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Salt in the mist. Many of the aerosols that seed precipitation in the Amazon Basin grow around tiny bits of potassium-rich salt spewed skyward by plants and fungi in the rainforest ecosystem, a new study suggests.

C. Pöhlker/MPI for Chemistry

Amazon Seeds Its Own Rain

The Amazon rainforest makes its own rain. That's the conclusion of a new study, which finds that microscopic bits of potassium-rich salt spewed skyward by trees and fungi may be seeding much of the region’s precipitation. Because aerosols also scatter light back into space, they can cool Earth’s surface as well.

Unless temperatures are extremely cold, raindrops don’t just form in thin air; molecules of water vapor must actually aggregate around a tiny core. Those seeds can either be particles such as mineral dust, soot, salt spray from the ocean—even airborne bacteria—or droplets such as the sulfur dioxide spewed by volcanoes. Scientists previously knew that the organic-rich particles of haze floating above the Amazon Basin acted as the seeds for much of the rainfall there, but what had served to trigger the growth of those particles—known scientifically as secondary organic aerosols—was a big mystery, says Christopher Pöhlker, an atmospheric chemist at the Max Planck Institute for Chemistry in Mainz, Germany.

Now, field studies by Pöhlker and his colleagues hint that the forest itself is the ultimate source of its own precipitation. The team’s detailed analyses of tiny aerosols slurped from the air above the rainforest during the rainy season at a remote site 150 kilometers northwest of Manaus, Brazil, revealed that a tiny chunk of potassium-rich salt lies at the core of most of the particles. For aerosols measuring about 0.15 micrometers across, the salty core accounted for as much as 20% of the particle’s weight; in larger aerosols, the fraction of weight due to the core was much smaller. Together, these trends suggest that the bits of salt act as the seeds upon which the organic-rich haze particles grow, the researchers report online today in Science.

Although scientists have identified several sources of airborne salt bits, a number of them are unlikely in this case, Pöhlker says. For one thing, potassium-rich particles often show up in smoke from wildfires or fires used to clear land, but the team’s samples didn’t include soot particles typically found in such smoke, and satellite images didn’t discern any fires upwind of the site where the aerosols were collected. Second, the potassium-rich bits probably didn’t come from salt spray above the Atlantic, which is located about 1000 kilometers upwind of the study site. Also, Pöhlker notes, the cores had a chemical composition distinctly different from sea salt.

Evidence suggests the salty cores of the aerosols originated in the forest itself. Previous studies have shown that plants and fungi release salts into the air, Pöhlker says. In particular, when fungi expel spores into the air, they also spew droplets that contain carbohydrates and potassium and chloride ions. Besides the aerosols, the air samples that the team collected over the Amazon also included large numbers of fungal spores.

Pöhlker and his colleagues “have found a new significant source for primary aerosol particles, which will enhance cloud droplet concentration,” says Markku Kulmala, an atmospheric scientist at University of Helsinki. “The connection between biogenic particle emissions and cloud properties in the tropical rainforest ecosystem appears even stronger and more direct than previously assumed.”

Because tropical rainforests have a large influence on atmospheric chemistry, the biological activity and diversity of the ecosystem’s particle-emitting organisms have played an important role in past climate and will likely do so in the future, Kulmala says. The findings may also help explain how secondary organic aerosols form in other parts of the world, although in many of those regions the cores of those particles are presumed to be dust, soot, or manmade pollutants.