Aerial photo of Manaus, Brazil
Atmospheric scientists studied cloud formation above Manaus, Brazil, to compare and contrast the air above the rural rain forest and the urban center. Credit: Neil Palmer/CIAT, CIFOR/Flickr, CC BY-NC-ND 2.0


Cover of April 2022 issue of Eos

When we talk about natural processes—especially atmospheric—nothing is too small to be irrelevant. Recently, researchers in Brazil and the United States found that nanoparticles of pollution play an outsize role in cloud formation and disruption, altering rain cycles even in pristine forest areas.

The study, published in Science Advances, showed that human-made aerosols smaller than 10 nanometers, previously thought to be too tiny to act as cloud condensation nuclei or have any influence on climate processes, can become climatically active as they swell on their way to the upper layers of Earth’s atmosphere.

The team used a Gulfstream 1 research airplane from the U.S. Department of Energy to fly about 5 kilometers above Manaus, Brazil, an urban patch of 2 million residents surrounded by the Amazon rain forest, on 13 and 17 March 2014. The plan was to find out how aerosol-cloud interactions played out not only over urban areas but also over relatively unperturbed forest regions.

An Intricate Process

Paulo Artaxo, a professor at the University of São Paulo and a coauthor of the study, explained that cloud condensation nuclei (CCN) are very small particles that form rain clouds. “Despite their nanometric scale, [CCN] must reach a certain minimum size—at least 60 or 70 nanometers—to add to cloud formation,” he said.

Volatile organic compounds (VOCs) of natural origin (such as limonene, emitted by coniferous forests) or anthropogenic sources (such as benzene and toluene, emitted from burning fossil fuels) can accumulate on other particles, including pollution particles, forming CCN.

In the Amazon, strong convection winds blow water vapor and other gases to the upper layers of the atmosphere. The same happens to nanoparticles of pollution, and the temperature change causes them to swell. “As these particles go up, the temperature drops. At lower temperatures, VOCs condense on the surface of these pollution nanoparticles, causing them to swell. When they reach at least 70 nanometers, they can pull water vapor and form a cloud droplet,” explained Artaxo.

This swelling process of pollution nanoparticles was mostly unknown until now. Daniel Rosenfeld, a professor at the Hebrew University of Jerusalem who did not take part in the study, said the deposition of VOCs on anthropogenic nanoparticles is an amplifier to processes of human-made CCN disrupting rain cycles. “If it weren’t for the interaction between pollution nanoparticles and natural gases, these types of CCN would be less abundant,” he said.

Oxidation and anthropogenic nanoparticles disrupt the rain cycle in two main ways, according to Artaxo. In the first outcome, the processes can lead to large amounts of lighter droplets forming shallow clouds, reducing the amounts of rainfall. In the second outcome, the nanoparticles can accumulate into larger CCN and form thicker cumulonimbus clouds, causing rain to pour more intensely.

Research data showed that rain clouds affected by pollution during the wet season around Manaus were 10%–40% smaller in diameter and had up to 1,000% more cloud droplets.

Research Perspectives

“You definitely cannot separate natural from disturbed environments any longer.”

The team of researchers saw this process playing out across the Amazon, and Rosenfeld believes the findings can be applicable to tropical settings anywhere. “You definitely cannot separate natural from disturbed environments any longer. The interesting thing about Manaus is that it is an island of anthropogenic perturbation surrounded by a pristine setting, and this is why the city has been a focus for projects of this nature,” he said.

To Rosenfeld, the study opens very interesting and important paths for research but doesn’t account for the effects of smoke from forest fires. Such fires emit their own VOCs and nitrous oxides. “We did the study outside the fire season because we wanted to analyze the impact of urban emissions in Manaus on cloud formation. However, the mechanism we describe can be used to study forest fire areas,” Artaxo said.

The aim now is to survey higher regions of Earth’s atmosphere. Current plans are to bring a jet airplane from the Max Planck Institute in Germany to fly about 15 kilometers over Manaus in December 2022. “We want to look, for example, at what controls the development of the cumulonimbus clouds in the strong wind settings we saw,” Artaxo said.

—Meghie Rodrigues (@meghier), Science Writer

Citation: Rodrigues, M. (2022), Pollution is disrupting rain cycles in the Amazon, Eos, 103, Published on 4 March 2022.
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