Atmospheric aerosol particles are crucial for the formation of clouds and precipitation, impacting the Earth’s energy balance, water cycle, and climate. However, the source of aerosol particles in pristine air over the Amazon rainforest during the wet season has been poorly understood. A recent study conducted by the Max Planck Institute for Chemistry in Mainz has revealed that heavy rainfall in the Amazon rainforest induces bursts of newly formed nanoparticles in the air above the forest canopy, which play a key role in cloud condensation nuclei formation.
During the wet season in the Amazon rainforest, frequent heavy rainfall occurs in the afternoon, essential for cloud and precipitation formation. Cloud condensation nuclei are crucial for water vapor to condense and create cloud droplets, but their origins have been unclear. An international research team from Germany, Brazil, Sweden, and China analyzed long-term data from the Amazon Tall Tower Observatory (ATTO), located in the heart of the rainforest. The observatory is equipped with advanced instrumentation and measurement towers up to 325m high, providing valuable insights into the interactions between aerosol particles, gases, and meteorological conditions.
The study led by Luiz Machado at ATTO demonstrated that rainfall plays a vital role in triggering bursts of nanoparticles that can evolve into cloud condensation nuclei. The removal of aerosol particles by rainfall introduces ozone into the forest canopy, which then oxidizes plant-emitted volatile organic compounds like terpenes. These oxidation products enhance the formation of new particles, resulting in temporary bursts of nanoparticles that contribute to cloud condensation nuclei formation. The researchers found that nanoparticle concentrations were highest just above the forest canopy and decreased with altitude throughout the wet season.
Christopher Pöhlker, a co-author of the study and research group leader at the Max Planck Institute for Chemistry, highlighted the importance of low volatile molecules in the formation and growth of natural nanoparticles in the atmosphere. Oxygen- and nitrogen-containing organic compounds created from the oxidation of isoprene, terpenes, and other volatile organic compounds emitted by plants play a significant role in particle formation. These molecules can grow through the uptake of low volatile compounds and act as cloud condensation nuclei, impacting cloud formation and precipitation in the region.
Previous studies had suggested a downward flux of newly formed nanoparticles in the upper troposphere, particularly in the outflow of convective clouds. However, the findings of this study indicate an upward flux of nanoparticles just above the forest canopy during the wet season, challenging previous assumptions. Ulrich Pöschl, the co-author and director at the Max Planck Institute for Chemistry, emphasized that these results signify a shift in the scientific understanding of the complex interactions between the rainforest, aerosols, clouds, and precipitation in the Amazon region. These interactions are critical for both regional and global climate systems, highlighting the importance of further research in this area.