A coalition of environmental groups announced its intent to sue the corporate owner of a Dickerson power plant yesterday, charging that the plant's nitrogen emissions last winter violated federal law.

The Dickerson Generating Station, a coal-burning power plant in northwest Montgomery County that supplies electricity to Pepco, produced nearly 500 tons of nitrogen oxide this past winter, exceeding daily limits established by the Clean Air Act, according to the coalition, which includes the Chesapeake Bay Foundation, the Maryland Public Interest Research Group and the Chesapeake Climate Action Network. The plant is owned by Atlanta-based Mirant Corp.

The coalition said that the tonnage is equal to the amount of smog released annually by 40,000 sport-utility vehicles. The emissions spark asthma attacks and contribute to heart and lung disease, and they also threaten marine life when the particles are deposited in the Chesapeake Bay watershed, activists said.

"It's an enormous amount of pollution in an area that has way too much nitrogen oxide already," said Eric Schaeffer, director of the Environmental Integrity Project, the fourth group in the coalition. Federal law requires 60 days advance notice of the suit.

Mirant received permission from the state last year to build natural gas-fired plants, which would nearly double its production capacity. In exchange, it agreed to meet year-round nitrogen oxide emission levels by employing a pollution control technology called select overfired air, or SOFA.

Mirant said it is not violating the agreement because the conditions of its permit do not become operative until the new gas fire generators begin operating, company spokesman Steven Arabia said. The Dickerson plant has not yet broken ground on the facilities, he said.

"What they're talking about is a requirement that is not yet in effect . . . that is black-and-white in the permit," Arabia said. "I think a reasonable person will look at that and say that these people's complaint is completely off the mark."

Officials at the Environmental Protection Agency said they did not know enough about the Dickerson plant to comment on the matter.

State officials responsible for enforcing the permit said Mirant is not violating any clean air laws. "The Dickerson plant is currently operating in compliance with all of Maryland's air quality requirements, and they have been for at least the past three years," said Julie Oberg, spokeswoman for the state Department of the Environment.

The plant is already using SOFA controls to limit nitrogen oxide emissions from the coal units in preparation for building the gas units, Arabia said. The company signed a consent decree last year to reduce nitrogen pollution from its four Washington area plants: Dickerson, Chalk Point in Prince George's County, the Morgantown generator in Charles County and a plant in Alexandria.

Dickerson uses its SOFA controls more heavily in the summer because nitrogen oxide is a greater concern in warmer weather, Arabia said. During the winter, the plant decreases its SOFA controls.

"Emissions from the Dickerson plant can be controlled, are controlled in the summertime and should be controlled year-round," said Jon A. Mueller, director of litigation at the Chesapeake Bay Foundation.

The plant's emissions were higher than the permit allows for every day between Dec. 11, 2004, through March 31, according to the Environmental Integrity Project, which reviewed data that Mirant filed in a monitoring report.

"Plain as the nose on your face, we think they're violating it," Schaeffer said. "We're asking them to drive the speed limit, basically."

Mirant filed for protection under Chapter 11 federal bankruptcy law in July 2003 but says it expects to reorganize and emerge.