbon (eBC) and number size distributions to investigate their correlation as function of particle size. The work was conducted in two main parts. The first part consisted of a short laboratory experiment to compare observed total particle light absorption (σ_abs) with that observed according to particle size by using a combination of a Differential Mobility Analyzer (DMA) and a Particle Soot Absorption Photometer (PSAP). The laboratory study confirmed strong similarities between the observed and derived σ_abs. In the second part the statistical approach using correlation between the σ_abs and the dN of each bin of the number size distribution was tested on long-term data ranging from 2002 to 2010 observed at Zeppelin station, Ny-Ålesund Svalbard. The data was clustered according to the number size distribution and grouped in four major categories: Washout, Nucleation, Intermediate and Polluted. Each category presented different features with respect to the derived eBC mass distributions, the Intermediate category showed similarities to the few available Single Particle Soot Photometer (SP2) observations in the Arctic. Overall, the statistical distribution of eBC, according to particle size, presented a larger dynamical range in the location of the mode(s). To check for consistency, the eBC mass distributions were transformed into number based eBC size distribution and compared to the observed total number size distribution. Whereas the Washout, Nucleation and Intermediate categories presented plausible number distributions, the Polluted category displayed a mode at small sizes (about 50 nm) that was significantly exaggerated.

  Read more about Using correlations between observed equivalent black carbon and aerosol size distribution to derive size resolved BC mass concentration

• A global analysis of climate-relevant aerosol properties retrieved from the network of Global Atmosphere Watch (GAW) near-surface observatories

  2020. Paolo Laj (et al.). Atmospheric Measurement Techniques 13 (8), 4353-4392

  Article

  Aerosol particles are essential constituents of the Earth's atmosphere, impacting the earth radiation balance directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei. In contrast to most greenhouse gases, aerosol particles have short atmospheric residence times, resulting in a highly heterogeneous distribution in space and time. There is a clear need to document this variability at regional scale through observations involving, in particular, the in situ near-surface segment of the atmospheric observation system. This paper will provide the widest effort so far to document variability of climate-relevant in situ aerosol properties (namely wavelength dependent particle light scattering and absorption coefficients, particle number concentration and particle number size distribution) from all sites connected to the Global Atmosphere Watch network. High-quality data from almost 90 stations worldwide have been collected and controlled for quality and are reported for a reference year in 2017, providing a very extended and robust view of the variability of these variables worldwide. The range of variability observed worldwide for light scattering and absorption coefficients, single-scattering albedo, and particle number concentration are presented together with preliminary information on their long-term trends and comparison with model simulation for the different stations. The scope of the present paper is also to provide the necessary suite of information, including data provision procedures, quality control and analysis, data policy, and usage of the ground-based aerosol measurement network. It delivers to users of the World Data Centre on Aerosol, the required confidence in data products in the form of a fully characterized value chain, including uncertainty estimation and requirements for contributing to the global climate monitoring system.

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• A global model-measurement evaluation of particle light scattering coefficients at elevated relative humidity

  2020. Maria A. Burgos (et al.). Atmospheric Chemistry And Physics 20 (17), 10231-10258

  Article

  The uptake of water by atmospheric aerosols has a pronounced effect on particle light scattering properties, which in turn are strongly dependent on the ambient relative humidity (RH). Earth system models need to account for the aerosol water uptake and its influence on light scattering in order to properly capture the overall radiative effects of aerosols. Here we present a comprehensive model-measurement evaluation of the particle light scattering enhancement factor f (RH), defined as the particle light scattering coefficient at elevated RH (here set to 85 %) divided by its dry value. The comparison uses simulations from 10 Earth system models and a global dataset of surface-based in situ measurements. In general, we find a large diversity in the magnitude of predicted f (RH) amongst the different models, which can not be explained by the site types. Based on our evaluation of sea salt scattering enhancement and simulated organic mass fraction, there is a strong indication that differences in the model parameterizations of hygroscopicity and model chemistry are driving at least some of the observed diversity in simulated f (RH). Additionally, a key point is that defining dry conditions is difficult from an observational point of view and, depending on the aerosol, may influence the measured f (RH). The definition of dry also impacts our model evaluation, because several models exhibit significant water uptake between RH = 0% and 40 %. The multisite average ratio between model outputs and measurements is 1.64 when RH = 0% is assumed as the model dry RH and 1.16 when RH = 40% is the model dry RH value. The overestimation by the models is believed to originate from the hygroscopicity parameterizations at the lower RH range which may not implement all phenomena taking place (i.e., not fully dried particles and hysteresis effects). This will be particularly relevant when a location is dominated by a deliquescent aerosol such as sea salt. Our results emphasize the need to consider the measurement conditions in such comparisons and recognize that measurements referred to as dry may not be dry in model terms. Recommendations for future model-measurement evaluation and model improvements are provided.

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• Frequent new particle formation over the high Arctic pack ice by enhanced iodine emissions

  2020. Andrea Baccarini (et al.). Nature Communications 11 (1)

  Article

  In the central Arctic Ocean the formation of clouds and their properties are sensitive to the availability of cloud condensation nuclei (CCN). The vapors responsible for new particle formation (NPF), potentially leading to CCN, have remained unidentified since the first aerosol measurements in 1991. Here, we report that all the observed NPF events from the Arctic Ocean 2018 expedition are driven by iodic acid with little contribution from sulfuric acid. Iodic acid largely explains the growth of ultrafine particles (UFP) in most events. The iodic acid concentration increases significantly from summer towards autumn, possibly linked to the ocean freeze-up and a seasonal rise in ozone. This leads to a one order of magnitude higher UFP concentration in autumn. Measurements of cloud residuals suggest that particles smaller than 30nm in diameter can activate as CCN. Therefore, iodine NPF has the potential to influence cloud properties over the Arctic Ocean. Which vapors are responsible for new particle formation in the Arctic is largely unknown. Here, the authors show that the formation of new particles at the central Arctic Ocean is mainly driven by iodic acid and that particles smaller than 30nm in diameter can activate as cloud condensation nuclei.

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• From a polar to a marine environment: has the changing Arctic led to a shift in aerosol light scattering properties?

