"We really want to have clean air at national parks all the time, and not just at average times," Shepherd said in a telephone interview. "All of our national parks have impaired visibility. . . . It would really be a setback in trying to make progress."
While the government has made progress in reducing haze-producing sulfur dioxide and nitrogen oxide pollution in recent decades, many of the nation's best-known parks still have poor visibility and air quality.
In October, the Park Service published a 10-year analysis of air quality trends that found that sulfate concentrations in precipitation have declined on the East Coast because of the federal acid rain program, but that Western parks have not experienced similar reductions. The concentrations of ozone smog over an eight-hour period are worsening across almost all of the interior West, including "some of the most remote places in the nation," said Vicki Patton, deputy general counsel for the Environmental Defense Fund.