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 a whopping 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?
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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. In part because plants play a role in cleaning up the air, nobody has found a way yet to calculate the possible net benefits that trees and other plants provide for the Valley.
For now, nobody is advising people to chop down their trees to save the air. But scientists do say people should plant trees such as the Modesto ash, oleander and Bradford pear, which don't emit a lot of hydrocarbons, and avoid the high-emitting trees -- the sycamore, eucalyptus, weeping willow and cottonwood, they say.
Indeed, there is some truth to President Ronald Reagan's often-criticized statement that "trees pollute." But trees and plants also remove ozone, trap global-warming carbon dioxide, filter microscopic debris and lower temperatures, which slows creation of ozone.
City temperatures would no doubt climb if many Valley trees were eliminated, says 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.
"I think you would actually have a bigger air-quality problem in Fresno without trees," said McPherson, an urban forest researcher for 25 years.
Added Sharon Kelly, project director of the nonprofit Tree Fresno: "Trees are one of our biggest solutions to dealing with air-quality problems."
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 achieve the federal health standard for ozone by 2017, though the official cleanup target is 2024. The Valley ranks alongside the Los Angeles area with the country's worst ozone problems.
The ozone fight will get 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 together 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 the skin, eyes and lungs. It can trigger asthma as well as other problems, especially in children and the elderly. Fresno County has the highest childhood asthma rate in California.
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.
By current estimates, about 40% of the region's hydrocarbons come from nature. They also are known as volatile organic compounds such as isoprene, terpenes, 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.
An 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.
Valley farmers grow more than 200 crops and often change crops from season to season -- meaning different types and amounts of hydrocarbons are released. So the ozone levels could vary widely each season, depending on the crop.
Another complication: Hydrocarbons increase with plant damage, which happens during harvest. The same thing happens when you mow your lawn or trim your shrubbery.
Other factors include time of year, wind and rainfall, said John DaMassa, chief of modeling and meteorology at the California Air Resources Board in Sacramento.
Spread these issues over the 25,000-square-mile Valley, scientists say, and any estimate is suspect.
But the regional air district needs to understand such a major hydrocarbon source to devise strategies for meeting the stringent ozone standard in the future. The strategies wouldn't focus on plants and crops. Instead, they probably would continue to attack the other main ozone gas -- oxides of nitrogen.
DaMassa said science has known about nature's hydrocarbons for decades, and research has helped to describe the problem.
"We have better tools to do the research now," he said, "but we still need to do a lot more work."
Added environmental scientist Ray Chavira of the U.S. Environmental Protection Agency in San Francisco: "There is no straight answer."
Scientists said hydrocarbons coming from the Sierra Nevada are not likely to harm air quality in the Valley. But further research is needed in the mountains as well, they said.
The air-cleaning benefits of crops and various plants are known. Research shows an acre of a typical orchard or a cotton field absorbs up to a half pound of ozone in July.
But there is no Valley-wide figure for the amount of ozone being removed by millions of farmland acres. For years, scientists have focused on describing how ozone stunts plant growth and limits harvests, not defining the amount of ozone being removed.
In cities, research on the advantages of trees is well-developed and expanding.
Forest Service researcher McPherson said that for every dollar a city invests in a properly maintained tree, the residents could get benefits equal to $2 to $5.
The benefits equate to energy savings, air quality improvement, enhanced property value and reduction in storm water runoff, he said.
Studies of trees in parking lots show that they lower temperatures and prevent gasoline in cars from evaporating. The evaporating gasoline would release more hydrocarbons.
"There is more research going on right now in Sacramento on the net effect of having trees in the city," he said.
"The idea is to see how things change when a city expands the tree canopy. If trees are a legitimate air pollution reduction measure, then there's justification for planting them."