  2020. Dominic Heslin-Rees (et al.). Atmospheric Chemistry And Physics 20 (21), 13671-13686

  Article

  The study of long-term trends in aerosol optical properties is an important task to understand the underlying aerosol processes influencing the change of climate. The Arctic, as the place where climate change manifests most, is an especially sensitive region of the world. Within this work, we use a unique long-term data record of key aerosol optical properties from the Zeppelin Observatory, Svalbard, to ask the question of whether the environmental changes of the last 2 decades in the Arctic are reflected in the observations. We perform a trend analysis of the measured particle light scattering and backscattering coefficients and the derived scattering Angstrom exponent and hemispheric backscattering fraction. In contrast to previous studies, the effect of in-cloud scavenging and of potential sampling losses at the site are taken explicitly into account in the trend analysis. The analysis is combined with a back trajectory analysis and satellite-derived sea ice data to support the interpretation of the observed trends. We find that the optical properties of aerosol particles have undergone clear and significant changes in the past 2 decades. The scattering Angstrom exponent exhibits statistically significant decreasing of between -4.9 % yr(-1) and -6.5 % yr(-1) (using wavelengths of lambda = 450 and 550 nm), while the particle light scattering coefficient exhibits statistically significant increasing trends of between 2.6 % yr(-1) and 2.9 % yr(-1) (at a wavelength of lambda = 550 nm). The magnitudes of the trends vary depending on the season. These trends indicate a shift to an aerosol dominated more by coarse-mode particles, most likely the result of increases in the relative amount of sea spray aerosol. We show that changes in air mass circulation patterns, specifically an increase in air masses from the south-west, are responsible for the shift in aerosol optical properties, while the decrease of Arctic sea ice in the last 2 decades only had a marginal influence on the observed trends.

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• Influence of Organic Acids on the Surface Composition of Sea Spray Aerosol

  2020. Isaak Unger (et al.). Journal of Physical Chemistry A 124 (2), 422-429

  Article

  Recent studies on sea spray aerosol indicate an enrichment of Ca2+ in small particles, which are often thought to originate from the very surface of a water body when bubbles burst. One model to explain this observation is the formation of ion pairs between Ca2+(aq) and surface-active organic species. In this study, we have used X-ray photoelectron spectroscopy to probe aqueous salt solutions and artificial sea spray aerosol to study whether ion pairing in the liquid environment also affects the surface composition of dry aerosol. Carboxylic acids were added to the sample solutions to mimic some of the organic compounds present in natural seawater. Our results show that the formation of a core-shell structure governs the surface composition of the aerosol. The core-shell structure contrasts previous observations of the dry sea spray aerosol on substrates. As such, this may indicate that substrates can impact the morphology of the dried aerosol.

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• Multidecadal trend analysis of in situ aerosol radiative properties around the world

  2020. Martine Collaud Coen (et al.). Atmospheric Chemistry And Physics 20 (14), 8867-8908

  Article

  In order to assess the evolution of aerosol parameters affecting climate change, a long-term trend analysis of aerosol optical properties was performed on time series from 52 stations situated across five continents. The time series of measured scattering, backscattering and absorption coefficients as well as the derived single scattering albedo, backscattering fraction, scattering and absorption Angstrom exponents covered at least 10 years and up to 40 years for some stations. The non-parametric seasonal Mann-Kendall (MK) statistical test associated with several pre-whitening methods and with Sen's slope was used as the main trend analysis method. Comparisons with general least mean square associated with autoregressive bootstrap (GLS/ARB) and with standard least mean square analysis (LMS) enabled confirmation of the detected MK statistically significant trends and the assessment of advantages and limitations of each method. Currently, scattering and backscattering coefficient trends are mostly decreasing in Europe and North America and are not statistically significant in Asia, while polar stations exhibit a mix of increasing and decreasing trends. A few increasing trends are also found at some stations in North America and Australia. Absorption coefficient time series also exhibit primarily decreasing trends. For single scattering albedo, 52 % of the sites exhibit statistically significant positive trends, mostly in Asia, eastern/northern Europe and the Arctic, 22 % of sites exhibit statistically significant negative trends, mostly in central Europe and central North America, while the remaining 26 % of sites have trends which are not statistically significant. In addition to evaluating trends for the overall time series, the evolution of the trends in sequential 10-year segments was also analyzed. For scattering and backscattering, statistically significant increasing 10-year trends are primarily found for earlier periods (10-year trends ending in 2010-2015) for polar stations and Mauna Loa. For most of the stations, the present-day statistically significant decreasing 10-year trends of the single scattering albedo were preceded by not statistically significant and statistically significant increasing 10-year trends. The effect of air pollution abatement policies in continental North America is very obvious in the 10-year trends of the scattering coefficient - there is a shift to statistically significant negative trends in 2009-2012 for all stations in the eastern and central USA. This long-term trend analysis of aerosol radiative properties with a broad spatial coverage provides insight into potential aerosol effects on climate changes.

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• Physical and chemical properties of aerosol particles and cloud residuals on Mt. angstrom reskutan in Central Sweden during summer 2014

  2020. Emelie Linnéa Graham (et al.). Tellus. Series B, Chemical and physical meteorology 72 (1)

  Article

  The size distribution, volatility and hygroscopicity of ambient aerosols and cloud residuals were measured with a differential mobility particle sizer (DMPS) and a volatility-hygroscopicity tandem differential mobility analyser (VHTDMA) coupled to a counterflow virtual impactor (CVI) inlet during the Cloud and Aerosol Experiment at Are (CAEsAR) campaign at Mt. Areskutan during summer 2014. The chemical composition of particulate matter (PM) and cloud water were analysed offline using thermo-optical OC/EC analysis and ion chromatography. The importance of aerosol particle size for cloud droplet activation and subsequent particle scavenging was clearly visible in the measured size distributions. Cloud residuals were shifted towards larger sizes compared to ambient aerosol, and the cloud events were followed by a size distribution dominated by smaller particles. Organics dominated both PM (62% organic mass fraction) and cloud water (63% organic mass fraction) composition. The volatility and hygroscopicity of the ambient aerosols were representative of homogeneous aged aerosol with contributions from biogenic secondary organics, with median volume fraction remaining (VFR) of 0.04-0.05, and median hygroscopicity parameter kappa of 0.16-0.24 for 100-300 nm particles. The corresponding VFR and kappa for the cloud residuals were 0.03-0.04 and 0.18-0.20. The chemical composition, hygroscopicity and volatility measurements thus showed no major differences between the ambient aerosol particles and cloud residuals. The VFR and kappa values predicted based on the chemical composition measurements agreed well with the VHTDMA measurements, indicating the bulk chemical composition to be a reasonable approximation throughout the size distribution. There were indications, however, of some more subtle changes in time scales not achievable by the offline chemical analysis applied here. Further, online observations of aerosol and cloud residual chemical composition are therefore warranted.

  Read more about Physical and chemical properties of aerosol particles and cloud residuals on Mt. angstrom reskutan in Central Sweden during summer 2014

• The MILAN Campaign: Studying Diel Light Effects on the Air–Sea Interface

  2020. Christian Stolle (et al.). Bulletin of The American Meteorological Society - (BAMS) 101 (2), E146-E166

  Article

  The sea surface microlayer (SML) at the air–sea interface is <1 mm thick, but it is physically, chemically, and biologically distinct from the underlying water and the atmosphere above. Wind-driven turbulence and solar radiation are important drivers of SML physical and biogeochemical properties. Given that the SML is involved in all air–sea exchanges of mass and energy, its response to solar radiation, especially in relation to how it regulates the air–sea exchange of climate-relevant gases and aerosols, is surprisingly poorly characterized. MILAN (Sea Surface Microlayer at Night) was an international, multidisciplinary campaign designed to specifically address this issue. In spring 2017, we deployed diverse sampling platforms (research vessels, radio-controlled catamaran, free-drifting buoy) to study full diel cycles in the coastal North Sea SML and in underlying water, and installed a land-based aerosol sampler. We also carried out concurrent ex situ experiments using several microsensors, a laboratory gas exchange tank, a solar simulator, and a sea spray simulation chamber. In this paper we outline the diversity of approaches employed and some initial results obtained during MILAN. Our observations of diel SML variability show, for example, an influence of (i) changing solar radiation on the quantity and quality of organic material and (ii) diel changes in wind intensity primarily forcing air–sea CO₂ exchange. Thus, MILAN underlines the value and the need of multidiciplinary campaigns for integrating SML complexity into the context of air–sea interaction.

