Thunderstorm clouds over the western part of the Amazon rainforest. Photo: Radovan Krejci
Thunderstorm clouds over the western part of the Amazon rainforest. Photo: Radovan Krejci

Understanding how tiny particles in the air called aerosol emitted by anthropogenic activities including cars and factories affect Earth's climate and hydrological cycle requires understanding of their role in natural undisturbed environment.

The scientists measured the number and size of particles at various altitudes above a pristine region of the Amazon rainforest. The collected airborne data together with a wide range of instruments on the ground allowed to follow how particles at ground level change during and after rainfall and how the wind profile inside the storm change. Before a rainstorm instruments measured a lot of big particles and few small. After the rain, it was the opposite, the big particles were removed from atmosphere by rain and at the same time we see a lot of small particles, similar to those observed by airplane high up above boundary layer in so called “free troposphere”. Additional analyses confirmed that rapid downdrafts associated with precipitation carried sufficient numbers of small particles from the lower free troposphere to the boundary layer to create a fresh population of the particles that would eventually seed the formation of new clouds.

The study also reveals that in the skies above the Amazon, large numbers of small aerosol particles that form naturally in the upper atmosphere are carried to the lowermost part of it, known as the "boundary layer," by rapid downdrafts associated with raining convective clouds.

Why and how is this process important?

Every cloud is made of trillions of small cloud droplets or ice crystals. In the core of each of them is an aerosol particle providing a seed on which water vapour condenses. Thus, aerosols do have a direct influence on how the cloud will look like, how much it will affect the amount of energy from Sun reaching Earth surface and how much heat from the surface will escape to space. Clouds are one of the key players in the Earth climate and due to the complexity of their distribution and properties they also represent largest uncertainty of future climate model predictions.  

Additional research needed

In addition to direct emissions from natural and anthropogenic sources aerosol population in boundary layer is replenished by process known as new particle formation, when molecules of certain gases at favourable conditions stick together and form new particles.

“This process is common elsewhere, but in pristine tropics is largely missing. Now we know more on how the new particles get to boundary layer in tropics, but we have to still find out why we do not see new particle formation there similar to boreal forest ecosystem for example” says Radovan Krejci from Department of Environmental Science and Analytical Chemistry (ACES) at Stockholm University, one of the authors of the study.

Tropical rainforest ecosystem heavily depends on rainfall. Intensity and distribution of rainfall is primarily driven by clouds.

“Our findings add to understanding how biosphere interact with atmosphere and hydrological cycle in pristine environment of tropical rainforest. It is a complex puzzle of interlinked processes, where we are still learning to understand it and there are likely many new surprising findings ahead” concludes Radovan Krejci. 

The paper is published in Nature by scientists from USA, Brazil, Sweden, Germany and Finland lead by Jian Wang and team from Brookhaven National Laboratory.