WASHINGTON, June 12— Saying ''we will make the 1990's the era for clean air,'' President Bush today proposed an array of costly measures to strengthen the nation's law for fighting air pollution.

In a speech at the White House, the President asked that the Clean Air Law of 1970 be amended to cut almost in half sulfur dioxide emissions from coal-burning power plants. Sulfur dioxide is a leading component of acid rain.Mr. Bush also recommended other efforts to combat urban smog and reduce emissions of toxic chemicals.

Mr. Bush's proposal marked a break with the Reagan Administration, which for eight years essentially opposed all efforts to impose antipollution requirements on industry. With Mr. Bush's support, the initiative is expected to break the deadlock in Congress that has blocked any change in the Clean Air Act since 1977. Economists' Argument Prevails

For the first time, the arguments of economists have apparently found a receptive audience at the White House. Mr. Bush's proposals would allow companies to buy and sell the right to pollute and thus let the market decide the cheapest way to contain smokestack emissions. The adoption of such a market-oriented approach would alter the thrust of antipollution efforts for decades to come.

If enacted as described by the President, the proposals could significantly lower threats to health and the environment now posed by air pollution.

They could also require heavy investments by industry, including electric utilities and the automotive and oil businesses. William K. Reilly, Administrator of the Environmental Protection Agency, estimated that the costs to industry of complying with the President's clean air program could reach $14 billion to $18 billion a year when it is in full effect.

Mr. Reilly, who urged the President to propose far-reaching measures, appears to have prevailed largely, although not entirely. His views were opposed by many Presidential advisers who cautioned that the costs would burden the economy.

''It won't be easy; some hard choices will have to be made,'' Mr. Reilly said at a briefing for reporters. But he added: ''Today is a historic moment for the environment. I believe we can look forward to the fulfilling the promise of the Clean Air Act.''

Mr. Bush outlined his proposals to members of Congress, state officials and environmentalists, who generally endorsed the President's goals as representing an important change from the previous Administration. Some, however, expressed doubt that the means proposed by the President were adequate to reach the goals.

While industry groups also spoke favorably of the President's initiative, there were some complaints, particularly from electric utilities and the oil industry, which said certain requirements were excessive or unnecessary. President's Proposals

Among the specific proposals offered by the President for cleaning the nation's air was a 10-million-ton reduction in emissions of sulfur dioxide from coal-burning electric power plants by the year 2000, somewhat less than half the amount currently emitted, and cutting emissions of nitrogen oxides by two million tons a year, or about a one-tenth reduction.

The oxides of sulfur and nitrogen change chemically as they move through the atmosphere and fall to earth as acidic rain, snow, fog or dust. Such pollution, much of which originates in the Midwest, harms life in lakes and streams and is suspected of damaging forests and injuring human health, particularly in the northeastern United States and in Canada.

Half the reductions would have to be accomplished by 1995 and the rest by 2000, under the proposal. Companies would be free to decide how to reach the goal. They could use scrubbers, special equipment on smokestacks that removes sulfur by a chemical reaction; they could burn coal with lower sulfur content; they could encourage their customers to conserve electricity, thus reducing the amount of coal they burn, or they could adopt new technology that allows them to burn coal in ways that do not emit pollution. Those who choose the new technology would have until 2003 to meet the final deadline to allow time for the equipment to be developed. Selling Pollution Rights

The first phase of required cuts would affect some 107 power plants in 18 states. As an incentive to reaching the goals, plants within the same state would be allowed to trade pollution rights; that is, if they exceeded the required reductions, they could sell rights to emit extra pollution to other companies or transfer rights to other plants within the same company.

Senator Robert C. Byrd, Democrat of West Virginia, said the President's proposal would ''decimate'' the companies that mine high-sulfur coal, which are largely in Eastern states like his, because power plants would take the easiest route to achieving the reductions and switch to low-sulfur coal, which comes mainly from mines in the West.

When he was Senate majority leader, Senator Byrd effectively opposed acid rain legislation for many years and he remains a powerful voice on issues that affect the coal industry. Urban Air Quality

The President proposed an extensive list of measures to deal with urban smog and carbon monoxide that threaten the health of more than 100 million Americans in some 100 cities.