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• A global view on the effect of water uptake on aerosol particle light scattering

  2019. María A. Burgos (et al.). Scientific Data 6

  Article

  A reference dataset of multi-wavelength particle light scattering and hemispheric backscattering coefficients for different relative humidities (RH) between RH = 30 and 95% and wavelengths between lambda = 450 nm and 700 nm is described in this work. Tandem-humidified nephelometer measurements from 26 ground-based sites around the globe, covering multiple aerosol types, have been re-analysed and harmonized into a single dataset. The dataset includes multi-annual measurements from long-term monitoring sites as well as short-term field campaign data. The result is a unique collection of RH-dependent aerosol light scattering properties, presented as a function of size cut. This dataset is important for climate and atmospheric model-measurement inter-comparisons, as a means to improve model performance, and may be useful for satellite and remote sensing evaluation using surface-based, in-situ measurements.

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• Composition, isotopic fingerprint and source attribution of nitrate deposition from rain and fog at a Sub-Arctic Mountain site in Central Sweden (Mt Åreskutan)

  2019. Carmen P. Vega (et al.). Tellus. Series B, Chemical and physical meteorology 71

  Article

  While dry and rain deposition of nitrate (NO3-) and ammonium (NH4+) are regularly assessed, fog deposition is often overlooked. This work assesses summer fog events contribution to nitrogen deposition and availability for forest ecosystems. Rain and fog samples were collected at Mt Areskutan, Sweden, during CAEsAR (Cloud and Aerosol Characterization Experiment), in 2014. NH4+ + NO3- represent (31 +/- 25) % of total rain ion amount, and (31 +/- 42) % in fog. Based on ion concentrations and the nitrate stable isotope signatures delta(N-15) and delta(O-18), it was possible to detect the plume generated by the Vastmanland forest fire; NOx emissions from oil rigs and Kola Peninsula; and the plume of Bardarbunga volcano, Iceland. Scavenging of ions by fog was more efficient than by rain. Rain NH4+ and NO3- deposition was (26 +/- 36) mu mol m(-2) d(-1) and (23 +/- 27) mu mol m(-2) d(-1), respectively. Fog NH4+ and NO3- contributed (77 +/- 80) % to total wet deposition of these species. Upscaling rain deposition fluxes to 1 year gave an inorganic nitrogen deposition of (18 +/- 16) mmol m(-2) a(-1) ((252 +/- 224) mg m(-2) a(-1) N equivalents), whereas fog deposition was estimated as (59 +/- 47) mmol m(-2) a(-1) ((826 +/- 658) mg m(-2) a(-1) N equivalents). Annual fog deposition was four times higher than previously reported for the area which only considered rain deposition. However, great uncertainty on the calculation of fog deposition need to be bear in mind. These findings suggest that fog should be considered in deposition estimates of inorganic nitrogen and major ions. If fog deposition is not accounted for, ion wet deposition may be greatly underestimated. Further sampling of wet and dry deposition is important for understanding the influence of nitrogen deposition on forest and vegetation development, as well as soil major ion loads.

  Read more about Composition, isotopic fingerprint and source attribution of nitrate deposition from rain and fog at a Sub-Arctic Mountain site in Central Sweden (Mt Åreskutan)

• Interactions between the atmosphere, cryosphere, and ecosystems at northern high latitudes

  2019. Michael Boy (et al.). Atmospheric Chemistry And Physics 19 (3), 2015-2061

  Article

  The Nordic Centre of Excellence CRAICC (Cryosphere-Atmosphere Interactions in a Changing Arctic Climate), funded by NordForsk in the years 2011-2016, is the largest joint Nordic research and innovation initiative to date, aiming to strengthen research and innovation regarding climate change issues in the Nordic region. CRAICC gathered more than 100 scientists from all Nordic countries in a virtual centre with the objectives of identifying and quantifying the major processes controlling Arctic warming and related feedback mechanisms, outlining strategies to mitigate Arctic warming, and developing Nordic Earth system modelling with a focus on short-lived climate forcers (SLCFs), including natural and anthropogenic aerosols. The outcome of CRAICC is reflected in more than 150 peer-reviewed scientific publications, most of which are in the CRAICC special issue of the journal Atmospheric Chemistry and Physics. This paper presents an overview of the main scientific topics investigated in the centre and provides the reader with a state-of-the-art comprehensive summary of what has been achieved in CRAICC with links to the particular publications for further detail. Faced with a vast amount of scientific discovery, we do not claim to completely summarize the results from CRAICC within this paper, but rather concentrate here on the main results which are related to feedback loops in climate change-cryosphere interactions that affect Arctic amplification.

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• The radiative impact of out-of-cloud aerosol hygroscopic growth during the summer monsoon in southern West Africa

  2019. Sophie L. Haslett (et al.). Atmospheric Chemistry And Physics 19 (3), 1505-1520

  Article

  Water in the atmosphere can exist in the solid, liquid or gas phase. At high humidities, if the aerosol population remains constant, more water vapour will condense onto the particles and cause them to swell, sometimes up to several times their original size. This significant change in size and chemical composition is termed hygroscopic growth and alters a particle's optical properties. Even in unsaturated conditions, this can change the aerosol direct effect, for example by increasing the extinction of incoming sunlight. This can have an impact on a region's energy balance and affect visibility. Here, aerosol and relative humidity measurements collected from aircraft and radiosondes during the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) campaign were used to estimate the effect of highly humid layers of air on aerosol optical properties during the monsoon season in southern West Africa. The effects of hygroscopic growth in this region are of particular interest due to the regular occurrence of high humidity and the high levels of pollution in the region. The Zdanovskii, Stokes and Robinson (ZSR) mixing rule is used to estimate the hygroscopic growth of particles under different conditions based on chemical composition. These results are used to estimate the aerosol optical depth (AOD) at lambda = 525 nm for 63 relative humidity profiles. The median AOD in the region from these calculations was 0.36, the same as that measured by sun photometers at the ground site. The spread in the calculated AODs was less than the spread from the sun photometer measurements. In both cases, values above 0.5 were seen predominantly in the mornings and corresponded with high humidities. Observations of modest variations in aerosol load and composition are unable to explain the high and variable AODs observed using sun photometers, which can only be recreated by accounting for the very elevated and variable relative humidities (RHs) in the boundary layer. Most importantly, the highest AODs present in the mornings are not possible without the presence of high RH in excess of 95 %. Humid layers are found to have the most significant impact on AOD when they reach RH greater than 98 %, which can result in a wet AOD more than 1.8 times the dry AOD. Unsaturated humid layers were found to reach these high levels of RH in 37% of observed cases. It can therefore be concluded that the high AODs present across the region are driven by the high humidities and are then moderated by changes in aerosol abundance. Aerosol concentrations in southern West Africa are projected to increase substantially in the coming years; results presented here show that the presence of highly humid layers in the region is likely to enhance the consequent effect on AOD significantly.

