Hydrocarbon chemicals -- one of the ingredients for smog -- are leaking from oil and natural gas facilities and polluting the air over Texas, Oklahoma and parts of Kansas, scientists have found. Air samples from Oklahoma City contained more hydrocarbons than did samples from New York City or Houston, says a report published online Monday in the Proceedings of the National Academy of Sciences.

"We just drove through the countryside and found that there were a lot of hydrocarbon gases," said Sherwood Rowland of the University of California, Irvine, co-leader of the study. "None of these are areas where you expect pollution."

The findings suggest that scientists may have significantly underestimated the amount of methane, a hydrocarbon and heat-trapping "greenhouse" gas, that is released into the atmosphere, he said. The Environmental Protection Agency has estimated that the U.S. natural gas industry puts about 6 million tons of methane into the air annually.

"It looks to us as though maybe there are 6 million tons right there in those three southwestern states," said Dr. Rowland, an atmospheric chemist and co-winner of the 1995 Nobel prize in chemistry.

Finding high levels of hydrocarbons isn't a surprise in a region so rich in oil- and natural gas-production facilities, he said. And the work builds on earlier findings, from the Texas Air Quality Study conducted in 2000, that suggested hydrocarbon emissions had been underestimated. "We simply haven't been in this business long enough to have a very good measure of the emission rate of a lot of these compounds," said Paul Goldan, senior scientist at the National Oceanic and Atmospheric Administration's Aeronomy Laboratory in Boulder, Colo., who participated in the 2000 Texas study.

Hydrocarbons include compounds like ethane, propane and butane, which can survive in the atmosphere up to several months. Such chemicals combine with nitrogen oxide compounds to form ozone and then smog. Areas downwind from the hydrocarbon pollution would form slightly more smog than usual, said Irvine chemist Donald Blake, the study's other leader.

The work began in August 1999 during a survey or urban atmospheres. The air of Oklahoma City, the scientists found, contained far more hydrocarbons than any of the five other cities surveyed. So the researchers sent a pack of graduate students out into the field. In September 2001, the students collected 85 air samples while driving 500 miles in eight directions from Oklahoma City. In spring 2002, they returned to the region, taking 261 air samples in a grid pattern across Texas, Oklahoma, New Mexico, Louisiana, Arkansas, Kansas and parts of Colorado, Missouri and Mississippi.

In Texas, the highest amount of hydrocarbon pollution was detected not near the petrochemical industries of Houston, but at a spot 40 miles east of Odessa, said Dr. Rowland. That suggests the source isn't individual power plants but rather the spread-out network of wells, storage tanks and pipes. "We're pretty sure that the gases escaping from these above-ground storage tanks are in part responsible for this," Dr. Blake said.

Still, the amount of leakage isn't that great compared with other sources of methane. In the United States, more methane comes from natural sources, such as wetlands, and sources such as landfills and livestock than from the oil and natural gas industries.

During the 2000 statewide survey of air quality, other researchers measured similar amounts of hydrocarbons across Texas, said chemist David Allen, director of the Center for Energy and Environmental Resources at the University of Texas at Austin. Air samples taken in Longview and Victoria, near oil and gas fields, contained high amounts of ethane, propane and butane, Dr. Allen said. But samples taken in La Grange, relatively far from an oil-producing region, had fewer hydrocarbons. The new study helps to pinpoint just how widespread that pollution is.

"All of our rural measurements, made over extended time periods but at only a few locations, are consistent with Dr. Rowland's findings, which were performed at many locations but over a more limited time frame," Dr. Allen wrote in an email interview.