Pristine Rainforests are “Biogeochemical Reactors;” Produce Fuel for Rainclouds

A multinational team that includes a North Carolina State University researcher has found another piece of the atmospheric puzzle surrounding the effects of aerosol particles on climate change. Their findings will contribute to our ability to more accurately measure human impact on climate, and to determine how much pollution may “mask” the actual rate of climate change.

Dr. Markus Petters, an NC State assistant professor of marine, earth and atmospheric sciences, traveled to the Amazon rainforest in a remote area of Brazil as part of a team that wanted to study how a rainforest behaved in the absence of any anthropogenic, or human, influence. They were particularly interested in the behavior of atmospheric aerosol particles, which play an important role in our climate system due to their ability to modify cloud formation and encourage or suppress precipitation.

Aerosol particles act as seeds for cloud formation and water condensation inside clouds. Some of these particles – about one in a million – form ice crystals inside the clouds, which initiate rainfall. In areas populated by humans, pollution serves as an additional source for these particles. In a pristine area such as the Amazon, where pollution is not a factor, the researchers found that the rainforest itself acted as a biogeochemical reactor, producing “fuel” for the rainclouds from organic molecules emitted by trees, as well as other biological matter such as plant debris, bacteria and pollen.

“The trees basically ‘sweat out’ organic molecules that react with compounds in the atmosphere, producing tiny particles that are around 20 to 200 nanometers in size,” Petters says. “These particles seed the clouds. In addition, other biological particles form the ice nuclei for the clouds.”

The researchers published their findings in the Sept. 17 issue of Science.

“We have to look at areas without anthropogenic influences and see what the aerosol budget is like without humans being involved,” Petters says. “The Amazon is unique in that it provides a good place for us to observe atmospheric interactions without human influence, and to catalog baseline activity for the rainforest. If we understand that baseline well enough around the world, we can run simulations that show how human aerosol emissions modulate climate change.”

The Department of Marine, Earth and Atmospheric Sciences is part of NC State’s College of Physical and Mathematical Sciences.

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Note to editors: Abstract of the paper follows.

“Rainforest Aerosols as Biogenic Nuclei of Clouds and Precipitation in the Amazon”
Authors: U. Poschl, B. Sinha, S.S.Gunthe, Max Planck Institute for Chemistry, Mainz, Germany; M.D. Petters, NC State University; et al
Published: Sept. 17, 2010, in Science

Abstract: The Amazon is one of the few continental regions where atmospheric aerosol particles and their effects on climate are not dominated by anthropogenic sources. During the wet season, the ambient conditions approach those of the pristine pre-industrial era. We show that the fine submicron particles accounting for most cloud condensation nuclei are predominantly composed of secondary organic material formed by oxidation of gaseous biogenic precursors. Supermicron particles, which are relevant as ice nuclei, consist mostly of primary biological material directly released from rainforest biota. The Amazon Basin appears to be a biogeochemical reactor, in which the biosphere and atmospheric photochemistry produce nuclei for clouds and precipitation sustaining the hydrological cycle. The prevailing regime of aerosol-cloud interactions in this natural environment is distinctly different from polluted regions.