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• Chemical composition and source analysis of carbonaceous aerosol particles at a mountaintop site in central Sweden

  2017. Vera Franke (et al.). Tellus. Series B, Chemical and physical meteorology 69

  Article

  The chemical composition of atmospheric particulate matter at Mt. angstrom reskutan, a mountaintop site in central Sweden, was analysed with a focus on its carbonaceous content. Filter samples taken during the Cloud and Aerosol Experiment at angstrom re (CAEsAR 2014) were analysed by means of a thermo-optical method and ion chromatography. Additionally, the particle light absorption and particle number size distribution measurements for the entire campaign were added to the analysis. Mean airborne concentrations of organic and elemental carbon during CAEsAR 2014 were OC= 0.85 +/- 0.8 mu gm(-3) and EC = 0.06 +/- 0.06 mu gm(-3), respectively. Elemental to organic carbon ratios varied between EC/OC = 0.02 and 0.19. During the study a large wildfire occurred in Vastmanland, Sweden, with the plume reaching our study site. This led to significant increases in OC and EC concentrations (OC = 3.04 +/- 0.03 mu gm(-3) and EC = 0.24 +/- 0.00 mu gm(-3)). The mean mass-specific absorption coefficient observed during the campaign was sigma(BC)(abs) = 9.1 +/- 7.3 m(2)g(-1) (at wavelength lambda= 637 nm). In comparison to similarly remote European sites, Mt. angstrom reskutan experienced significantly lower carbonaceous aerosol loadings with a clear dominance of organic carbon. A mass closure study revealed a missing chemical mass fraction that likely originated from mineral dust. Potential regional source contributions of the carbonaceous aerosol were investigated using modelled air mass back trajectories. This source apportionment pointed to a correlation between high EC concentrations and air originating from continental Europe. Particles rich in organic carbon most often arrived from highly vegetated continental areas. However, marine regions were also a source of these aerosol particles. The source contributions derived during this study were compared to emission inventories of an Earth system model. This comparison highlighted a lack of OC and EC point-sources in the model's emission inventory which could potentially lead to an underestimation of the carbonaceous aerosol reaching Mt. angstrom reskutan in the simulation of this Earth system model.

  Read more about Chemical composition and source analysis of carbonaceous aerosol particles at a mountaintop site in central Sweden

• Revising the hygroscopicity of inorganic sea salt particles

  2017. Paul Zieger (et al.). Nature Communications 8

  Article

  Sea spray is one of the largest natural aerosol sources and plays an important role in the Earth's radiative budget. These particles are inherently hygroscopic, that is, they take-up moisture from the air, which affects the extent to which they interact with solar radiation. We demonstrate that the hygroscopic growth of inorganic sea salt is 8-15% lower than pure sodium chloride, most likely due to the presence of hydrates. We observe an increase in hygroscopic growth with decreasing particle size (for particle diameters <150 nm) that is independent of the particle generation method. We vary the hygroscopic growth of the inorganic sea salt within a general circulation model and show that a reduced hygroscopicity leads to a reduction in aerosol-radiation interactions, manifested by a latitudinal-dependent reduction of the aerosol optical depth by up to 15%, while cloud-related parameters are unaffected. We propose that a value of kappa(s) = 1.1 (at RH = 90%) is used to represent the hygroscopicity of inorganic sea salt particles in numerical models.

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• Calcium enrichment in sea spray aerosol particles

  2016. Matthew E. Salter (et al.). Geophysical Research Letters 43 (15), 8277-8285

  Article

  Sea spray aerosol particles are an integral part of the Earth's radiation budget. To date, the inorganic composition of nascent sea spray aerosol particles has widely been assumed to be equivalent to the inorganic composition of seawater. Here we challenge this assumption using a laboratory sea spray chamber containing both natural and artificial seawater, as well as with ambient aerosol samples collected over the central Arctic Ocean during summer. We observe significant enrichment of calcium in submicrometer (<1m in diameter) sea spray aerosol particles when particles are generated from both seawater sources in the laboratory as well as in the ambient aerosols samples. We also observe a tendency for increasing calcium enrichment with decreasing particle size. Our results suggest that calcium enrichment in sea spray aerosol particles may be environmentally significant with implications for our understanding of sea spray aerosol, its impact on Earth's climate, as well as the chemistry of the marine atmosphere.

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• Effect of hygroscopic growth on the aerosol light-scattering coefficient: A review of measurements, techniques and error sources

  2016. G. Titos (et al.). Atmospheric Environment 141, 494-507

  Article

  Knowledge of the scattering enhancement factor,.f(RH), is important for an accurate description of direct aerosol radiative forcing. This factor is defined as the ratio between the scattering coefficient at enhanced relative humidity, RH, to a reference (dry) scattering coefficient. Here, we review the different experimental designs used to measure the scattering coefficient at dry and humidified conditions as well as the procedures followed to analyze the measurements. Several empirical parameterizations for the relationship between f(RH) and RH have been proposed in the literature. These parameterizations have been reviewed and tested using experimental data representative of different hygroscopic growth behavior and a new parameterization is presented. The potential sources of error in f(RH) are discussed. A Monte Carlo method is used to investigate the overall measurement uncertainty, which is found to be around 20-40% for moderately hygroscopic aerosols. The main factors contributing to this uncertainty are the uncertainty in RH measurement, the dry reference state and the nephelometer uncertainty. A literature survey of nephelometry-based f(RH) measurements is presented as a function of aerosol type. In general, the highest f(RH) values were measured in clean marine environments, with pollution having a major influence on f(RH). Dust aerosol tended to have the lowest reported hygroscopicity of any of the aerosol types studied. Major open questions and suggestions for future research priorities are outlined.

