Atmospheric aerosol particles play a crucial role in creating clouds and precipitation, which in turn affect the Earth’s energy balance, hydrological cycle, and climate. Despite this significance, the sources of aerosol particles in the unpolluted air above the Amazon rainforest during the wet season are not well understood. Recent research indicates that rainfall frequently triggers the formation of new nanoparticles in the air above the forest.
Atmospheric aerosol particles play a crucial role in generating clouds and precipitation, thus affecting the Earth’s energy balance, hydrological cycle, and climate. However, the sources of aerosol particles in the unpolluted air over the Amazon rainforest during the wet season remain unclear. A recent study conducted by researchers at the Max Planck Institute for Chemistry in Mainz found that rainfall often leads to the release of newly formed nanoparticles in the atmosphere above the forest canopy.
In the Amazon rainforest, heavy rain typically occurs in the afternoons during the wet season. The formation of clouds and precipitation depends on tiny airborne particles called cloud condensation nuclei, which allow water vapor to coalesce into cloud droplets. But where do these cloud condensation nuclei come from?
An international team of researchers from Germany, Brazil, Sweden, and China discovered that rainfall triggers surges of nanoparticles that can grow into cloud condensation nuclei. They examined extensive long-term data on aerosol particles, trace gases, and meteorological conditions collected from the Amazon Tall Tower Observatory (ATTO). This facility, which is equipped with advanced instruments and features towers reaching up to 325 meters in height, is located in the heart of the Amazon rainforest in northern Brazil, approximately 150 kilometers northeast of Manaus, and is cooperatively managed by German and Brazilian scientists.
Luiz Machado, the lead author of the study published in the journal Nature Geoscience, explains that “Rainfall cleans the atmosphere of aerosol particles and brings ozone into the forest canopy. Ozone can react with volatile organic compounds emitted by plants, particularly terpenes, leading to oxidation products that promote the creation of new particles, resulting in temporary spikes in nanoparticle concentrations.”
Nanoparticle concentrations peak just above the forest canopy
The research revealed that nanoparticle levels are highest immediately above the forest canopy and decline with increasing altitude. “This gradient remains consistent throughout the wet season, suggesting ongoing particle generation in the canopy and an upward movement of newly created particles that can further grow by absorbing low volatile molecules and function as cloud condensation nuclei,” states Christopher Pöhlker, co-author and leader of the research group at the Max Planck Institute for Chemistry. The low volatile molecules contributing to the formation and growth of natural atmospheric nanoparticles include oxygen- and nitrogen-rich organic compounds produced by the oxidation of isoprene, terpenes, and other volatile organic compounds emitted by plants that interact with ozone and hydroxyl radicals.
Previous studies had observed new particle formation in the outflow regions of convective clouds in the upper troposphere and had suggested a downward movement of newly formed nanoparticles instead.
“Our findings suggest a significant shift in the scientific understanding of how rainforests interact with aerosols, clouds, and precipitation in the Amazon, which is critical for both regional and global climate systems,” concludes Ulrich Pöschl, co-author and director at the Max Planck Institute for Chemistry.
