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Tiny pollutant particles strengthen thunderstorms

Scientists find that long-ignored ultrafine aerosols can make storms rainier and windier

by Sam Lemonick
January 29, 2018

Credit: Daniel Indiana/Shutterstock
Storm clouds over Manaus, Brazil.

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.



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M. Dalil Rahman (January 31, 2018 4:47 PM)
Very interesting and impotent finding. I am not surprised though, because in my research I found a very tiny amount and even less than 50nm size impurities make my product into a waste. It took me a lot of work and time to convince other people this truth.
stephen Lord (January 31, 2018 5:13 PM)
Very interesting. The overall effects would seem to be more rain, windier and more lightning.
Rain is very useful since it provides more fresh water for people and plans and the main climate concerns have been drought. High rain intensity could be a flooding issue and might require more reservoirs and flood control but that would also provide more stored water for people and irrigation. Windier could be useful for wind generation but might also cause damage depending on how much and where. The study seems to suggest wind production at high altitudes which would seem to have little direct effect.. Lightning can cause damage but is also a natural source of nitrate fertilizers which would increase plant growth and CO2 takeup. WE know that nature generates rain nucleating aerosols and it would be interesting to see if the naturally produced aerosols decrease when there large quantities of these aerosols.
Edgar Mueller (February 1, 2018 9:06 AM)
Interesting study, but shuld be placed into, and evaluated within the global model of the physical processes taking place in the atmosphere.
The atmosphere is a thermodynamic engine, converting the temperature difference between its lower and upper strates into mechanical energy in the form of atmospheric convection (wind, storm). At its lower strates, the atmosphere is heated up by the solar radiation incident onto the ground. At its upper strates(upper troposphere to lower stratosphere) the IR-active molecules (H2O - 0.1 to 5% - and CO2 - 0.04%) radiate the atmosphere's thermal energy into outer space (the symmetric diatomics N2, O2, and the atomic Ar cannot radiate any thermal energy). The radiative cooling eventually condenses the atmospheric water (rain, snow). The temperature difference between the lower stratosphere and the ground launches a vertical atmospheric convection. Via the earth rotation (Coriolis forces), this vertical convection is converted into horizontal convection (cyclones, anticyclones), thus into wind and storms. These latter carry the water - mainly evaporated over the oceans (2/3 of the earth surface) into the interior of the continents, and allow for vegetation to grow there.

Thus, the conclusion ist: the more water in the atmosphere, the more radiative cooling in the upper atmosphere. The more radiative cooling in the upper atmosphere, the more convection (wind, storms). The more convection, the more water in the interior of the continents. And the more water there, the more radiative cooling there, too. Climatic change is due to the overall increase of atmospheric water, and leads to a further overall increase of atmospheric water, until the new global equilibrium is reached.
Peter Daudey (February 4, 2018 10:38 AM)
The formation of tiny water droplets is not sufficient to stimulate rain. It is the formation of ice crystals that does this, for two reasons:
1) Ice has a lower vapor pressure than water, so it is more stable than water at sub zero temperature,leading to a faster growth of ice crystals compared to water droplets
2) Growing ice crystals generate large amounts of secondary nuclei, thus outnumbering heterogeneous nucleation of ice crystals.
To stimulate rain, one would need to have ice nucleation promoters, preferentially solid particles with some resemblance to ice. Silver chloride has been used for this in the 60-ties. It would be interesting to search for a link between UAP's and ice nucleation.

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