National Park Service data show that sulfate particle concentrations are about six times greater in the rural East than in the rural West; elemental carbon and organic particle concentrations are about twice as great in the rural East as in the rural West; and concentrations of fine soil dust and nitrates are about the same in both regions. In the East, airport data and NPS camera data reveal a strong summertime visibility minimum that is strongly linked to a summertime maximum in sulfate concentrations.

Airport data from the late 1940s through the early 1980s show coherent visibility trends over large regions. In the Northeast, visibility has moderately improved during the winter and moderately decreased during the summer. Visibility in the Southeast has worsened moderately during the winter and substantially during the summer. The greatest increases occurred during the 1950s and 1960s. It is likely that changes in SO2 emissions are largely responsible for these changes in visibility.

The average natural background visual range varies from about 150 km in the eastern United States to 230 km in the arid West. The major contributors to natural extinction are Rayleigh scattering, organics, water, and suspended dust. The main constituents of anthropogenic haze are sulfates, organics, elemental carbon, soil dust, nitrates, and water. The principal anthropogenic haze-causing emissions are sulfur dioxide (SO2, a precursor of sulfate particles), primary organic particles, gaseous volatile organic compounds (VOCs, precursors of secondary organic particles), primary elemental carbon, primary crustal material, ammonia (NH3, a precursor of ammonium nitrate), and nitrogen oxides (NOx, precursors of nitrate particles and NO2). Except for VOCs and primary organics, the anthropogenic emission inventories for these species are dominated by relatively few source categories.



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