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• LOAC: a small aerosol optical counter/sizer for ground-based and balloon measurements of the size distribution and nature of atmospheric particles - Part 1

  2016. Jean-Baptiste Renard (et al.). Atmospheric Measurement Techniques 9 (4), 1721-1742

  Article

  The study of aerosols in the troposphere and in the stratosphere is of major importance both for climate and air quality studies. Among the numerous instruments available, optical aerosol particles counters (OPCs) provide the size distribution in diameter range from about 100 nm to a few tens of mu m. Most of them are very sensitive to the nature of aerosols, and this can result in significant biases in the retrieved size distribution. We describe here a new versatile optical particle/sizer counter named LOAC (Light Optical Aerosol Counter), which is light and compact enough to perform measurements not only at the surface but under all kinds of balloons in the troposphere and in the stratosphere. LOAC is an original OPC performing observations at two scattering angles. The first one is around 12 degrees, and is almost insensitive to the refractive index of the particles; the second one is around 60 degrees and is strongly sensitive to the refractive index of the particles. By combining measurement at the two angles, it is possible to retrieve the size distribution between 0.2 and 100 mu m and to estimate the nature of the dominant particles (droplets, carbonaceous, salts and mineral particles) when the aerosol is relatively homogeneous. This typology is based on calibration charts obtained in the laboratory. The uncertainty for total concentrations measurements is +/- 20% when concentrations are higher than 1 particle cm 3 (for a 10 min integration time). For lower concentrations, the uncertainty is up to about +/- 60% for concentrations smaller than 10 2 particle cm(-3). Also, the uncertainties in size calibration are +/- 0.025 mu m for particles smaller than 0.6 mu m, 5% for particles in the 0.7-2 mu m range, and 10% for particles greater than 2 mu m. The measurement accuracy of sub-micronic particles could be reduced in a strongly turbid case when concentration of particles > 3 mu m exceeds a few particles cm(-3). Several campaigns of cross-comparison of LOAC with other particle counting instruments and remote sensing photometers have been conducted to validate both the size distribution derived by LOAC and the retrieved particle number density. The typology of the aerosols has been validated in well-defined conditions including urban pollution, desert dust episodes, sea spray, fog, and cloud. Comparison with reference aerosol mass monitoring instruments also shows that the LOAC measurements can be successfully converted to mass concentrations.

  Read more about LOAC

• LOAC: a small aerosol optical counter/sizer for ground-based and balloon measurements of the size distribution and nature of atmospheric particles - Part 2

  2016. Jean-Baptiste Renard (et al.). Atmospheric Measurement Techniques 9 (8), 3673-3686

  Article

  In the companion (Part I) paper, we have described and evaluated a new versatile optical particle counter/sizer named LOAC (Light Optical Aerosol Counter), based on scattering measurements at angles of 12 and 60A degrees. That allows for some typology identification of particles (droplets, carbonaceous, salts, and mineral dust) in addition to size-segregated counting in a large diameter range from 0.2aEuro-A mu m up to possibly more than 100aEuro-A mu m depending on sampling conditions (Renard et al., 2016). Its capabilities overpass those of preceding optical particle counters (OPCs) allowing the characterization of all kind of aerosols from submicronic-sized absorbing carbonaceous particles in polluted air to very coarse particles (> 10-20aEuro-A mu m in diameter) in desert dust plumes or fog and clouds. LOAC's light and compact design allows measurements under all kinds of balloons, on-board unmanned aerial vehicles (UAVs) and at ground level. We illustrate here the first LOAC airborne results obtained from a UAV and a variety of scientific balloons. The UAV was deployed in a peri-urban environment near Bordeaux in France. Balloon operations include (i) tethered balloons deployed in urban environments in Vienna (Austria) and Paris (France), (ii) pressurized balloons drifting in the lower troposphere over the western Mediterranean (during the Chemistry-Aerosol Mediterranean Experiment - ChArMEx campaigns), (iii) meteorological sounding balloons launched in the western Mediterranean region (ChArMEx) and from Aire-sur-l'Adour in south-western France (VOLTAIRE-LOAC campaign). More focus is put on measurements performed in the Mediterranean during (ChArMEx) and especially during African dust transport events to illustrate the original capability of balloon-borne LOAC to monitor in situ coarse mineral dust particles. In particular, LOAC has detected unexpected large particles in desert sand plumes.

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• An empirically derived inorganic sea spray source function incorporating sea surface temperature

  2015. Matthew E. Salter (et al.). Atmospheric Chemistry And Physics 15 (19), 11047-11066

  Article

  We have developed an inorganic sea spray source function that is based upon state-of-the-art measurements of sea spray aerosol production using a temperature-controlled plunging jet sea spray aerosol chamber. The size-resolved particle production was measured between 0.01 and 10 mu m dry diameter. Particle production decreased non-linearly with increasing seawater temperature (between -1 and 30 degrees C) similar to previous findings. In addition, we observed that the particle effective radius, as well as the particle surface, particle volume and particle mass, increased with increasing seawater temperature due to increased production of particles with dry diameters greater than 1 mu m. By combining these measurements with the volume of air entrained by the plunging jet we have determined the size-resolved particle flux as a function of air entrainment. Through the use of existing parameterisations of air entrainment as a function of wind speed, we were subsequently able to scale our laboratory measurements of particle production to wind speed. By scaling in this way we avoid some of the difficulties associated with defining the white area of the laboratory whitecap - a contentious issue when relating laboratory measurements of particle production to oceanic whitecaps using the more frequently applied whitecap method. The here-derived inorganic sea spray source function was implemented in a Lagrangian particle dispersion model (FLEXPART - FLEXible PARTicle dispersion model). An estimated annual global flux of inorganic sea spray aerosol of 5.9 +/- 0.2 Pg yr(-1) was derived that is close to the median of estimates from the same model using a wide range of existing sea spray source functions. When using the source function derived here, the model also showed good skill in predicting measurements of Na+ concentration at a number of field sites further underlining the validity of our source function. In a final step, the sensitivity of a large-scale model (NorESM - the Norwegian Earth System Model) to our new source function was tested. Compared to the previously implemented parameterisation, a clear decrease of sea spray aerosol number flux and increase in aerosol residence time was observed, especially over the Southern Ocean. At the same time an increase in aerosol optical depth due to an increase in the number of particles with optically relevant sizes was found. That there were noticeable regional differences may have important implications for aerosol optical properties and number concentrations, subsequently also affecting the indirect radiative forcing by non-sea spray anthropogenic aerosols.

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• Low hygroscopic scattering enhancement of boreal aerosol and the implications for a columnar optical closure study

