NOGALES - Hundreds of big trucks cross the U.S.-Mexican border here every day, many more than once. On a busy day, the rigs idle for long stretches of time, waiting in line at the border, spewing exhaust through often-outdated mufflers that contribute to unhealthy levels of pollution.

For air-quality regulators, the border creates a legal barrier. State and federal agencies can't force vehicles manufactured and bought in Mexico to comply with U.S. emissions rules, even though the trucks cross into this country.

So the Arizona Department of Environmental Quality tried a different approach, offering to pay Mexican truck owners to replace old mufflers with new catalytic converters that will reduce harmful diesel emissions by up to 30 percent. The project in effect circumvents the more lax Mexican rules about exhaust systems.

Using federal grant money, the state agency installed the new converters on 55 trucks last year and will refit about the same number by the middle of this year. The project is part of a broader initiative to improve air quality on both sides of the border, where towns and cities often lack resources or expertise to do it themselves.

"It's about establishing this relationship on environmental issues," said ADEQ Director Henry Darwin. "It's especially important on air quality because you can't stop the air from moving across the border."

On a hazy day in Ambos Nogales - Spanish for "Both Nogaleses," the name used to describe Nogales, Ariz., and Nogales, Sonora - most of the pollution hanging in the air originates on the Mexican side of the border. Fewer than 21,000 people live in Nogales, Ariz.; an estimated 300,000 live in the Sonoran city.

The pollution is a toxic mix of dust particles, vehicle exhaust and volatile compounds that produce ground-level ozone, all elements linked to respiratory ailments and other health problems.

The air is bad enough that the region fails to meet limits for coarse-dust pollution set by the U.S. Environmental Protection Agency.

The dust is generated by industrial activity, trash fires and hundreds of miles of unpaved roads.

Yet while Arizona communities suffer the ill effects of the bad air, U.S. regulators can only encourage Mexico to address the issue. The diesel-fueled trucks that crossed the border on short hauls, moving goods back and forth between warehouses and factories, offered a rare opportunity for a state or federal agency to intervene.

"There's not a lot of low-hanging fruit in these areas," Darwin said. "There really aren't a lot of industrial sources that we can require to install equipment. We have to be creative."

Using EPA grant money, the state offered to refit the trucks with the new exhaust systems, replacing factory-installed mufflers with converters similar to what is required for U.S. trucks. The process takes two or three hours to complete at a cost per truck of about $1,600.

"It's a definite upgrade," said Larry Boch, an account manager for Auto Safety House, the Phoenix-based company doing the work. The old mufflers were all that was required down there, still are."

The company worked with a Nogales businessman who had extensive contacts with truck operators on the Sonoran side of the border. He sold the idea among some of the larger trucking companies and provided space for the work on a gravel lot just a few hundred yards from the Mariposa border crossing, where most truck traffic is funneled.

The truck, without its trailer, pulls onto the lot, which sits across a narrow street from a busy Shell gas station. The mechanics remove the old muffler and install the catalytic converter. If the system is mounted on the side of the cab, as many are, the work is easy.

The entire cost - parts and labor - is paid by the EPA grant through ADEQ.

Ari-Son Trucking, based in Nogales, Sonora, has had 17 of its trucks fit with the new systems, said Lourdes Puebla Maldonado, administrative manager for the company.

"It was an excellent opportunity for us," she said. "We feel it is for the community and for the benefit of both cities."

Mexican authorities have begun addressing air quality on their side of the border. The government has started paving dirt roads, which produce nearly 9,000 tons of dust pollution each year in the Nogales region.

Other sources present more complicated issues. Trash fires, vehicles and industrial operations spew enormous amounts of dust. On back-to-back March afternoons, the skies filled with smoke from a fire at a recycling plant on the Mexican side and from a wildfire that burned 7,500 acres on the Arizona side.

The scope of the problem is still difficult to quantify, in part because Mexican officials have done little to monitor air quality until recently. With loans and grant money from international funding agencies, Sonoran authorities have installed air monitors that will help regulators on both sides of the border identify and measure pollution sources.

"That's really the first step," Darwin said. "Unless you know where you're starting from, you can't get anywhere."