Amazon boundary-layer aerosol concentration sustained by vertical transport during rainfall

 

Authors

Jian Wang — Washington University in St. Louis
Radovan Krejci — Department of Applied Environmental Science (ITM) Stockholm
Scott Giangrande — Brookhaven National Laboratory
Chongai Kuang — Brookhaven National Laboratory
Henrique de Melo Jorge Barbosa — University of Maryland, Baltimore County
Joel Brito —
Samara Carbone — University of Sao Paulo
Xuguang Chi — Nanjing University
Jennifer M. Comstock — Pacific Northwest National Laboratory
Florian Ditas — Max Planck Institute for Chemistry
Jost Valentin Lavric — Max Planck Institute of Chemistry
Hanna Manninen — University of Helsinki
Fan Mei — Pacific Northwest National Laboratory
Moran-Zuloaga Daniel — Max Planck Institute for Chemistry
Christopher Pohlker — Max Planck Institute for Chemistry
Mira Pöhlker — Max Planck Institute for Chemistry
Jorge Saturno — Max Planck Institute of Chemistry
Beat Schmid — Pacific Northwest National Laboratory
Rodrigo Augusto Souza — Universidade do Estado do Amazonas
Stephen R. Springston — Brookhaven National Laboratory
Jason Tomlinson — Pacific Northwest National Laboratory
Tami Fairless — Pacific Northwest National Laboratory
David Walter — Max Planck Institute for Chemistry
Daniela Wimmer — University of Helsinki
James Smith — University of California, Irvine
Markku Kulmala — University of Helsinki

Paulo Artaxo — University of Sao Paulo
Meinrat Andreae — Max Planck Institute of Chemistry
Tuukka Taneli Petaja — University of Helsinki
Scot T. Martin — Harvard University

Category

GoAmazon – Clouds and aerosols in Amazonia

Description

Processes that help maintain the CCN population in the Amazonian atmospheric boundary layer under natural conditions. Condensational and coagulational growth of new particles formed in the outflow region of earlier convective clouds leads to high concentrations of small particles in the free troposphere. These particles descend from the free troposphere into the boundary layer during rainfall. In the boundary layer, the small particles grow into CCN of larger diameter. The mass for the particle growth derives from the oxidation of biogenic volatile organic compounds (BVOCs) emitted by the forest.
A necessary prerequisite of cloud formation, aerosol particles represent one of the largest uncertainties in computer simulations of climate change, in large part because of a poor understanding of processes under natural conditions. The Amazon rainforest is one of the few continental regions where aerosol particles and their precursors can be studied under near-natural conditions. Cloud condensation nuclei (CCN) in clean Amazonia are mostly produced by the growth of smaller particles in the boundary layer. However, the origin of these smaller particles remains unclear. Here, using recent aircraft measurements above central Amazonia, we show high concentrations of small particles (diameters of less than 50 nm) in the lower free troposphere. These high-concentration small particles are injected from the free troposphere into the boundary layer during precipitation events, both by strong convective downdrafts and by weaker downward motions in the trailing stratiform region. This rapid vertical transport helps maintain the population of particles, and ultimately CCN, in the boundary layer, thereby playing an important role in controlling the climate state under natural conditions. In contrast, this process becomes a net sink of particles in the boundary layer under polluted conditions, which sometimes prevail in Amazonia as well as over other tropical continental regions. This represents a heretofore unobserved mechanism in the life cycle of aerosol in pristine regions of the atmosphere and is important if human impacts on the pre-industrial atmosphere are to be properly understood.