  2015. Paul Zieger (et al.). Atmospheric Chemistry And Physics 15 (13), 7247-7267

  Article

  Ambient aerosol particles can take up water and thus change their optical properties depending on the hygroscopicity and the relative humidity (RH) of the surrounding air. Knowledge of the hygroscopicity effect is of crucial importance for radiative forcing calculations and is also needed for the comparison or validation of remote sensing or model results with in situ measurements. Specifically, particle light scattering depends on RH and can be described by the scattering enhancement factor f(RH), which is defined as the particle light scattering coefficient at defined RH divided by its dry value (RH < 30-40 %). Here, we present results of an intensive field campaign carried out in summer 2013 at the SMEAR II station at Hyytiala, Finland. Ground-based and airborne measurements of aerosol optical, chemical and microphysical properties were conducted. The f(RH) measured at ground level by a humidified nephelometer is found to be generally lower (e.g. 1.63 +/- 0.22 at RH = 85% and lambda = 525 nm) than observed at other European sites. One reason is the high organic mass fraction of the aerosol encountered at Hyytiala to which f(RH) is clearly anti-correlated (R-2 approximate to 0.8). A simplified parametrization of f(RH) based on the measured chemical mass fraction can therefore be derived for this aerosol type. A trajectory analysis revealed that elevated values of f(RH) and the corresponding elevated inorganic mass fraction are partially caused by transported hygroscopic sea spray particles. An optical closure study shows the consistency of the ground-based in situ measurements. Our measurements allow to determine the ambient particle light extinction coefficient using the measured f(RH). By combining the ground-based measurements with intensive aircraft measurements of the particle number size distribution and ambient RH, columnar values of the particle extinction coefficient are determined and compared to columnar measurements of a co-located AERONET sun photometer. The water uptake is found to be of minor importance for the column-averaged properties due to the low particle hygroscopicity and the low RH during the daytime of the summer months. The in situ derived aerosol optical depths (AOD) clearly correlate with directly measured values of the sun photometer but are substantially lower compared to the directly measured values (factor of similar to 2-3). The comparison degrades for longer wavelengths. The disagreement between in situ derived and directly measured AOD is hypothesized to originate from losses of coarse and fine mode particles through dry deposition within the canopy and losses in the in situ sampling lines. In addition, elevated aerosol layers (above 3 km) from long-range transport were observed using an aerosol lidar at Kuopio, Finland, about 200 km east-northeast of Hyytiala. These elevated layers further explain parts of the disagreement.

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• Influence of water uptake on the aerosol particle light scattering coefficients of the Central European aerosol

  2014. Paul Zieger (et al.). Tellus. Series B, Chemical and physical meteorology 66, 22716

  Article

  The influence of aerosol water uptake on the aerosol particle light scattering was examined at the regional continental research site Melpitz, Germany. The scattering enhancement factor f(RH), defined as the aerosol particle scattering coefficient at a certain relative humidity (RH) divided by its dry value, was measured using a humidified nephelometer. The chemical composition and other microphysical properties were measured in parallel. f(RH) showed a strong variation, e.g. with values between 1.2 and 3.6 at RH = 85% and lambda = 550 nm. The chemical composition was found to be the main factor determining the magnitude of f(RH), since the magnitude of f(RH) clearly correlated with the inorganic mass fraction measured by an aerosol mass spectrometer (AMS). Hysteresis within the recorded humidograms was observed and explained by long-range transported sea salt. A closure study using Mie theory showed the consistency of the measured parameters.

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• Investigation of the Planetary Boundary Layer in the Swiss Alps Using Remote Sensing and In Situ Measurements

  2014. C. Ketterer (et al.). Boundary-layer Meteorology 151 (2), 317-334

  Article

  The development of the planetary boundary layer (PBL) has been studied in a complex terrain using various remote sensing and in situ techniques. The high-altitude research station at Jungfraujoch (3,580 m a.s.l.) in the Swiss Alps lies for most of the time in the free troposphere except when it is influenced by the PBL reaching the station, especially during the summer season. A ceilometer and a wind profiler were installed at Kleine Scheidegg, a mountain pass close to Jungfraujoch, located at an altitude of 2,061 m a.s.l. Data from the ceilometer were analyzed using two different algorithms, while the signal-to-noise ratio of the wind profiler was studied to compare the retrieved PBL heights. The retrieved values from the ceilometer and wind profiler agreed well during daytime and cloud-free conditions. The results were additionally compared with the PBL height estimated by the numerical weather prediction model COSMO-2, which showed a clear underestimation of the PBL height for most of the cases but occasionally also a slight overestimation especially around noon, when the PBL showed its maximum extent. Air parcels were transported upwards by slope winds towards Jungfraujoch when the PBL was higher than 2,800 m a.s.l. during cloud-free cases. This was confirmed by the in situ aerosol measurements at Jungfraujoch with a significant increase in particle number concentration, particle light absorption and scattering coefficients when PBL-influenced air masses reached the station in the afternoon hours. The continuous aerosol in situ measurements at Jungfraujoch were clearly influenced by the local PBL development but also by long-range transport phenomena such as Saharan dust or pollution from the south.

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• Reconciling aerosol light extinction measurements from spaceborne lidar observations and in situ measurements in the Arctic

  2014. Matthias Tesche (et al.). Atmospheric Chemistry And Physics 14 (15), 7869-7882

  Article

  In this study we investigate to what degree it is possible to reconcile continuously recorded particle light extinction coefficients derived from dry in situ measurements at Zeppelin station (78.92 degrees N, 11.85 degrees E; 475 m above sea level), Ny-lesund, Svalbard, that are recalculated to ambient relative humidity, as well as simultaneous ambient observations with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. To our knowledge, this represents the first study that compares spaceborne lidar measurements to optical aerosol properties from short-term in situ observations (averaged over 5 h) on a case-by-case basis. Finding suitable comparison cases requires an elaborate screening and matching of the CALIOP data with respect to the location of Zeppelin station as well as the selection of temporal and spatial averaging intervals for both the ground-based and spaceborne observations. Reliable reconciliation of these data cannot be achieved with the closest-approach method, which is often used in matching CALIOP observations to those taken at ground sites. This is due to the transport pathways of the air parcels that were sampled. The use of trajectories allowed us to establish a connection between spaceborne and ground-based observations for 57 individual overpasses out of a total of 2018 that occurred in our region of interest around Svalbard (0 to 25 degrees E, 75 to 82 degrees N) in the considered year of 2008. Matches could only be established during winter and spring, since the low aerosol load during summer in connection with the strong solar background and the high occurrence rate of clouds strongly influences the performance and reliability of CALIOP observations. Extinction coefficients in the range of 2 to 130 Mm(-1) at 532 nm were found for successful matches with a difference of a factor of 1.47 (median value for a range from 0.26 to 11.2) between the findings of in situ and spaceborne observations (the latter being generally larger than the former). The remaining difference is likely to be due to the natural variability in aerosol concentration and ambient relative humidity, an insufficient representation of aerosol particle growth, or a misclassification of aerosol type (i.e., choice of lidar ratio) in the CALIPSO retrieval.

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• Seasonal variation of aerosol water uptake and its impact on the direct radiative effect at Ny-Alesund, Svalbard

  2014. Narges Rastak (et al.). Atmospheric Chemistry And Physics 14 (14), 7445-7460

  Article

  In this study we investigated the impact of water uptake by aerosol particles in ambient atmosphere on their optical properties and their direct radiative effect (ADRE, W m(-2)) in the Arctic at Ny-Alesund, Svalbard, during 2008. To achieve this, we combined three models, a hygroscopic growth model, a Mie model and a radiative transfer model, with an extensive set of observational data. We found that the seasonal variation of dry aerosol scattering coefficients showed minimum values during the summer season and the beginning of fall (July-August-September), when small particles (< 100 nm in diameter) dominate the aerosol number size distribution. The maximum scattering by dry particles was observed during the Arctic haze period (March-April-May) when the average size of the particles was larger. Considering the hygroscopic growth of aerosol particles in the ambient atmosphere had a significant impact on the aerosol scattering coefficients: the aerosol scattering coefficients were enhanced by on average a factor of 4.30 +/- 2.26 (mean +/- standard deviation), with lower values during the haze period (March-April-May) as compared to summer and fall. Hygroscopic growth of aerosol particles was found to cause 1.6 to 3.7 times more negative ADRE at the surface, with the smallest effect during the haze period (March-April-May) and the highest during late summer and beginning of fall (July-August-September).

