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.

Another cause may lie with all those cars jammed on freeways Friday and Saturday nights headed for Las Vegas, dinner and a movie or the beach. Studies show greater traffic on weekend nights than on weeknights, and measurements taken between 200 and 5,000 feet above the Los Angeles Basin reveal that a large pool of ozone and its precursor chemicals persist in the night sky.

Early Saturday and Sunday mornings, the pollution descends, and it lingers until fed by other ozone-forming pollutants later in the day, according to the air board.

Other possible explanations involve the complex chemistry of smog. The most hotly debated theory, advocated by the auto industry and accepted by many experts in atmospheric chemistry, is that the problem on the weekend is, paradoxically, a shortage of a certain pollutant.

That explanation harks back to an on-again, off-again debate in the quest for clean air over which pollutants must be cut most: hydrocarbon fumes from gasoline, solvents and paint; or nitrogen oxide gases, a byproduct of fuel combustion. The two substances mix in the presence of sunlight to create smog, and for 30 years, California’s strategy has been to attack them both aggressively.

Industries Argue for Different Strategy

On weekends, emissions of both pollutants drop, but nitrogen oxide concentrations fall more, because less traffic, fewer buses and big trucks and fewer factories at work all mean much less fuel being burned.

The unequal decline is significant, because nitrogen oxides have a split personality when it comes to ozone. They can promote ozone formation if released into an environment that has a lot of hydrocarbon fumes. But they can also gobble up ozone if hydrocarbon fumes are scarce.

Over the years, the auto industry, oil companies and utilities have argued that the way to knock down ozone is to take advantage of the ozone-eating potential of nitrogen oxides and allow more of them into the air. The way to do that would be with more controls on reactive vapors--from gas stations, consumer products and the like--and fewer controls on tailpipes, auto industry officials say.

“We think there’s a point where decreasing nitrogen oxides actually increases ozone, and that’s what you see on weekend days,” said Steven Douglas, director of environmental affairs for the Alliance of Automobile Manufacturers.

Lawson, the Department of Energy scientist, who has been conducting a study of weekend smog, agrees. “The way to reduce ozone is to reduce hydrocarbons. If you reduce nitrogen oxides, you’ll actually increase ozone,” he said. “Nitrogen oxide reductions in the South Coast Air Basin make it more difficult to attain the ozone standard.”

Other experts dispute that.

“Thirty years of reducing nitrogen oxides and hydrocarbons has resulted in a two-thirds reduction in ozone, which leads us to believe there’s a flaw in [the auto industry’s] theory,” said Richard Varenchik, spokesman for the California air board. “We’re not ready to look at this one little phenomenon and scrap our whole approach to pollution control.”

Engines Also Emit Airborne Particles

Joseph M. Norbeck, director of the Center for Environmental Research and Technology at UC Riverside, said that reduced emissions from big trucks are probably contributing to the weekend effect, but not enough to warrant going soft on tailpipe exhaust.

“It doesn’t mean we should increase pollution,” he said. “It’s an interesting phenomenon, but it’s not going to affect public health to a large extent and it’s not going to delay attainment [of clean-air standards] at all.”

Those who want to continue the push for nitrogen oxide reductions point out that, in addition to contributing to ozone, engine emissions are a major source of the region’s other big air pollution problem: minuscule airborne particles that obscure blue skies and threaten public health.

Medical studies link particles to cancer, premature death and lung disease. Deep cuts in nitrogen oxides will be needed to restore visibility, reduce health risks and meet new, tighter federal health standards, said John Bachmann, associate director for science policy at the U.S. Environmental Protection Agency.

While scientists continue to investigate, air quality officials are beginning to recognize that they need tailor-made strategies to tackle weekend ozone. None are in place today.

Next year, the AQMD for the first time will consider specific strategies for weekend smog.

Meanwhile in Sacramento, the state air board is using computer models to simulate day-by-day changes in the Los Angeles Basin’s smog. Findings from that work are expected to lead to strategies to tackle the problem, Varenchik said, but first scientists have to figure out what is happening on weekends.

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Weekend Smog

Ozone concentrations in much of Southern California are about 25% higher on weekends than on weekdays. Levels of other pollutants, such as microscopic particles, are lower.

Percentage of midweek average for pollutants

Source: California Air Resources Board, 1996-98