Study Links New Culprit to Smog in Mexico City

Mexico City’s notorious smog--for years blamed on automobile traffic and factories--is largely caused by millions of household fuel tanks for stoves and heaters that are leaking gases “on a massive scale,” California scientists reported today.

Liquefied petroleum gas, Mexico’s predominant fuel for cooking and heating, is “a large, previously unrecognized source” of the city’s smog-causing fumes, according to UC Irvine’s renowned atmospheric chemist F. Sherwood Rowland and senior research associate Donald Blake.

The leaking hydrocarbons mix with nitrogen oxides and react with sunlight to form ozone, the main pollutant that fouls the skies of Mexico City as well as the Los Angeles Basin.

Unburned gases such as butenes and propane that spew from the residential tanks “play a major (perhaps the dominant) role in ozone production in the valley of Mexico,” the scientists reported in an article published today in the journal Science.

The researchers suspect that residential fuel might also be a key pollutant in other smoggy cities around the world, including Athens, Taipei, Taiwan; Santiago, Chile, and some U.S. cities. A notable exception is the Los Angeles Basin, where household fuel is a negligible source, and cars and trucks remain the major culprit.

In Mexico City, about 5 million households have rooftop canisters of liquefied petroleum gas, or LPG, and leaks are believed to be common during use and refilling, Rowland said.

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The findings could prompt Mexico to broaden its controversial strategy for battling smog in its capital city, where more than 15 million people are breathing severely polluted air that exceeds health standards on nearly a daily basis.

After meeting with the UC Irvine scientists, the Mexican government and petroleum industry recently began funding further research into the scope of pollution from LPG and ways to reduce it, Rowland said.

Mexico’s environmental officials were not available for comment Thursday.

“Their procedures for trying to control smog have emphasized automobiles and industry, and that is clearly the right direction to go, but it should be expanded to include LPG,” Rowland said in an interview Thursday. “They have a terrible smog problem, and I’m encouraged that they will try” to address the household pollution.

Mexico City’s smog, which has grown dramatically over two decades, now mirrors the levels suffered in the Los Angeles area during the 1960s and 1970s.

Although the four-county Los Angeles Basin still suffers the worst smog in the United States, its air has improved markedly since the 1970s while Mexico City’s problem has surged.

Until now, it was assumed that Mexico City’s ozone was mostly due to its clogged traffic.

“When Mexico City expanded and automobile traffic grew, they got bad smog, so it is certainly a natural thing to believe it is coming from the automobiles,” Rowland said.

But the first detailed analysis of the hydrocarbons in the city’s air showed otherwise. Blake and Rowland found very high amounts of alkane hydrocarbons--mostly propane, butanes and butenes--that could not have come from vehicles and factories.

“We saw these very large amounts of propane and it was quite clear that this is not automobile pollution,” Rowland said.

Instead, the team wrote, the source “lies in LPG--the major energy source of cooking and heating in urban households in Mexico City--leaking in unburned form into the atmosphere on a massive scale.”

The conclusions of the study funded by the U.S. Department of Energy were based on about 180 air samples collected in 50 spots during five trips between 1991 and 1995. Seventy-five of the samples were taken during a severe smog episode in February, 1993.

Air was tested in central areas as well as in outlying locations miles from traffic and industry.

In a midday sample taken this May, unburned fumes from LPG made up an astonishing 45% of hydrocarbon reactivity at Mexico City’s bustling Plaza de la Constitucion.

Surprised by the data, other air pollution experts say it could have a profound effect on smog control in Mexico.

“This nominally is a relatively easy fix, or it could be,” said Roger Atkinson, interim director of UC Riverside’s Statewide Air Pollution Research Center. “If you could find a source of LPG that did not have the butenes in it, you could make a fairly easy and immediate effect on reactive emissions.”

LPG manufacturers could switch the fuel’s butenes, which are highly reactive, to less damaging propane. Residents also could check their canisters for leaks--loss of fuel is a waste of money--and gas companies could take greater care in filling them, he said. The tanks are filled monthly, with about 200,000 deliveries of LPG made daily in Mexico City.

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Rowland and Atkinson stressed that Mexico City, like Los Angeles, must keep battling the other important component of smog--nitrogen oxides--which comes largely from vehicles.

“Their automobiles are still a very important source obviously,” Rowland said.

The Mexican government curtails automobile use and industrial production on highly polluted days, and closed down an oil refinery in the city. But it does not restrict the content or handling of LPG.

Most Mexico City residents rely on individual canisters of LPG for cooking and heating rather than gas pipelines because of the severe threat of earthquakes.

Rowland and Blake met with Julia Carabias, Mexico’s minister of the environment, in May to discuss their findings.

Previous studies in Mexico, the UC Irvine researchers say, vastly underestimated the role of residential fuel.

Last year, a report by the Los Alamos National Laboratory and a Mexican institute estimated that half of the 625,000 annual tons of hydrocarbons in the city’s air came from motor vehicles. They estimated residential sources as contributing about 330 tons per year.

But that study did not perform a sophisticated chemical analysis like the one performed by the UC Irvine team. Given the city’s annual sales of about 2 million tons of LPG per year, the figure is probably “very much too small,” Rowland and Blake reported.

The data probably has no impact on the Los Angeles region, where most residential heaters and stoves are fueled by cleaner burning sources. LPG here does not contain much of the highly reactive butene.

In fact, recent research by the California Air Resources Board concludes that motor vehicles are playing an even larger role in producing Los Angeles area smog than previously estimated. Between half and three-quarters of ozone-forming pollutants come from cars, trucks and other vehicles, according to the state agency.

Mexico City, which lies in a high valley surrounded by pollution-trapping mountains, suffers some of the world’s worst smog concentrations. Its air exceeded Mexico’s ozone health standard on 358 days in 1992.

The pollutant that year peaked at a hazardous concentration of 0.48 parts per million--50% higher than recent peaks in the Los Angeles Basin and four times greater than the U.S. health standard.

Ozone causes eye and throat irritation and shortness of breath. It also is believed to lead to premature aging of the lungs and can aggravate respiratory diseases such as asthma and chronic bronchitis.

For 20 years, Rowland has conducted pioneering global research, mostly on depletion of the Earth’s protective ozone layer. In the upper atmosphere, ozone provides a shield from harmful radiation, but as a pollutant in the lower atmosphere, it is unhealthful to inhale.

Rowland and a colleague in the 1970s were the first to identify chlorofluorocarbons as the major source of ozone damage. The findings led to an international treaty phasing out CFCs, which have been widely used as refrigerants and industrial cleaning solvents.