Officials in cities across the U.S. are trying to figure out how to get cleaner air. They’ve discovered that when auto exhaust levels go down on weekends, ozone?the main ingredient of smog?increases. They’re worried that as air pollution from car exhaust decreases with the introduction of new, lower-emissions cars, smog levels will climb even higher.

Researchers Robert Harley and Douglas Lawson are studying this “weekend effect.” In, Amanda Onion quotes Harley as saying, “Atmosphere chemistry is different wherever you are. But I don’t think you can dismiss this effect. It has been spreading to other cities.”

Lawson says, “It seems counterintuitive, but it’s true. And it’s a double whammy because people spend more time outside on the weekends.”

The American Lung Association says half of Americans live in areas with health-threatening levels of ozone pollution. While smog levels dropped during the 1980s, they stayed the same throughout the 90s, despite efforts to reduce them. Pollution controls on power plants and new cars have led to a 12% drop in nitrogen oxide (ozone) emissions from 1993 to 2002, but the smog level remains about the same.

Ozone forms when oxygen, or O2, picks up an extra oxygen molecule from nitrogen dioxide, or NO2, in the presence of sunlight. Once ozone, or O3, is created, the presence of fresh nitrogen oxides (mostly in the form of NO) can cause it to convert back into the original NO2 and O2. This means that pollution in the form of nitrogen oxides not only creates ozone, it also controls it.

For ozone levels to drop, emissions of nitrogen oxides and other pollutants that contribute to ozone formation have to be reduced much further than they already are. “You can choose to get your car fixed or some people drive around without proper registration. That’s frowned upon. But we’re not out to take anyone’s car away,” says Gennet Paauwe of the California Air Resources Board.

The problem could be that people are driving about 50% more than they did 10 years ago, and they’re also driving more polluting cars. This will continue until 2007, when the EPA increases emissions standards for SUVs, minivans and light trucks.

Car companies claim the weekend effect proves that higher car emissions standards are not the solution. George Wolff, of General Motors, says, “I think [the weekend effect] should definitely influence policy if we want to lower the ozone concentration.”

Environmental researcher Joseph Norbeck says, “I think you just have to understand the chemistry, bear with it and know it will improve in the end.”

More openness?less conspiracy? Don’t bet on it! Here’s where it all started. Photo credits:

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