Montgomery County Executive Douglas M. Duncan urged Maryland officials yesterday to conduct extensive air quality tests around the state's three Mirant Corp. power plants after Virginia determined that the company's Alexandria plant sometimes exceeded federal pollution standards.

In a letter to Gov. Robert L. Ehrlich Jr. (R), Duncan (D) asked the state Department of the Environment to follow Virginia's lead by assuring that Mirant plants in Montgomery, Prince George's and Charles counties comply with federal standards.

"Virginia has taken decisive action in cracking down on Mirant and its polluting plants, and it is my hope and expectation that Maryland will take a similarly strong stand," wrote Duncan, who is trying to make air quality a key theme in his likely campaign for governor next year.

Despite warnings from District officials that the move could jeopardize the city's power supply, Atlanta-based Mirant decided to shut its Alexandria power plant Wednesday until it can find a way to meet federal air quality standards.

The closing of the coal-fired plant, which serves about 400,000 Pepco customers in the District and Maryland, followed a Virginia Department of Environmental Quality analysis of pollution levels in the plant's vicinity.

Tom Snyder, director of the Air and Radiation Management Administration of the Maryland Department of the Environment, said the state is trying to obtain a copy of Virginia's study. But Snyder said Maryland already continually monitors emissions from Mirant's plants.

"We have a very extensive network of both measurements at the plant as well as away from the plant in the path of the emission plume," Snyder said.

Steve Arabia, a Mirant spokesman, said the company has performed an analysis of ambient air quality at its plants in Dickerson in Montgomery and Chalk Point in Prince George's. Those tests uncovered no air quality problems, Arabia said.

"What is asked for in that letter has already been done," he said.

Environmentalists contend that such tests do not go far enough.

"I don't think the excessive pollution would be caught under normal procedures. It was caught [in Virginia] because they did the extra testing," said Gary Skulnik, director of the Clean Energy Partnership.

The report that led to the Alexandria plant's temporary closure 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.

The D.C. Public Service Commission is warning of a "potentially dangerous and security-threatening" interruption of electrical service in the Washington area unless the Alexandria plant reopens.

According to a complaint and petition filed with the U.S. Department of Energy and the Federal Energy Regulatory Commission on Wednesday night, the commission said that without the Alexandria plant, the District's power grid could fail in the event of extreme weather or another disaster.

"Without the power generated by [the plant], catastrophe could be very near, " the petition said.

Some environmentalists are pushing for the permanent closure of the Alexandria plant, which started operating in 1949, because they say it is outdated.

"The plant is part of the 20th-century, health-damaging, dirty, fossil-fuel-burning plants, and now it is time for a 21st century of wind farms and solar farms," said Mike Tidwell, executive director of the Chesapeake Climate Action Network.

Tidwell's group was among the first to call for Maryland to go further in testing around the Mirant plants.

Duncan's decision to join the effort comes as his campaign is trying to distinguish his record from that of his likely rivals.

Besides frequently criticizing Ehrlich's environmental policies, Duncan has been needling Baltimore Mayor Martin O'Malley, Duncan's likely opponent for the Democratic nomination, over the mayor's reluctance to endorse state legislation to crack down on toxic emissions from coal-fired power plants.

The measure died in a Senate committee in the spring.

The O'Malley camp has pointed out that Baltimore has joined a federal lawsuit aimed at cleaning up mercury pollution from power plants. Montgomery has not joined the suit.