CHARACTERISTICS AND ORIGINS OF HAZE IN THE CONTINENTAL UNITED-STATES

Estimates of visibility under natural conditions, spatial and temporal trends in atmospheric aerosol, and extinction are examined at a national level. It is found that the best visibility is in the inner mountain west, while the poorest visibility is east of the Mississippi River and in the urban areas of California. Aerosol concentrations' spatial trends generally follow this East-West trend, and fine mass concentrations are about a factor of four greater in the East than West, with sulfates and organics being the largest constituents. Sulfates are the largest single contributor to visibility degradation, with nitrates and organics contributing significantly in some regions of the country. In both the East and the West, historical changes in visibility show a high degree of correlation with increases in sulfur dioxide emissions. Source receptor modeling shows that in the Southwest, southern California, copper smelters in southern Arizona, and large sulfur dioxide sources along the Gulf Coast of Mexico contribute to elevated sulfate levels. A coal fired power plant and wood processing facilities contribute to elevated sulfate in the Northwest, while a nickel smelter in Canada and coal fired power plants in the New York and Philadelphia area are the cause of high sulfate concentrations in the Northeast. Coal fired power plants along the Ohio River Valley are one of the single largest contributors to elevated sulfate in both Shenandoah and Great Smoky Mountains National Parks.