Tiny pollutant particles strengthen thunderstorms

Researchers studying thunderstorms in the Amazon rainforest have discovered that tiny pollution particles—each about 10,000 times as small as the period at the end of this sentence—can boost the power of storms (Science 2018, DOI: 10.1126/science.aan8461).

Sulfates, nitrogen oxides, volatile organic compounds, and other molecules can conglomerate into these ultrafine aerosol particles (UAPs), which are less than 50 nm across. Scientists have long known that aerosol particles larger than 50 nm serve as starting points for clouds, but most researchers have discounted the role of UAPs in storm clouds because they thought the particles were too small to nucleate water droplets from moisture in the air.

Jiwen Fan of the Pacific Northwest National Laboratory and colleagues found otherwise. While larger particles nucleate droplets at the base of storm clouds, UAPs can nucleate droplets 2.5–3 km up. When the water condenses, it releases heat, which can push the tops of clouds higher and increase wind speeds.

The Amazon has very low levels of UAPs naturally, but the Brazilian city of Manaus in the heart of the Amazon basin produces a plume of pollution that flows with the prevailing winds over the pristine rainforest. In 2014 and 2015, an international team of researchers led by Scot Martin of Harvard University tracked air quality in Manaus’ vicinity from planes and ground stations.

Fan’s group looked at cloud data collected in that effort. They found that vertical wind velocity and reflectivity—a proxy for the amount of precipitation—as measured by radar increased with growing concentrations of UAPs.

Fan says studying UAPs has long been a challenge because it’s difficult to separate their effects from other variables in storm clouds, such as humidity, temperature, and wind. Thanks to shifting wind patterns in the Amazon, his team could compare thunderstorms that were nearly identical except for the presence or absence of UAPs from Manaus.

“In this study we have this perfect setting,” Fan says. “During the wet season, many of the storms are happening under very similar meteorological conditions, except for aerosol.”

Joel Thornton, an atmospheric chemist at the University of Washington who was not involved in the study, points out that these small particles last only for a couple days at most in the lower atmosphere, so their effects have limits. But with human activity in almost every corner of the globe, there are plenty of places where UAPs could affect storms. Fan notes that researchers have found more lightning strikes in the Indian Ocean where ships—which emit UAPs—sail.

Fan and Thornton both agree that the paper will spur others to take a closer look at UAPs. “What this paper does is raise the stakes in needing to develop a deeper more accurate understanding of the sources and fates of atmospheric UAPs than we currently have,” says Thornton.