Ultrafine aerosols can help form clouds under certain conditions
Though they be but little, they are fierce.
Airborne particles smaller than 50 nanometers across can intensify storms, particularly over relatively pristine regions such as the Amazon rainforest or the oceans, new research suggests. In a simulation, a plume of these tiny particles increased a storm’s intensity by as much as 50 percent.
Called ultrafine aerosols, the particles are found in everything from auto emissions to wildfire smoke to printer toner. These aerosols were thought to be too small to affect cloud formation. But the new work suggests they can play a role in the water cycle of the Amazon Basin — which, in turn, has a profound effect on the planet’s hydrologic cycle, researchers report in the Jan. 26 Science.
“I have studied aerosol interactions with storms for a decade,” says Jiwen Fan, an atmospheric scientist at the Pacific Northwest National Laboratory in Richland, Wash., who led the new study. “This is the first time I’ve seen such a huge impact” from these minute aerosols.
Larger aerosol particles greater than 100 nanometers, such as soot or black carbon, are known to help seed clouds. Water vapor in the atmosphere condenses onto these particles, called cloud condensation nuclei, and forms tiny droplets. But water vapor doesn’t condense easily around the tinier particles. For that to be possible, the air must contain even more water vapor than is usually required to form clouds, reaching a very high state of supersaturation.
Such a state is rare — larger aerosols are usually also present to form water droplets, removing that extra water from the atmosphere, Fan says. But in humid places with relatively low background air pollution levels, such as over the Amazon, supersaturation is common, she says.
From 2014 to 2015, Brazilian and U.S. research agencies collaborated on a field experiment to collect data on weather and pollution conditions in the Amazon Basin. As part of the experiment, several observation sites tracked plumes of air pollution traveling from the city of Manaus out across the rainforest. During the warm, wet season, there is little difference day to day in most meteorological conditions over the rainforest, such as temperature, humidity and wind direction, Fan says. So a passing pollution plume represents a distinct, detectable perturbation to the system.
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The international team examined vertical wind motion, or updrafts, and aerosol concentration data from one of these stations from March to May 2014. When a large plume of aerosols with an abundance of ultrafine particles passed by an observation station, the researchers observed a corresponding, more powerful vertical wind motion and heavier rain. Such updrafts intensify storms, helping to drive stronger circulation.
Next, the researchers conducted simulations of an actual storm that occurred on March 17, 2014, matching its temperature, wind and water vapor conditions, as well as a low level of background aerosols in the atmosphere. Then, the team introduced several pollution scenarios to interact with the storm, including no plume and a typical plume from the Manaus metropolis. The results suggested that the ultrafine aerosol particles, in particular, were not only acting as cloud condensation nuclei over the Amazon Basin, but also that the water droplets the aerosols created significantly strengthened the gathering storm.
If the conditions are right, the sheer abundance of the ultrafine particles in such a plume would rapidly create a very large number of cloud droplets. The formation of those droplets would also suddenly release a lot of latent heat — released from a substance as it changes from a vapor to a liquid — into the atmosphere. The heat would rise, creating updrafts and quickly strengthening the storm.
Aside from the Amazon, Fan notes that such pristine, humid conditions can also exist over large swaths of the oceans. One recent study in Geophysical Research Letters that she points to found a link between well-traveled shipping lanes, which would contain abundant exhaust including ultrafine aerosols, and an increase in lightning strikes. “This mechanism may have been at play there,” she says.
Atmospheric scientist Joel Thornton of the University of Washington in Seattle, who led the study on the shipping exhaust, says it’s possible that ultrafine particles play a role in that scenario. “What this paper does is raise the stakes in needing to develop a deeper, more accurate understanding of the sources and fates of atmospheric ultrafine particles,” Thornton says.
Meteorologist Johannes Quaas of the University of Leipzig in Germany, who was not involved in either study, agrees. “It’s a very interesting hypothesis.”
But the observations described in the new study don’t definitively demonstrate that ultrafine aerosols alone drive updrafts, Quaas adds. The weather conditions may appear highly consistent from day to day, but such systems are still highly chaotic. Everything from wind to temperature to how the land surface interacts with incoming solar radiation may be variable, he notes. “In reality, it’s not just the aerosols that change.”
Editor's note: This story was updated January 25, 2018, to correct the description of the season during which researchers performed their field experiment.
J. Fan et al. Substantial convection and precipitation enhancements by ultrafine aerosol particles. Science. Vol. 359, January 26, 2018, p. 411. doi: 10.1126/science.aan8461.
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