During the rainy season in the Amazon rainforest, heavy afternoon rainfall is common. The formation of clouds and precipitation depends on tiny particles in the air called cloud condensation nuclei, which provide a surface for water vapor to condense into cloud droplets. But where do these cloud cores come from? An international team of researchers from Germany, Brazil, Sweden and China has found that rainfall regularly triggers bursts of nanoparticles that can grow into cloud condensation nuclei.
The scientists reached this conclusion by analyzing a large amount of long-term data on aerosol particles, trace gases and meteorological conditions provided by the Amazon Alto Tower Observatory (ATTO). ATTO is located deep in the Amazon rainforest about 150 kilometers northeast of Manaus, Brazil, and is equipped with advanced instruments and a 325-meter-high measurement tower. It is run by scientists from Germany and Brazil.
"Rainfall clears aerosol particles and introduces ozone from the atmosphere into the forest canopy," explains Luiz Machado, first author of the study now published in the journal Nature Geoscience. "Ozone can oxidize volatile organic compounds emitted by plants, especially terpenes, and the oxidation products can promote the formation of new particles, leading to temporary bursts of nanoparticles."
The researchers found that nanoparticle concentrations were highest above the forest canopy and decreased with height. Co-author Christopher Pöhlker, leader of the research group at the Max Planck Institute for Chemistry, added: "This gradient persists throughout the rainy season, indicating that particles are constantly forming in the canopy. Newly formed particles flow upward, grow by absorbing further low-volatile molecules, and become cloud condensation nuclei."
Low-volatility molecules involved in the formation and growth of natural nanoparticles in the atmosphere include oxygen- and nitrogen-containing organic compounds formed by the oxidation of isoprene, terpenes, and other volatile organic compounds that are naturally emitted by plants and oxidized by ozone and hydroxyl radicals in the air.
Earlier studies had detected the formation of new particles in outflows from convective clouds in the upper troposphere and suggested that newly formed nanoparticles flowed downward, not upward.
Co-author Ulrich Pöschl, Director of the Max Planck Institute for Chemistry, concluded: "Our findings represent a paradigm shift in the scientific understanding of the interactions between the Amazon rainforest, aerosols, clouds and precipitation, which is important for both regional and global climate."
Compiled from /ScitechDaily