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• Effects of relative humidity on aerosol light scattering: results from different European sites

  2013. Paul Zieger (et al.). Atmospheric Chemistry And Physics 13 (21), 10609-10631

  Article

  The effect of aerosol water uptake on the aerosol particle light scattering coefficient (sigma(sp)) is described in this study by comparing measurements from five European sites: the Jungfraujoch, located in the Swiss Alps at 3580 m a.s.l.; Ny-angstrom lesund, located on Spitsbergen in the Arctic; Mace Head, a coastal site in Ireland; Cabauw, a rural site in the Netherlands; and Melpitz, a regional background site in Eastern Germany. These sites were selected according to the aerosol type usually encountered at that location. The scattering enhancement factor f(RH, lambda) is the key parameter to describe the effect of water uptake on the particle light scattering. It is defined as the sigma(sp)(RH) at a certain relative humidity (RH) and wavelength lambda divided by its dry value. f(RH) at the five sites varied widely, starting at very low values of f(RH = 85%, lambda = 550 nm) around 1.28 for mineral dust, and reaching up to 3.41 for Arctic aerosol. Hysteresis behavior was observed at all sites except at the Jungfraujoch (due to the absence of sea salt). Closure studies and Mie simulations showed that both size and chemical composition determine the magnitude of f(RH). Both parameters are also needed to successfully predict f(RH). Finally, the measurement results were compared to the widely used aerosol model, OPAC (Hess et al., 1998). Significant discrepancies were seen, especially at intermediate RH ranges; these were mainly attributed to inappropriate implementation of hygroscopic growth in the OPAC model. Replacement of the hygroscopic growth with values from the recent literature resulted in a clear improvement of the OPAC model.

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• Effect of Long-Range Transported Fire Aerosols on Cloud Condensation Nuclei Concentrations and Cloud Properties at High Latitudes

  2024. S. M. Kommula (et al.). Geophysical Research Letters 51 (6)

  Article

  Active vegetation fires in south-eastern (SE) Europe resulted in a notable increase in the number concentration of aerosols and cloud condensation nuclei (CCN) particles at two high latitude locations—the SMEAR IV station in Kuopio, Finland, and the Zeppelin Observatory in Svalbard, high Arctic. During the fire episode aerosol hygroscopicity κ slightly increased at SMEAR IV and at the Zeppelin Observatory κ decreased. Despite increased κ in high CCN conditions at SMEAR IV, the aerosol activation diameter increased due to the decreased supersaturation with an increase in aerosol loading. In addition, at SMEAR IV during the fire episode, in situ measured cloud droplet number concentration (CDNC) increased by a factor of ∼7 as compared to non-fire periods which was in good agreement with the satellite observations (MODIS, Terra). Results from this study show the importance of SE European fires for cloud properties and radiative forcing in high latitudes.

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• Highly Hygroscopic Aerosols Facilitate Summer and Early-Autumn Cloud Formation at Extremely Low Concentrations Over the Central Arctic Ocean

  2024. P. Duplessis (et al.). Journal of Geophysical Research - Atmospheres 129 (2)

  Article

  Arctic clouds are sensitive to atmospheric particles since these are sometimes in such low concentrations that clouds cannot always form under supersaturated water vapor conditions. This is especially true in the late summer, when aerosol concentrations are generally very low in the high Arctic. The environment changes rapidly around freeze-up as the open waters close and snow starts accumulating on ice. We investigated droplet formation during eight significant fog events in the central Arctic Ocean, north of 80 degrees, from August 12 to 19 September 2018 during the Arctic Ocean 2018 expedition onboard the icebreaker Oden. Calculated hygroscopicity parameters (kappa) for the entire study were very high (up to kappa = 0.85 +/- 0.13), notably after freeze-up, suggesting that atmospheric particles were very cloud condensation nuclei (CCN)-active. At least one of the events showed that surface clouds were able to form and persist for at least a couple hours at aerosol concentrations less than 10 cm-3, which was previously suggested to be the minimum for cloud formation. Among these events that were considered limited in CCN, effective radii were generally larger than in the high CCN cases. In some of the fog events, droplet residuals particles did not reactivate under supersaturations up to 0.95%, suggesting either in-droplet reactions decreased hygroscopicity, or an ambient supersaturation above 1%. These results provide insight into droplet formation during the clean late-summer and fall of the high Arctic with limited influence from continental sources. The Arctic atmosphere can be very clean in the summer, to the point that clouds cannot form because there are insufficient particles present for the water vapor to condense upon. This has important implications for the radiation budget, which is highly dependent on clouds. As part of the Arctic Ocean 2018 expedition in the central Arctic Ocean near the North Pole, we investigated the ability of particles to turn into droplets throughout the whole cruise (August 12 to 19 September 2018), and during eight significant fog events. Overall, we found that after the sea ice started to freeze, the particles were more capable of turning into cloud droplets. During one fog event, we observed fog droplets forming when the particle concentrations were lower than the limit that past studies had suggested that fog/cloud could be sustained. During several fog events, the dried fog droplets did not always re-form droplets when exposed to cloud-like conditions, which suggests that the original droplets must have formed under extreme conditions. Our results show that in the summer/fall in the high Arctic, liquid droplets sometimes form under unusual circumstances that are likely not always considered in models. Aerosol hygroscopicity was greater after surface water freeze-up than beforeHygroscopicity of Aitken mode particles was generally greater than accumulation mode particlesCloud droplet effective radii during aerosol-limited periods were larger generally than periods with higher aerosol concentrations

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• Impact of Biomass Burning on Arctic Aerosol Composition

  2024. Yvette Gramlich (et al.). ACS Earth and Space Chemistry

  Article

  Emissions from biomass burning (BB) occurring at midlatitudes can reach the Arctic, where they influence the remote aerosol population. By using measurements of levoglucosan and black carbon, we identify seven BB events reaching Svalbard in 2020. We find that most of the BB events are significantly different to the rest of the year (nonevents) for most of the chemical and physical properties. Aerosol mass and number concentrations are enhanced by up to 1 order of magnitude during the BB events. During BB events, the submicrometer aerosol bulk composition changes from an organic- and sulfate-dominated regime to a clearly organic-dominated regime. This results in a significantly lower hygroscopicity parameter κ for BB aerosol (0.4 ± 0.2) compared to nonevents (0.5 ± 0.2), calculated from the nonrefractory aerosol composition. The organic fraction in the BB aerosol showed no significant difference for the O:C ratios (0.9 ± 0.3) compared to the year (0.9 ± 0.6). Accumulation mode particles were present during all BB events, while in the summer an additional Aitken mode was observed, indicating a mixture of the advected air mass with locally produced particles. BB tracers (vanillic, homovanillic, and hydroxybenzoic acid, nitrophenol, methylnitrophenol, and nitrocatechol) were significantly higher when air mass back trajectories passed over active fire regions in Eastern Europe, indicating agricultural and wildfires as sources. Our results suggest that the impact of BB on the Arctic aerosol depends on the season in which they occur, and agricultural and wildfires from Eastern Europe have the potential to disturb the background conditions the most. 

