Afternoon thundershowers freshen city air, but they can also make air pollution worse by spreading smog across large areas, according to a professor of atmospheric chemistry from the University of Maryland in College Park.

Since 1985, Russell R. Dickerson and scientists from Colorado and Washington have sampled air pollutants in thunder clouds of 30 storms. "The carbon monoxide was highest up there," Dickerson said in a telephone interview. "That means the air pollution was highest up there.""The conclusion we drew is that thunderstorms are very important in long range and global distribution of air pollution," he added.

Air currents in a storm front lift smog up to six miles into the air, where wind carries the pollutants long distances. Higher up, there's more sunshine and less moisture, so the pollutants stay airborne much longer, said Dickerson.

Some pollutants, such as oxides of nitrogen, cause other gases in the air to react and form ozone. It takes just a few molecules of these oxides to make lots of ozone. Because these gases last longer at such high altitudes, even more ozone can be produced, Dickerson explained.

Ozone in the very high layers of the atmosphere, more than 10 miles up, shields life on earth from damaging radiation from the sun. "But if you get any ozone in the lower atmosphere, then it's a noxious pollutant," said Dickerson. It irritates the eyes and lungs and damages plants, costing farmers billions of dollars from crop loss.

"It's also a greenhouse gas," he added. Like carbon dioxide, ozone traps sunlight and may contribute to global warming.

When exhaust gases and byproducts from power plant pollutants remain close to Earth, less than a half-mile from the surface, they do not travel very far or last very long before settling out or being washed out of the air by rain. Thus their effects are limited.

Storms that wash out pollutants away help dilute atmospheric acids and reduce acid rain. But when these particles catch a ride on the gusts of spring storms, they travel long distances and last much longer.