The EPA has proposed sharp cuts in emissions from cars and increasingly popular sport utility vehicles that could significantly drive down the amount of pollution entering the Bay.

For the first time, the new standards would force vans, light trucks and other sport utility vehicles — which account for almost half of all vehicles sold — to eventually meet the same emission requirements as cars.

The new standards proposed in May would be phased in over a period of five years beginning in 2004. Larger sport utility vehicles would be given more time to meet the standards, but would have to meet a series of increasingly tighter standards in the interim.

When fully phased in, NOx emissions from cars will be about 77 percent less than what is the case today. Emissions from SUVs weighing less than 6,000 pounds will be reduced 86 percent. Emissions from large SUVs, those between 6,000 and 8,500 pounds, will be up to 95 percent cleaner than today.

Some benefits from those reductions would be offset by increased driving. But the EPA expects the new standards to continue the overall downward trend in vehicle emissions for years to come.

Despite increased travel, past air pollution control regulations have resulted in a steady decline in overall vehicle emissions within the Bay watershed. From 1987 to 1997, NOx emissions in the watershed decreased by 18 percent even as the number of miles driven increased by 41 percent, according to Bay Program figures.

Motor vehicles account for about a third of all the NOx emissions in the Bay region. Power plants, industries and a host of smaller sources — from lawn mowers and chainsaws to boats and bulldozers — make up the rest.

Air pollution is estimated to contribute about a quarter of all the nitrogen that enters the Bay.

Although automakers will install improved pollution control devices on vehicles — roughly at a cost of $100 to $200 each — much of the burden will actually fall on refiners. Citing evidence that sulfur in gasoline hinders the effectiveness of catalytic converters on cars, the EPA wants refiners to reduce the average sulfur level to 30 parts per million by 2004, down from the current average of more than 300 ppm. That will increase the price of gas by about 1 to 2 cents per gallon, according to the EPA

Automakers had argued that they could not do much more to reduce emissions unless the sulfur content in gasoline was dramatically reduced. As a result, automakers are in the odd position of siding with environmental groups in supporting the regulations.

Refiners, though, oppose the sulfur reductions, saying they will have to spend billions of dollars to change the refining process. At the least, they say the low sulfur gas should only be sold in the East, where smog is a major problem. The EPA rejected that argument, in part because anyone visiting a state selling high-sulfur gas would damage their catalytic converter.

The EPA plans to adopt final emission regulations by the end of the year.