Overlooked Tiny Air Pollutants Can Have Major Climate Impact

Study finds once-ignored small aerosol particles can be like steroids for rain clouds, fueling more potent thunderstorms.

Pollution in the form of tiny aerosol particles—so small they've long been overlooked—may have a significant impact on local climate, fueling thunderstorms with heavier rainfall in pristine areas, according to a study released Thursday.

The study, published in the journal Science, found that in humid and unspoiled areas like the Amazon or the ocean, the introduction of pollution particles could interact with thunderstorm clouds and more than double the rainfall from a storm.

The study looked at the Amazonian city of Manaus, Brazil, an industrial hub of 2 million people with a major port on one side and more than 1,000 miles of rainforest on the other.  As the city has grown, so has an industrial plume of soot and smoke, giving researchers an ideal test bed.

"It's pristine rainforest," said Jiwen Fan, an atmospheric scientist at the Pacific Northwest National Laboratory and the lead author of the study.  "You put a big city there and the industrial pollution introduces lots of small particles, and that is changing the storms there."

Fan and her co-authors looked at what happens when thunderstorm clouds—called deep convective clouds—are filled with the tiny particles.  They found that the small particles get lifted higher into the clouds, and get transformed into cloud droplets.  The large surface area at the top of the clouds can become oversaturated with condensation, which can more than double the amount of rain expected when the pollution is not present.  "It invigorates the storms very dramatically," Fan said—by a factor of 2.5, the research showed.

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