When clouds and aerosols collide

Air pollution and climate are linked, but defining the details of that connection is tricky. One uncertainty is exactly what happens when aerosols -- the tiny airborne particles in mist, smoke and smog -- meet clouds. Aerosols and clouds can have both cooling and warming effects on Earth's atmosphere. They can reflect sunlight away from Earth and they can also trap heat. The situation becomes even more complicated when aerosols and clouds interact with one another and trigger a variety of physical and chemical changes in the atmosphere.

In the first study of its kind [1], Dr. Nadine Unger, of NASA's Goddard Institute for Space Studies in New York, N.Y., and colleagues used a state-of-the-art computer program to discover what interactions between aerosols and clouds do to the chemical makeup of the troposphere. The troposphere is the lowest part of the atmosphere that stretches up from Earth's surface past all but the very highest clouds.

Specifically, they wanted to see how aerosol-cloud interactions influence changes in worldwide levels of methane (a powerful greenhouse gas), the production of sulfates in clouds, ozone near the ground and fine particulates in the air.

For the study, they simulated two different time periods: from the pre-industrial period to the present day and from the present day to the year 2050. For 2050, they used one of the more moderate climate-change scenarios for the future developed by the Intergovernmental Panel on Climate Change (IPCC).

The computer simulations show that the effects of aerosol-cloud interactions on our atmosphere are relatively small on a global scale, but large and significant for individual regions. Globally, the interplay between aerosols and clouds causes methane to linger in the atmosphere longer, and causes sulfate particles -- which cool the climate -- to disappear more quickly. Regional effects are much more pronounced. In areas where pollution emissions are higher, aerosol-cloud interactions cause a 20 percent increase in the amount of sulfate produced in clouds, significant increases in the amount of acid rain, and more surface ozone and sulfate-particulate pollution.

What this tells us, Unger says, is that the interaction between aerosols and clouds is important for our climate, and should be considered in projections of future air quality and worldwide methane trends.

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Rosemary Sullivant NASA / Jet Propulsion Laboratory

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Research paper: [1] N. Unger, et al., "Impacts of aerosol-cloud interactions on past and future changes in tropospheric composition," Atmos. Chem. Phys., 9, 4115-4130 (2009).