E-mail story
Printer friendly page
Subscribe to PrintBy Russell Clemings / The Fresno Bee
The ozone plan that the San Joaquin Valley Air Pollution Control District approved earlier this year is a sharp -- and perhaps risky -- shift from the region's previous smog control efforts.Blame the complex chemistry of air pollution.Smog has two main building blocks: hydrocarbons, such as unburned gasoline, and nitrogen oxides, or NOx, which are formed when hydrocarbons burn.In the past, regulators have focused on reducing both types of emissions about equally. Not any more.The new plan calls for reducing NOx emissions by almost three times as much as hydrocarbons. District officials say the justification lies in computer modeling that simulates smog formation and pinpoints the most effective way to stop it.But the strategy could backfire.Both hydrocarbons and NOx help create ozone, but the reaction isn't one-way. Instead, the two pollutants enter into reactions with other gases, and even with ozone itself.In some cases, rather than creating ozone, NOx actually removes it from the air. In Los Angeles and other major cities, ozone levels have risen when NOx emissions drop. It's called the "weekend effect" because it has been seen on weekends when diesel truck traffic -- a major NOx source -- is reduced.One scientist who has studied the weekend effect is Erik Fujita of Reno's Desert Research Institute. He concluded that ozone can rise if the ratio of hydrocarbons to NOx falls too low."My position would be that if you're going to control NOx, you should also continue to control hydrocarbons," Fujita said. "You should control it equally is what I would say."Two University of California scientists did their own computer modeling of San Joaquin Valley conditions and reported the results in 2002.They found a weekend effect, at least in the northern end of the Valley. On the computer, a 25% cut in NOx emissions triggered an ozone increase of 5% to 25% southwest of Stockton.But district officials point to modeling by the state Air Resources Board that suggests conditions in most of the Valley are different from Southern California and other areas where the weekend effect has been seen."What our problem seems to be is the sort of long-term kind of stagnation in pockets of the San Joaquin Valley," said Scott Nester, the air district's planning director. Ozone levels rise slower here than in Southern California and reach their peaks farther from the Valley's cities."They looked to see if they could discern a weekend effect but couldn't really see it," Nester said.At least, that's what the model says.