Smallest pollution particles affect rainfall in the Amazon

A new study published in the journal Science Advances has found that even extremely small pollution particles resulting from human activities such as the burning of fossil fuels can significantly change cloud formation and rainfall patterns in the Amazon. 

According to an international team of scientists, oxidation leads small aerosols expelled by factories or automobiles to grow rapidly in the atmosphere, reaching up to 400 times their original size, and thus affecting raindrop formation.

“Understanding cloud and rain formation mechanisms in the Amazon is a major challenge because of the complexity of the non-linear physical and chemical processes that occur in the atmosphere,” said study co-author Paulo Artaxo, a professor of Environmental Physics at the University of São Paulo (USP) in Brazil.

By examining data collected by instruments on board of a special aircraft surveying the city of Manaus, in Brazil’s Amazon region, Professor Artaxo and his colleagues have found that aerosols of less than 10 nanometers emitted by factories, power plants, or vehicle exhaust form a pollution plume over Manaus that is blown by winds in a southwesterly direction.

“Little was known about the role played by these nanoparticles in the rainfall regime,” said study co-author Luiz Augusto Machado, an expert in Meteorology at USP.  

“It so happens that the Manaus area is unique in the world in the sense that it’s an open-air laboratory, a mega-city surrounded by forest at a great distance from other cities where we can investigate how a metropolitan area changes an environment similar to that of the pre-industrial era.”

“It’s very hard to estimate the effect of particulate matter on rainfall because of the large number of atmospheric variables that influence this interaction.”

“We therefore compared the pollution line with nearby areas that lie outside the plume. We found that the particles rapidly grow in size. By the time they’re 10 km out of Manaus, they’re larger, and at 30 km they can reach a large enough size to become condensation nuclei, affecting the formation of raindrops.”

The researchers discovered that, as the pollution nanoparticles grow and become condensation nuclei, they can influence rainfall according to what types of clouds they interact with. If the particles meet small, warm clouds, scant rainfall will result. However, if they pass through dense, vertical clouds such as cumulonimbi, the aerosols could increase precipitation, and occasionally even fuel fierce storms. 

“In other words, even these small particles of pollution influence the rainfall regime,” concluded Professor Machado. 

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By Andrei Ionescu, Earth.com Staff Writer