The improvement is due to fewer emissions of nitrogen oxides and volatile organic compounds, which together undergo a chemical reaction in heat and sunlight that forms smog. Some of the major sources are motor vehicle exhaust, industrial plants, gas vapors and chemical solvents.
EPA said that the amount of nitrogen oxides emitted from power plants and other industrial sources in the 19 states had fallen to 593,000 tons in 2004, a nearly 50 percent drop from the 1.2 million tons emitted in 2000. That reduced summer ozone over the four years by 10 percent, EPA said.
EPA Administrator Stephen Johnson said that has improved air quality for more than 100 million people. He called it "yet another example of how market-based trading programs are significantly reducing emissions of air pollutants."
EPA's approach sets caps on allowable levels of pollution, then lets companies trade among themselves any unused amounts.
Jeff Holmstead, EPA's outgoing top official for air quality, called it "a very significant reduction of ozone" concentrations. Holmstead, whose last day at the agency was Friday, attributed most of the improvement to technology improvements that the EPA ordered at coal-burning power plants.
The Clinton administration in 1998 ordered ground-level ozone in the East cleaned up. That EPA regulation affected the District of Columbia and 19 states: Alabama, Connecticut, Delaware, Illinois, Indiana, Kentucky, Massachusetts, Maryland, Michigan, North Carolina, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee, Virginia and West Virginia.
EPA officials said further improvements should result from a new program the agency announced in March requiring states in the East, South and Midwest, plus the District of Columbia, to reduce power plant pollutants that form smog and soot and drift downwind.
States have until September 2006 to submit plans for achieving the pollution reductions. If they miss that deadline, the EPA has said that it would write the plans for them.
Under those regulations, by 2015, nitrogen oxides pollution in 30 states will have to be reduced by 1.9 million tons annually, or 61 percent below 2003 levels, EPA said. Sulfur dioxide pollution must drop by 5.4 million tons, a 57 percent reduction, the agency said.
By John Heilprin