The following maps display U.S. visibility trends derived from such data. The greater the haze, the poorer the visibility. The dark blue color represents the best visibility and red represents the worst visibility. Overall, these maps show that visibility impairment in the eastern U.S. increased greatly between 1970 and 1980, and decreased slightly in 1990. This follows the overall trends in SOx emissions during these periods.
Sulfur oxides are a major source of fine particles.
The effort has been expanded to incorporate other federal and regional monitoring programs. The network is the largest in the country devoted to fully characterizing visibility. Sulfates are the largest single contributor to haze, or visibility reduction, in many parts of the U.S. Data from this monitoring network reveals that sulfates account for 68 percent of the visibility reduction in the Appalachian Mountains in the East. Organic carbon, the next-largest contributor, causes 16 percent of visibility reduction. In most areas of the western U.S. and Alaska, sulfates and organic particles contribute equally to haze.