Continuous aerosol particle formation over the Amazon rainforest

A hidden mechanism continuously forms new aerosol particles from gaseous precursors in the Amazon rainforest, challenging previous assumptions about the sources of cloud condensation nuclei and their role in the hydrological cycle and climate.

Aerosols are tiny particles suspended in the air, which influence the climate and the hydrological cycle by serving as nuclei for clouds and precipitation. In clean Amazonian rainforest air, however, new particle formation was rarely observed. The origin of most aerosol particles remained enigmatic and was tentatively attributed to distant sources and long-range transport. High-precision long-term measurements at the Amazon Tall Tower Observatory (ATTO) have now led to the discovery of a process called Quiet New Particle Formation, which continuously converts condensable vapors into nanometer-sized organic aerosol particles. This previously undetected mechanism can explain a large fraction of the nanometer-sized aerosol particles observed during the wet season in the Amazon rainforest.

Atmospheric aerosol particles play a key role in the formation of clouds and precipitation and in the evolution of the hydrological cycle and climate. However, their origin in the pristine air of the Amazon rainforest has long remained unclear. A new study led by the Max Planck Institute for Chemistry and the University of São Paulo now shows that aerosol particles can form continuously in the forest's lower atmosphere, even under conditions previously considered unfavorable for particle formation. Although the rates of particle formation and growth are relatively small compared to other environments, they become highly relevant under the exceptionally clean conditions of the Amazon wet season, where particle number concentrations, that is the number of particles per unit volume, are very low.

Particle size distributions at ATTO analyzed over ten years

An international team of researchers analyzed particle size distributions from high-precision long-term measurements at the Amazon Tall Tower Observatory (ATTO) over a period of ten years to obtain unique insights into how nanometer-sized particles behave under pristine atmospheric conditions. This approach allowed them to identify subtle but persistent signals of new particle formation that are typically hidden within atmospheric variability. The results reveal that nanoparticle formation is not limited to rare or intense atmospheric events but also occurs regularly in a subtle and previously overlooked manner.
“We used to think that new particle formation in the Amazon rarely occurred near the forest canopy and was mainly driven by processes in the upper troposphere or rain-related events and downward transport,” says Bruno Backes Meller, team leader at the Max Planck Institute for Chemistry and first author of the study. “Now we show that a hidden mechanism produces nearly half of the particles in the 10 to 25 nanometer size range in the planetary boundary layer without relying on downdrafts or long-range transport.” The planetary boundary layer extends from the surface to an altitude of one to two kilometers. 

The study suggests that the growth of these newly formed clusters is driven by low-volatility organic compounds produced from the oxidation of gases emitted by vegetation. These compounds can condense onto molecular clusters, enabling them to grow to sizes relevant for cloud formation.

Atmospheric models need to be refined

“Our recent findings change traditional views about the sources of nuclei for clouds and precipitation in the Amazon”, says Ulrich Pöschl, co-author and director at the Max Planck Institute for Chemistry. “The discovery of a continuous and widespread source of newly formed particles in tropical rainforest air suggests that atmospheric models need to be refined for an accurate representation of aerosol-cloud and atmosphere-biosphere interactions in the Earth system”.

“With higher temperatures and a very different chemical environment near the forest, the compounds and chemical mechanisms driving this type of particle formation are likely distinct from those observed in the upper troposphere,” says Paulo Artaxo, corresponding author from the University of Sao Paulo. “Understanding these processes will be crucial for predicting how the Amazonian atmosphere and rainforest ecosystem respond to environmental and climate change.”