Rainfall triggers bursts of natural nanoparticles, which contribute to cloud formation and additional precipitation over the Amazon rainforest.
During the wet season in the Amazon rainforest, heavy afternoon rainfall is common. The formation of clouds and precipitation depends on tiny airborne particles called cloud condensation nuclei, which provide a surface for water vapor to condense and form cloud droplets. But where do these nuclei originate?
An international team of researchers from Germany, Brazil, Sweden, and China has discovered that rainfall regularly triggers bursts of nanoparticles, which can grow into cloud condensation nuclei. The scientists reached this conclusion by analyzing extensive long-term data on aerosol particles, trace gases, and meteorological conditions from the Amazon Tall Tower Observatory (ATTO). Equipped with advanced instruments and measurement towers reaching up to 325 meters in height, ATTO is located deep in the Amazon rainforest, about 150 kilometers northeast of Manaus, Brazil. It is jointly operated by scientists from Germany and Brazil.
Luiz Machado, first author of the study now published in the journal Nature Geoscience, explains: “Rainfall removes aerosol particles and introduces ozone from the atmosphere into the forest canopy. Ozone can oxidize plant-emitted volatile organic compounds, especially terpenes, and the oxidation products can enhance the formation of new particles, leading to temporary bursts of nanoparticles.”
Nanoparticle concentrations are highest just above the forest canopy
The researchers discovered that nanoparticle concentrations are highest just above the forest canopy and decrease with increasing altitude. “This gradient persists throughout the wet season, indicating continuous particle formation in the canopy and an upward flux of newly formed particles that can grow by further uptake of low volatile molecules and serve as cloud condensation nuclei,” adds Christopher Pöhlker, co-author and research group leader at the Max Planck Institute for Chemistry.
Among the low volatile molecules involved in the formation and growth of natural nanoparticles in the atmosphere are oxygen- and nitrogen-containing organic compounds that are formed upon oxidation of isoprene, terpenes, and other volatile organic compounds, which are naturally emitted by plants and oxidized by ozone and hydroxyl radicals in the air.
Earlier studies had detected new particle formation in the outflow of convective clouds in the upper troposphere and suggested a downward flux rather than an upward flux of newly formed nanoparticles.
“Our findings imply a paradigm shift in the scientific understanding of interactions between the rainforest, aerosols, clouds, and precipitation in the Amazon, which are important for regional and global climate,” concludes Ulrich Pöschl, co-author and director at the Max Planck Institute for Chemistry.
Reference: “Frequent rainfall-induced new particle formation within the canopy in the Amazon rainforest” by Luiz A. T. Machado, Gabriela R. Unfer, Sebastian Brill, Stefanie Hildmann, Christopher Pöhlker, Yafang Cheng, Jonathan Williams, Harder Hartwig, Meinrat O. Andreae, Paulo Artaxo, Joachim Curtius, Marco A. Franco, Micael A. Cecchini, Achim Edtbauer, Thorsten Hoffmann, Bruna Holanda, Théodore Khadir, Radovan Krejci, Leslie A. Kremper, Yunfan Liu, Bruno B. Meller, Mira L. Pöhlker, Carlos A. Quesada, Akima Ringsdorf, Ilona Riipinen, Susan Trumbore, Stefan Wolff, Jos Lelieveld and Ulrich Pöschl, 8 November 2024, Nature Geoscience.
DOI: 10.1038/s41561-024-01585-0
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