Weekend Peaks in L.A. Basin's Ozone Mystify Experts

The Southland's smog is working overtime these days, reaching peaks on weekends--not during weekdays.

The shift puzzles experts and has led to new scrutiny of strategies that have been the bedrock of California's clean-air effort for a generation.

Overall, air pollution across Southern California has declined markedly in recent years. But what experts call the "ozone weekend effect" is growing worse.

Across the region, average weekend smog levels are now about one-quarter higher than those during the week. Similar, although less dramatic, trends have been seen in the Bay Area, as well as in Chicago, New York City and Detroit.

Several theories have been advanced to explain the weekend phenomenon. Some are more plausible than others, although none appears definitive.

More than a scientific curiosity, the weekend peaks have rekindled debate over the strategies used in California's successful but controversial anti-smog campaign. The auto industry, in particular, has seized the issue as a potential weapon against new regulations targeting tailpipe exhaust.

"It has major policy implications," said Douglas R. Lawson, principal scientist at the U.S. Department of Energy's National Renewable Energy Laboratory in Colorado.

Heavy weekend smog is counterintuitive, because many sources of emissions are lower on Saturdays and Sundays than during the workweek: Factories are mostly idle, freeways are less clogged with commuters at rush hour, and fewer big trucks and buses are in use.

For the average person, weekend smog is also a more serious potential health threat. Ozone in large but brief doses can literally sear lung tissue, and more of it lingers over the weekend at soccer fields, baseball diamonds and recreation pools than people may suspect.

"More people are outside on weekends than during the week, so they are more likely to be out gardening or engaged in recreational activities. People need to be concerned about air pollution levels," said Bill Kelly, spokesman for the South Coast Air Quality Management District.

What's causing the weekend peaks? A recent draft study by the California Air Resources Board offered a number of possibilities.

First, although emissions are lower overall on weekends, that may not be true in every community. Lynwood, for example, home to some of the worst carbon monoxide pollution in the nation, gets its highest levels of the pollutant about midday Saturday, the study says. More barbecues, boats, recreational vehicles and lawn mowers are used on weekends, which may contribute to high ozone in some areas.