Tiny airborne pollutants that scatter sunlight may play a major role in creating smog. The finding, reported  in today's issue of Science, could help researchers develop a better understanding of how to combat the haze that chokes many cities. It also suggests that the smoggiest areas in the United States could reap unforeseen benefits from a controversial government mandate to reduce particule pollution.
Scientists have long known that cars, factories, forest fires, and other pollution sources belch a stream of tiny particles and droplets--less than 2.5 micrometers in diameter--into the air, where the particles mix with gases to form aerosols. Researchers generally assumed that these aerosols either play no role in smog formation, or actually slow it by absorbing or reflecting the sunlight that powers the chemical reactions that create smog. Smog forms when the sun's ultraviolet rays bombard two common air pollutants: gases called nitrogen oxides and volatile organic compounds. Both types of gases react to form ozone, a corrosive form of oxygen that is smog's main ingredient and a health hazard.
Now, Russell Dickerson of the University of Maryland, College Park, and five colleagues have discovered that aerosols actually promote smog production rather than hamper it. The team reached this conclusion after comparing computer-model predictions with measurements during a severe smog episode that blanketed the northeastern United States in July 1995. The measurements confirmed predictions that high smog levels would be accompanied by high concentrations of light-scattering aerosols that are rich in sulfur and organic particles.
The team estimated that radiation striking these relatively clear particles, found mainly in the United States and other countries that burn a lot of fossil fuels, has a 96% chance of being scattered--not absorbed or reflected, as previously thought. The longer, bouncing path to Earth's surface hikes the odds that photons will collide with a molecule of one of the smog-producing gases. The researchers calculated that the aerosols increase ozone concentrations by about one-third during a smog episode. They note, however, that aerosols in other parts of the world--where weather conditions are different and darker soot particles are more common--could absorb light and inhibit smog.
The study "does a very nice job of reconciling theory with real-world observations" and should prompt researchers to reconsider the smog-formation models that currently guide pollution-control efforts, says Alan Hansen of the Electric Power Research Institute in Palo Alto, California. The findings also lend support to efforts to reduce particle pollution, which the Environmental Protection Agency says contributes to more than 15,000 premature U.S. deaths each year. After a lengthy battle with Congress and agency critics, the EPA last summer imposed strict new limits on fine particle emissions, primarily to reduce asthma and other breathing disorders. Dickerson notes that, by reducing the particles that promote haze, the new rules could deliver an additional health benefit in the 100 U.S. counties that regularly violate smog standards.