Thursday, Apr. 17, 2008

Valley's greenery gives off pollutant

Scientists looking at role of hydrocarbons in the ozone problem

FRESNO -- An astonishing fact is buried in dirty-air data: Valley trees and plants produce far more hydrocarbons than vehicles do.

In the summer, crops, trees, lawns and the rest of nature release 360 tons daily of the key smog component. That's four times more hydrocarbons than from cars and trucks.

Hydrocarbons combine with other gases in vehicle exhaust to form the San Joaquin Valley's ozone, considered one of the nation's worst air problems.

Between the valley's millions of farmland acres and the Sierra Nevada's sprawling forests, this region has a lot of plants. How big a role does all this greenery play in the valley's bad air?

Scientists need the answer to help meet smog cleanup targets over the next 20 years. But it is difficult to know how much of nature's hydrocarbons are involved in creating ozone. Because plants play a role in cleaning the air, nobody has found a way to calculate the possible net benefits that trees and other plants provide for the valley.

Don't chop down those trees

For now, nobody is advising people to chop down trees to save the air. But scientists say people should plant trees such as Modesto ash, oleander and Bradford pear, which don't emit a lot of hydrocarbons, and avoid high-emitting trees such as sycamore, eucalyptus, weeping willow and cottonwood.

There is some truth to President Reagan's often-criticized statement that "trees pollute." But trees and plants remove ozone, trap carbon dioxide, filter microscopic debris and lower temperatures, which slows the creation of ozone.

City temperatures would climb if many valley trees were eliminated, said Greg McPherson, director of the U.S. Forest Service's Center for Urban Forest Research at the University of California at Davis. And that would mean more ozone.

The San Joaquin Valley Air Pollution Control District continues to examine plant pollution contributions to the air, though officials say air agencies do not regulate such emissions.

The district needs to understand as much as possible about every source of pollution, officials said.

"Anything that does not help air quality needs to be investigated," said James Sweet, air quality analyst and project planner. "We need a clearer picture."

The district is pushing to meet the federal health standard for ozone by 2017, though the official cleanup target is 2024. The valley ranks alongside the Los Angeles area as having the nation's worst ozone problems.

The ozone fight is getting tougher. Federal officials have announced plans for a stricter ozone standard, and the valley may not achieve it until 2030.

Ozone forms as hydrocarbons and oxides of nitrogen cook in sunny weather. With hot, often stagnant summers and a bowl shape, the valley is an ideal place to form this corrosive gas.

The pollutant attacks skin, eyes and lungs. It can trigger asthma and other problems, especially in children and seniors.

Diesel trucks are the biggest source of oxides of nitrogen. Aside from nature, many tons of hydrocarbons come from dairies, gasoline, paint and other fumes.

An estimated 40 percent of the region's hydrocarbons come from nature. They also are known as volatile organic compounds such as isoprene, ter- penes, alkanes, alkenes, alcohols, esters and carbonyls.

But state and federal air officials say they have trouble figuring out how much of nature's hydrocarbons are involved in creating smog.

For example, estimates show more than 30 tons of hydrocarbons are coming from farm fields. But are they the type of hydrocarbons that will remain in the air long enough to drift and form ozone in a city? Some will, others won't.