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• Increase in precipitation scavenging contributes to long-term reductions of light-absorbing aerosol in the Arctic

  2024. Dominic Heslin-Rees (et al.). Atmospheric Chemistry And Physics 24 (4), 2059-2075

  Article

  We investigated long-term changes using a harmonised 22-year data set of aerosol light absorption measurements, in conjunction with air mass history and aerosol source analysis. The measurements were performed at Zeppelin Observatory, Svalbard, from 2002 to 2023. We report a statistically significant decreasing long-term trend for the light absorption coefficient. However, the last 8 years of 2016–2023 showed a slight increase in the magnitude of the light absorption coefficient for the Arctic haze season. In addition, we observed an increasing trend in the single-scattering albedo from 2002 to 2023. Five distinct source regions, representing different transport pathways, were identified. The trends involving air masses from the five regions showed decreasing absorption coefficients, except for the air masses from Eurasia. We show that the changes in the occurrences of each transport pathway cannot explain the reductions in the absorption coefficient observed at the Zeppelin station. An increase in contributions of air masses from more marine regions, with lower absorption coefficients, is compensated for by an influence from high-emission regions. The proportion of air masses en route to Zeppelin, which have been influenced by active fires, has undergone a noticeable increase starting in 2015. However, this increase has not impacted the long-term trends in the concentration of light-absorbing aerosol. Along with aerosol optical properties, we also show an increasing trend in accumulated surface precipitation experienced by air masses en route to the Zeppelin Observatory. We argue that the increase in precipitation, as experienced by air masses arriving at the station, can explain a quarter of the long-term reduction in the light absorption coefficient. We emphasise that meteorological conditions en route to the Zeppelin Observatory are critical for understanding the observed trends.

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• Polar Aerosol Atmospheric Rivers: Detection, Characteristics, and Potential Applications

  2024. Remy Lapere (et al.). Journal of Geophysical Research - Atmospheres 129 (2)

  Article

  Aerosols play a key role in polar climate, and are affected by long-range transport from the mid-latitudes, both in the Arctic and Antarctic. This work investigates poleward extreme transport events of aerosols, referred to as polar aerosol atmospheric rivers (p-AAR), leveraging the concept of atmospheric rivers (AR) which signal extreme transport of moisture. Using reanalysis data, we build a detection catalog of p-AARs for black carbon, dust, sea salt and organic carbon aerosols, for the period 1980-2022. First, we describe the detection algorithm, discuss its sensitivity, and evaluate its validity. Then, we present several extreme transport case studies, in the Arctic and in the Antarctic, illustrating the complementarity between ARs and p-AARs. Despite similarities in transport pathways during co-occurring AR/p-AAR events, vertical profiles differ depending on the species, and large-scale transport patterns show that moisture and aerosols do not necessarily originate from the same areas. The complementarity between AR and p-AAR is also evidenced by their long-term characteristics in terms of spatial distribution, seasonality and trends. p-AAR detection, as a complement to AR, can have several important applications for better understanding polar climate and its connections to the mid-latitudes. The extreme transport of aerosol-containing air masses, from the mid-latitudes to the polar regions, can be characterized and quantified by leveraging polar Aerosol Atmospheric Rivers (p-AARs). This is similar to the Atmospheric Rivers (ARs) which carry large amounts of water to the poles and affect the overall stability of polar ecosystems. In this work, we establish a detection algorithm for p-AARs and evaluate it for different well-known aerosol intrusions or AR events. The areas most affected by p-AARs are described, their trends are investigated and we discuss the potential applications of p-AAR detection for a better understanding of polar climate. A catalog of polar aerosol atmospheric rivers (p-AAR) is provided for 1980-2022 by adapting an atmospheric river (AR) detection schemeImportant p-AAR events, representing rapid poleward transport of aerosol-enriched air masses, are presentedCombining AR and p-AAR can improve our understanding of the links between mid- and polar-latitudes, in the past, present and future climate

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• Aerosol and dynamical contributions to cloud droplet formation in Arctic low-level clouds

  2023. Ghislain Motos (et al.). Atmospheric Chemistry And Physics 23 (21), 13941-13956

  Article

  The Arctic is one of the most rapidly warming regions of the globe. Low-level clouds and fog modify the energy transfer from and to space and play a key role in the observed strong Arctic surface warming, a phenomenon commonly termed “Arctic amplification”. The response of low-level clouds to changing aerosol characteristics throughout the year is therefore an important driver of Arctic change that currently lacks sufficient constraints. As such, during the NASCENT campaign (Ny-Ålesund AeroSol Cloud ExperimeNT) extending over a full year from October 2019 to October 2020, microphysical properties of aerosols and clouds were studied at the Zeppelin station (475 m a.s.l.), Ny-Ålesund, Svalbard, Norway. Particle number size distributions obtained from differential mobility particle sizers as well as chemical composition derived from filter samples and an aerosol chemical speciation monitor were analyzed together with meteorological data, in particular vertical wind velocity. The results were used as input to a state-of-the-art cloud droplet formation parameterization to investigate the particle sizes that can activate to cloud droplets, the levels of supersaturation that can develop, the droplet susceptibility to aerosol and the role of vertical velocity. We evaluate the parameterization and the droplet numbers calculated through a droplet closure with in-cloud in situ measurements taken during nine flights over 4 d. A remarkable finding is that, for the clouds sampled in situ, closure is successful in mixed-phase cloud conditions regardless of the cloud glaciation fraction. This suggests that ice production through ice–ice collisions or droplet shattering may have explained the high ice fraction, as opposed to rime splintering that would have significantly reduced the cloud droplet number below levels predicted by warm-cloud activation theory. We also show that pristine-like conditions during fall led to clouds that formed over an aerosol-limited regime, with high levels of supersaturation (generally around 1 %, although highly variable) that activate particles smaller than 20 nm in diameter. Clouds formed in the same regime in late spring and summer, but aerosol activation diameters were much larger due to lower cloud supersaturations (ca. 0.5 %) that develop because of higher aerosol concentrations and lower vertical velocities. The contribution of new particle formation to cloud formation was therefore strongly limited, at least until these newly formed particles started growing. However, clouds forming during the Arctic haze period (winter and early spring) can be limited by updraft velocity, although rarely, with supersaturation levels dropping below 0.1 % and generally activating larger particles (20 to 200 nm), including pollution transported over a long range. The relationship between updraft velocity and the limiting cloud droplet number agrees with previous observations of various types of clouds worldwide, which supports the universality of this relationship.

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