Important insights on black carbon in Arctic clouds

Clouds over Svalbard. Photo: Paul Zieger

Black carbon are tiny airborne particles that are able to absorb solar radiation, making them, despite their size, an important part of the climate system. Black carbon particles are humanmade, being generated from forms of incomplete combustion for example forest fires and car exhausts. Once emitted, they are able to reside in the atmosphere for a few days, enough time to be transported up to remote places like the Arctic. 

Black carbon significant impacting Arctic climate

The Arctic region is of particular interest since it is witnessing unprecedented changes, with rates of warming far exceeding the global average, making it a worthwhile and important place to do research. Black carbon is considered a significant player when it comes to impacting the Arctic climate, not only can they absorb solar radiation whilst in the atmosphere, but once deposited they are able to reduce the albedo of the Arctic’s frozen surface, and thus enhance warming and possibly accelerate the melting of ice. Another aspect to black carbon, which has been understudied, is their interaction with cloud droplets.

In order to form, cloud droplets require a surface for water to condense on, and black carbon can provide that surface, given the right meteorological conditions. Understanding the extent to which black carbon is present in these low-level and persistent Arctic clouds, is key to getting to grips with how they come to be rained out by clouds. These particles’ ability to be rained out, also called deposited, can tell us about how long they are likely to hang around for in the atmosphere.  

First direct and long-term observational dataset

The counterflow virtual impactor inlet (or the "cloud hoover") installed on the roof of Zeppelin Observatory, Svalbard. Photo: Paul Zieger

An international team of scientists has now studied to what extend black carbon is found in Arctic clouds. Within this work, recently published in the journal of Nature Communications, the interaction of black carbon with low-level clouds is studied using the first direct and long-term observational dataset of black carbon found inside and outside of clouds, observed at the Zeppelin Observatory on Svalbard, Norway. An incredible feat given the difficulties in performing measurements in such remote locations.

“These are very unique observations that, for the first time, show directly over the course of all seasons, how much black carbon can be found inside Arctic clouds," says Radovan Krejci, researcher at the Department of Environmental Science (ACES) at Stockholm University and co-author of the study.

“We see that black carbon particles are indeed taken up by clouds and that this uptake follows a clear seasonal cycle, with a maximum in spring during the Arctic haze phenomenon, followed by cleaner summer months with very low concentrations.”

Collected data for a period of four years

The researchers have used a special inlet, called the counterflow virtual impactor (CVI) inlet, which uniquely separates cloud particles and those particles which have not become part of a cloud droplet. The sampled cloud droplets or ice crystals are then dried and the black carbon content is then measured using an optical method in the laboratory of the observatory. Thus, the researchers were able to ascertain information about the black carbon content within and outside of clouds. Within this study, the researchers have collected data of clouds for an unprecedented long period of four years.

Paul Zieger.
Photo: Stella Papadopoulou

“The uptake of black carbon by clouds is not observed to the same extend throughout the year”, says Paul Zieger, associate professor at ACES and lead author of the study, and further adds “In the winter, at colder temperatures, we see less black carbon inside cloud droplets or ice crystals, probably due to mixed-phase cloud processes something we did not expect to observe from the beginning.” Mixed-phase is the term used to describe when a cloud consists of both liquid cloud droplets and ice crystals, a common characteristic of Arctic clouds.

“Air mass trajectory analysis have helped us to identify the potential sources of black carbon, which revealed that air masses during cloudy times resided relatively longer over ocean surfaces compared to continental areas. The analysis of arriving air masses also pointed towards the potential importance of anthropogenic sources of black carbon from marine environments such as shipping or gas flaring,” says Dominic Heslin-Rees, PhD student at ACES and co-author of the study

The newly gained insights are an important contribution to better understand on how clouds and aerosol particles interact in the Arctic environment, an area of the globe where climate change manifests most. The findings will be needed to further evaluate and improve model climate predictions.

Article in Nature Communications: Black carbon scavenging by low-level Arctic clouds