Source contributions to visibility impairment in the southeastern and western United States.

The 1999 Regional Haze Rule requires states to complete comprehensive technical analyses of air pollutants that impair visibility and to define long-term strategies to improve visibility in the nation's 156 visibility-protected federal Class I national parks and wilderness areas. Class I areas in the southeastern United States are among the most impacted in the country; fine particle loadings in the western United States are a fraction of those in the East. In the Southeast, (NH4)2SO4 (ammonium sulfate) predominantly from SO2 (sulfur dioxide) emissions from electric generating utilities and industrial sources contributes 60-70% of the light extinction on the 20% haziest days; particulate organic matter (POM) predominantly from biogenic emissions and biomass burning is the second largest contributor. In the West, the mix of source contributions is more complex. At Class I areas downwind of major urban areas (e.g., California), ammonium nitrate (NH4NO3), predominantly because of mobile sources, is the dominant contributor to haze. For many western Class L areas, POM from wildland fires and fine particles from windblown dust, largely uncontrollable sources, are significant contributors to haze. International emissions are an additional uncontrollable and significant contribution to total sulfate (SO4) and nitrate (NO3) concentrations at the western Class I areas. In the Southeast, SO2 emissions reductions are projected to result in nearly 1:1 regional SO4 reductions; oxides of nitrogen (NO(x)) emissions reductions have minimal impact on NO3 concentrations and haze. In the West, SO2 emissions reductions result in incremental SO4 reductions, whereas mobile NO(x) emissions reductions are projected to reduce NO3 and improve visibility at Class I areas affected by urban areas. Because wildfire, dust, and international emissions have large contributions to the haziest days and are mostly uncontrollable in the West, reductions from anthropogenic sources in the West have less effect in improving visibility compared with the Southeast.