Opponents of more air traffic at National Airport have been told by Virginia Attorney General J. Marshall Coleman that the state has the power to stop expansion of the already heavily used facility.

But in delivering some good news to Virginians for Dulles, the group that has been trying to put a ceiling on National's growth, Coleman also had some bad news: The State Air Pollution Control Board doesn't want to have that power any more.

The state's weapon is called a permit for indirect sources of air pollution. In theory, if the state thinks any new development will generate auto traffic that will lead to violation of pollution standards, it can prevent construction from going forward. Coleman, in a clarification sought by Virginians for Dulles, has said in an "advisory opinion," which is not legally binding, that the state has the authority to require federal facilities - such as National - to have such a permit.

"I'm very pleased," said Sen. Clive L. DuVal II (D-Fairfax), chairman of the Dulles group, "but I'm bothered by the opinion too. If the state doesn't do it (sue to stop any expansion of National), we'll see if we can't go forward with a suit."

At issue is the tremendous increase in air traffic that is taking place at National. Many residents who live in the airport's noise zone - including DuVal's constituents in McLean and North Arlington - want a lid put on the airport's passenger volume. They want traffic shifted to Dulles and Baltimore-Washington International Airports.

National, which now handles 14.7 million passengers a year, was expected to have 18 million passengers annually by 1990, according to a draft environmental impact study published by the Federal Aviation Administration, which operates the airport.

But deregulation of air carriers, among other factors, has forced FAA to revise the 1990 date to 1983 or 1984, "if everything continues as it is," according to Charles C. Erhard, environmental and noise abatement officer for FAA's Metropolitan Washington Airports Office.

"We can handle 18 million passengers with what we have here," Erhard said. "But we'd like to rebuild and modernize." Presumably, such construction would be covered by the indirect pollution source permit.

DuVal and his Virginians for Dulles claim that 18 million passengers would generate a 35 percent increase in air pollution in the area.

But while the Dulles group, which has lost earlier legal efforts to halt National's expansion, wants the state to unsheath its antipollution weapon, the State Air Pollution Control Board says the weapon is useless and just generates mounds of paper work.

"We've never denied a permit for an indirect source," said John M. Daniel, assistant executive director of the state board.

The only type of pollution that could be measured for the permit is carbon monoxide. The monitoring station that is closest to National - at Aurora Hills south of the Pentagon Metro stop - is currently measuring a "pretty low one-hour level" of one to 2 1/2 parts of carbon monoxide per million parts of air, according to John Doherty, regional director of the state air board. The one-hour standard is 27 parts per million, not counting the assumed background count of 8 ppm.

Doherty said, "My judgment is that it would be questionable" whether an increase in annual passenger traffic volume at National would lead to violations of the carbon monoxide standard.