The Reagan administration is again in court defending its refusal to do anything about acid rain in Canada, while at the same time it has signed a new international accord limiting one of acid rain's main constituents.

The Environmental Protection Agency said the accord signed Tuesday in Sofia, Bulgaria, by agency administrator Lee M. Thomas means "no additional regulatory actions are required" by the United States against oxides of nitrogen.These combustion products become acid rain in the atmosphere and help form urban smog. One of them is responsible for the color of the "brown cloud" that hangs over smoggy cities such as Los Angeles.

Lawsuits were filed Tuesday in the U.S. Circuit Court of Appeals for the District of Columbia seeking to force the EPA to require states to revise their air pollution control plans to eliminate damage to Canada.

This is the same question that the plaintiffs, the Canadian province of Ontario, the Izaak Walton League and the Sierra Club Legal Defense Foundation, lost in a previous court round.

Though the EPA under the Carter Administration had declared that acid rain originating in the United States was harming Canada, the court ruled that such a declaration was not enough to force the states to act. Formal EPA regulations are required, the court said.

The EPA has argued that not enough is known about acid rain to draw up a control program.

"We cannot afford to hold our breath waiting for the U.S. Environmental Protection Agency to wake up and smell the sulfur," Ontario environment minister Jim Bradley told the provincial legislature in announcing the new lawsuit.

The major component of acid rain in the eastern United States and Canada is sulfuric acid formed from sulfur dioxide emitted by utility and factory boilers.

The New York state attorney general's office said it was preparing a renewal of a related suit on behalf of New York, New Jersey, Minnesota and the six New England states.

In Bulgaria, the United States joined a nitrogen oxides annex to the United Nations-sponsored treaty on Long-Range Transboundary Air Pollution.

In negotiation, the United States sought a 20 percent credit against emissions limits to compensate for reductions in U.S. emissions that had not yet been matched by European countries.

In a compromise, the negotiators chose a "pick your limit" requirement. Emissions in any year through 1987, the particular year chosen by each signing country, will be the limit in the 1990s. In addition, the annual average for the period 1987-1996 may not exceed 1987 emissions.

For the United States, this amounts to about a 5 percent credit. Nitrogen oxides peaked in 1978 at 22.4 million tons and are now about 20.3 million tons. Forty-four percent comes from transportation, mostly motor vehicles, and a little under half from boilers.

Though new cars emit only a quarter of the nitrogen oxides that they did in the 1960s, travel has grown so much that total emissions are declining only slowly, and EPA expects the total to turn up again in about 1995, reaching 20.2 million tons in 2000.

The United States refused to sign an earlier sulfur dioxide agreement pledging 30 percent emissions reductions. It said that would be unfair since U.S. emissions have fallen by a much larger amount than those in European countries.