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.