We’ve all heard that our atmosphere contains increasing levels of carbon dioxide and other greenhouse gases. These greenhouse gases trap solar energy in the earth’s atmosphere and contribute to global warming. But new research indicates that certain kinds of air pollutants are changing the chemistry of clouds so that they reflect more sunlight back into space and less to earth. This phenomenon could offset the warming effects of greenhouse gases.

Robert Charlson, an atmospheric chemist at the University of Washington, has proposed that aerosols (particles so small they remain suspended in the atmosphere) from industrial pollution affect the formation of clouds. Working with scientists from Consilio Nazionale delle Ricerche, he analyzed clouds over the Po River Valley, an industrial region in northern Italy. The researchers collected large amounts of water from clouds and then evaporated the water, reversing the process by which the clouds formed in the first place.

When Charlson and his colleagues analyzed the evaporated clouds, they found the water droplets had 30 percent less surface tension than normal. Surface tension is the force that pulls water molecules toward the center and away from the surface. In water, the hydrogen atoms have a negative charge and the oxygen atoms have a positive charge. This creates a strong attraction between the hydrogens and oxygens, causing the water molecules to bond more tightly to each other than to other molecules.

When other molecules, such as certain air pollutants, mix with the water, they disrupt some of these hydrogen bonds and reduce the surface tension. In the case of cloud formation, this lower surface tension would result in smaller droplets, but more of them. The researchers calculate that a 30 percent drop in surface tension would produce about 20 percent more droplets and reduce the size of the droplets by about 6 percent. (1)

Charlson compares the effect to mixing water with sugar versus mixing it with detergent. If you add sugar to a jar of water, no bubbles will form, no matter how much sugar you add and how much you shake the jar. But adding just a few drops of detergent will give you a jar full of suds. Try it at home and you’ll notice how light reflects off of individual soap bubbles. Apparently, some air pollutants act more like sugar, while others act more like soap and make “sudsy” clouds that reflect more light.

According to Charlson and his colleagues, the greater number of cloud droplets caused about 1 percent more sunlight to be reflected away from earth. It’s important to remember that while the aerosols may counteract the effects of greenhouse gases, they are also changing the basic chemistry of the atmosphere. A NASA mission scheduled for 2003 will study the atmosphere in greater detail and should provide a clearer picture of how aerosols affect climate change.

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