For cloud formation, a little aerosol goes a long way

by Mary Caperton Morton
Wednesday, September 3, 2014

Clouds play a starring role in creating and controlling climate, but cloud physics are notoriously difficult to model, leaving wide gaps in understanding how cloud conditions have changed since the pre-industrial era. A new study looking at pristine regions of the sky in the South Pacific is shining some much-needed light on how particulate air pollution interacts with water vapor to form clouds.

“Usually when we’re studying particulates, we go to the most polluted places, such as over the Amazon during the burning season,” says Ilan Koren, a cloud physicist at the Weizmann Institute of Science in Rehovot, Israel, and lead author of the new study, published in Science. “Going to the most pristine places seems kind of counterintuitive, but [such places] can teach us a lot about how clouds form.”

Aerosols are particles suspended in the atmosphere that can both scatter sunlight and cool the planet, or absorb sunlight and warm it, depending on particulate concentrations and a host of chemical reactions. Man-made aerosols — including soot, chlorofluorocarbons and sulfates from vehicle emissions — are often characterized as pollutants, but not all aerosols are anthropogenic: Wind erosion, sea spray, volcanoes and biological emissions all contribute to natural background levels, which are always changing and generally not well quantified, Koren says. Aerosols, natural or anthropogenic, provide the necessary seeds for cloud formation. “Without aerosols, there are no clouds,” Koren says.

Koren and colleagues set out to test whether aerosols can invigorate clouds — a little-understood phenomenon in which the aerosol particles that serve as the nuclei for condensing water vapor further push clouds to become taller and larger.

Using 10 years of satellite data and meteorological reports, Koren’s team selected a region of the South Pacific, over water, with very low levels of aerosol particles where deep, convective clouds are still known to form. Even in places far from urban and industrial pollution, background levels of aerosols are still present, due to long-range transport through the atmosphere and from natural local sources like emissions from volcanoes, says Lorraine Remer, an atmospheric scientist at the University of Maryland, Baltimore County who was not involved in the new study.

“When you’re in a pristine area, clouds are very susceptible to change. Any amount of particles makes a big difference,” Remer says. “It’s almost like the clouds are starved for a seed. They have plenty of water vapor, and they’re looking for a way to condense into droplets, but there’s nothing to condense on. Add a few particles, and boom, you have a lot more clouds.”

Koren’s team found that even small amounts of aerosol particles — tens of particles per cubic centimeter, as compared to the hundreds per cubic centimeter often found in polluted cities — had a significant impact on the formation of clouds over their test area. The transition from pristine to slightly polluted air led to more and taller clouds over larger areas, and in some cases, more rainfall.

“This suggests that before industrialization, when the atmosphere had far fewer aerosols, clouds would have been in higher starvation levels with respect to aerosols,” Koren says. “Therefore, we expect pre-industrial times had completely different cloud patterns, with smaller and shallower warm convective clouds, than in modern times.”

The next step will be to figure out how to better include aerosol invigoration into global climate models, which is no small task given the complicated microphysics and dynamics involved in cloud formation. “Eventually, we need to find a way to include complicated cloud formation processes into the models used to predict climate change,” Koren says. “Without clouds, we are missing a very important piece of the climate puzzle.”

© 2008-2021. All rights reserved. Any copying, redistribution or retransmission of any of the contents of this service without the expressed written permission of the American Geosciences Institute is expressly prohibited. Click here for all copyright requests.