Pollutants behavior puzzle scientist throughout state

The Associated Press \
Friday October 27, 2000

LOS ANGELES — It’s a dirty-air puzzle that befuddles scientists: When other pollutants punch out for the week, smog-causing ozone starts working overtime in some parts of the state. 

In the Los Angeles area, ozone levels typically are 32 percent higher on Saturdays and Sundays than they are Monday through Friday when freeways are clogged with commuters. The difference is 25 percent in San Francisco. 

Other pollutants – even those that help form smog – drop on weekends. The so-called ozone weekend effect doesn’t happen everywhere, however. A draft report released this week by the California Air Resources Board found no statistically significant differences between weekend and weekday ozone levels in Sacramento and the San Joaquin Valley. 

The phenomenon in Los Angeles and San Francisco, first noticed in the 1980s, has four possible explanations, according to the report. Among the theories: 

l Weekends produce a different kind of traffic volume that could encourage smog production. Morning traffic is much higher on weekdays, but weekend traffic catches up later in the day, when nitrogen oxides can produce ozone more efficiently. 

l Ozone produced on weekdays could drift over the ocean, then return to shore over the weekend. 

l The atmosphere could contain more soot on weekdays than on weekends. That could help absorb some of the ultraviolet radiation that otherwise would help form ozone. 

Automakers support another possible reason that suggests government regulators could reduce weekend smog by allowing an increase in one smog-causing pollutant: nitrogen oxides. 

Experiments show the ratio between hydrocarbons and nitrogen oxides plays an important role in the formation of smog. Decreasing nitrogen oxides when levels of that pollutant are low compared to hydrocarbons actually increases smog, the experiments indicate. 

Researchers found that nitrogen oxide levels decrease at a greater rate than hydrocarbon levels on weekends. 

Even if further studies support the automakers’ theory, that would be a poor reason to turn back the clock on decades of regulation and technological improvements aimed at reducing nitrogen oxides in the air, Air Resources Board spokesman Richard Varenchik said. 

“The big picture overall is that ozone has been cut by two-thirds in the last 30 years,” Varenchik said. “The strategy we’ve been using is certainly the correct strategy.” 

The South Coast Air Basin, which includes Los Angeles, saw ozone levels of 200 parts per billion or higher about every other day 35 years ago. 


Cleaner air has prevailed in recent years.The basin hasn’t seen such a “Stage I episode” for two years. 

Nitrogen oxides need to be kept out of the air not just because of their role in forming smog but because they also contribute to pollution from particulate matter and toxic air contaminants, said Bart Croes, chief of the air board’s research division. 

“There’s only one component of air pollution that’s worse on the weekends,” he said. “We shouldn’t devise a control strategy based on one pollutant.”