A decades-long mystery: What is the origin of cloud-nucleating aerosol particles over the Amazon rainforest?

Atmospheric aerosol particles are essential for the formation of clouds and precipitation, affecting the Earth's energy budget, water cycle, and climate on regional and global scales. Cloud droplet formation and growth depend on the abundance of atmospheric aerosol particles that are sufficiently large to act as cloud condensation nuclei. Over the Amazon rainforest, most of these particles are thought to be formed by gas-to-particle conversion of precursor gases, i.e., by conversion of volatile organic compounds like isoprene or terpenes into condensable vapors like organic acids and other highly oxygenated organic molecules of low volatility. However, the exact mechanisms and conditions of new particle formation and growth, and thus the origin of the background aerosol population in pristine rainforest air during the wet season has remained a mystery for decades. Previous studies have shown new particle formation in the outflow of deep convective clouds and suggested a downward flux of aerosol particles during precipitation events. Meteorological considerations, however, indicate that the downward transport from the upper troposphere may not be sufficient to explain the aerosol concentrations and size distributions observed in the lower troposphere. Here we use comprehensive aerosol, trace gas, and meteorological data from the Amazon Tall Tower Observatory (ATTO) to show that rainfall regularly induces bursts of newly formed nanoparticles in the planetary boundary layer right above the forest canopy. These nanoparticle bursts provide a plausible explanation for the origin of cloud condensation nuclei, leading to the local formation of “Green Ocean” clouds and precipitation under pristine conditions in the wet season.

Interplay of rainfall, aerosols, gases, and new particle formation

During rain events, there is a marked decrease in the concentration of particles in the size range larger than about 40 nm, accompanied by an increase in the concentration of particles in the nucleation size range smaller than 40 nm. The decrease in larger particles due to precipitation reduces the loss of low-volatility gases due to condensation on existing particles, thus promoting new particle formation in the nucleation size range. A box model was used to evaluate the formation of new particles from the ground to the canopy, confirming the clustering process and subsequent atmospheric nucleation, and the effect of reducing the condensation sink.

Rainfall events significantly alter trace gas concentrations just above the canopy and produce bursts of nucleation particles that grow into the size range above 40 nm by early morning the next day. Figure 1 presents a schematic representation of the gas-aerosol processes occurring during and after rainfall events. The main nucleation particle bursts occur together with ozone (O₃) injections into the understory. The maximum concentration of both particles and O₃ occurs about 30 minutes after the precipitation peak. The evolution of trace gas concentrations from the time before the onset of rain up to 2 hours after the maximum precipitation suggests that the particle burst is produced by the reduction of the condensation sink and reactions involving O₃, terpenes and NO_x. Sesquiterpenes are likely to play a major role in new particle formation.

Overall, our study shows how precipitation can trigger new particle formation in the planetary boundary layer and forest canopy. It opens up a new perspective in the complex cascades of chemical-atmospheric, meteorological and aerosol-cloud interactions and feedback loops that characterize the Amazonian 'Green Ocean' atmosphere.