Mirant Corp. decided yesterday to shut down its power plant in Alexandria -- which provides electricity to hundreds of thousands of homes in the District and Maryland -- until the company can find a way to meet national air quality standards, state and company officials said.

Mirant informed the Virginia Department of Environmental Quality of its decision shortly after 4:30 p.m. The move to temporarily shutter the plant last night followed discussions yesterday with state officials on ways Mirant could try to correct problems revealed in a company analysis that showed that some pollutants in the vicinity of the coal-fired plant considerably exceed national standards.

"They looked at a number of scenarios for operating the plant, and they could not come up with anything that would meet the air-quality requirement," said Bill Hayden, spokesman for the Virginia Department of Environmental Quality. "We don't know how long that will take."

The company has said it will consider various solutions, including changing its technology or the type of coal it burns. "We're confident we'll be able to develop a solution and implement it," said Mirant spokesman Steve Arabia.

Officials with Pepco, which distributes electric power, characterized the plant as a vital link in the Washington power grid, but they said contingency plans have been in place to deal with a shutdown.

"It didn't catch us off guard," said Pepco spokeswoman Debbi Jarvis. "The lights will stay on. . . . But the shutdown of the plant still reduces the level of reliability below that which Pepco regularly maintains."

The plant, which began operation in 1949, employs about 120 people and supplies enough electricity to serve about 400,000 homes in the District and Maryland. It does not serve Virginia.

Neighbors of Mirant have complained for years about what they believed to be high levels of contaminants from the plant, on the banks of the Potomac River. The report that precipitated the shutdown looked at worst-case scenarios involving pollution, weather and operational capacity. Researchers found that under certain conditions, levels of sulfur dioxide, nitrogen dioxide and particles in the vicinity of the plant are higher than national ambient air quality standards allow.

As a result, state officials directed Mirant on Friday to take immediate steps to reduce pollution from the plant. They gave the company until last night to present plans for short- and long-term changes. Mirant responded Sunday by reducing the output of all five boilers from a maximum of 482 megawatts to 175 megawatts, or about 35 percent of capacity.

Rep. James P. Moran Jr. (D-Va.), who has pressured Mirant to come into compliance, characterized the decision as long overdue. "This shows government can represent the interests of the public," Moran said. "It's slow and cumbersome, but sometimes you get good victories like this one."

Atlanta-based Mirant, which filed for bankruptcy in 2003, operates four plants in the area, including three in Maryland: in Montgomery, Prince George's and Charles counties.

Yesterday, activists with the Chesapeake Climate Action Network, which supports renewable sources of energy, asked the Maryland Department of the Environment to examine Mirant's permits and operating procedures to see whether there are similar problems in Maryland.

"Given what's happened at the Alexandria plant, we're left to wonder: What else don't we know about other Mirant-owned plants?" said the group's director, Mike Tidwell. "We tip our caps to Mirant for doing the right thing, but we don't think they should ever reopen."

A jogging path runs past Mirant's Alexandria power plant, which is on the Potomac River. The plant has been found to violate air quality standards.

Coal is piled outside the Mirant power plant. Mirant says changing the type of coal is among the possible solutions to the plant's pollution problems.

The plant, which has five boilers, produces enough power for about 400,000 homes in the District and Maryland.