September 16, 2010
Aerosols control rainfall in the rainforest
Researchers show that precipitation-controlling aerosols over the Amazon rainforest mainly originate directly from the forest ecosystem as biological particles
In the rainy season, the Amazon rainforest makes its own aerosols – atmospheric particles which produce clouds and precipitation – mainly from organic substances. This is the main finding from a new collaborative study published in Science involving researchers from CIRES and the Dept. of Chemistry and Biochemistry at the University of Colorado-Boulder, and led by the Max Planck Institute for Chemistry in Mainz, Germany. These forest-derived aerosols provide insight on the functioning of the rainforest ecosystem: the high content of organic substances indicate that the Amazon region is largely a closed bioreactor during the rainy season. The results could help to establish more accurate climate models and to analyze how humans affect clouds and precipitation. (Science, 17 September 2010)
Almost nowhere in the world is the air as clean as over the Amazon. Thus the Amazon is an ideal place for climate scientists to study how clouds form naturally, free of human-interference – a crucial reference point for understanding how humans affect cloud formation, precipitation and climate. This international group of researchers has for the first time accurately characterized the chemical components of aerosols (tiny suspended particles in air) above the Amazon rainforest during the rainy season by electron microscopy and mass spectrometry. The measurements were taken during the AMAZE (“AMazonian Aerosol characterization Experiment”) project in the Brazilian Amazon, north of Manaus, Brazil in February and March of 2008.
Submicron particles are smaller than a thousandth of a millimeter in diameter, serve as condensation nuclei for cloud droplets, and consist of about 85% of secondary organic aerosol (“SOA”) components. These organic components arise from volatile organic compounds, which are released by the forest ecosystem and transformed in the atmosphere by photochemical reactions to less volatile substances that condense and form SOA. The remaining tenth of the submicron particles is mainly composed of salts, minerals, and soot particles, which are likely transported over the Atlantic from Africa.
Super-micron particles are larger, with diameters of more than one micron, and more than 80 percent of them are primary biological aerosol components such as fungal spores, pollen and plant fragments that are churned out of the rain forest to the atmosphere. These super-micron biological particles can serve as cores for ice nucleation in clouds, and play an important role in the formation of precipitation.
The fact that the aerosols over the Amazon rainforest are almost exclusively of biogenic origin suggests that the ecosystem is surprisingly important to climate-relevant atmospheric processes in the rainy season. "The Brazilian rainforest can be thought of as a bioreactor for the rainy season," says Ulrich Pöschl, the paper’s lead author. These biological aerosols are carried many km above the forest and can condense water vapor, which also comes from the forest, to form cloud droplets or ice crystals. In the clouds formed from these particles, further water condensation occurs until the droplets fall as precipitation back to Earth. While part of the precipitation evaporates, the rest enters the ecosystem and can be taken up by plants. As the plants grow, they release further organic material into the atmosphere, which again facilitates the formation of new clouds. “The ecosystem and the atmosphere are coupled in a feedback system”, says Delphine Farmer, a Research Scientist at CIRES and coauthor of the study.
The findings on the natural process of cloud formation provide researchers with new information on natural, as opposed to human influenced, cloud formation: "What we can say now is that the cloud droplet number over the Amazon rainforest is limited by aerosols – and the quantity of aerosols depends on how much organic material is released by the ecosystem" says Jose-Luis Jimenez, a CIRES Fellow and coauthor of the study. In contrast, in densely populated areas and during the Amazon’s dry season, when gases and aerosol particles from transport, industry and biomass burning dominate the atmosphere, cloud droplet number instead depends on how quickly particles are transported by updrafts.
Scientific research of the atmosphere over the Amazon rainforest will continue in the next few years. Researchers are currently building a 300 meter high measurement tower near Manaus, Brazil. "That's where we want to carry out further long-term and more comprehensive measurements," says Ulrich Pöschl. In addition to aerosols, the researchers will analyze the carbon and nitrogen cycle in more detail. "The better we understand the natural ecosystem of the rainforest, the more effectively we can describe the impact of humans on the climate."
The participation of the University of Colorado in this research was supported by the US National Science Foundation.
Original Paper:
U. Pöschl, S.T. Martin, B. Sinha, Q. Chen, S.S. Gunther, J.A. Huffman, S. Borrmann, D.K. Farmer, R.M. Garland, G. Helas, J.L. Jimenez, S.M. King, A. Manzi, E. Mikhailov, T. Pauliquevis, M.D. Petters, A.J. Prenni, P. Roldin, D. Rose, J. Schneider, H. Su, S.R. Zorn, P. Artaxo, M.O. Andreae. Rainforest biogenic aerosols as nuclei of clouds and precipitation in the Amazon. Science, 17 September 2010.
Contacts:
Dr. Ulrich Pöschl
Max Planck Institute for Chemistry, Mainz
Tel: +49-6131-305-422
E-mail: @ u.poschl mpic.de
Prof. Jose-Luis Jimenez
CIRES & Dept. of Chemistry and Biochemistry, University of Colorado, Boulder
Tel: 303-492-3557 ; E-mail: jose.jimenez@colorado.edu
Dr. Delphine Farmer
CIRES & Dept. of Chemistry and Biochemistry, University of Colorado, Boulder
Tel: 303-492-1351 ; E-mail: delphine.farmer@colorado.edu
