Regional haze and visibility impairment results from particles and gases in the atmosphere scattering and absorbing light.¹ The primary pollutants that affect visibility throughout much of the United States are sulfates and nitrates.² This Dialogue addresses the statutory commands, regulatory programs, and other forces that are likely to drive future regulation of industry in this evolving, highly complex area.

In looking at the regulatory requirements and the goals of the visibility and regional haze program, it is important to remember that the effect of the same amount of pollution on visibility will differ geographically, depending on the sensitivity and pollution levels in those respective areas.³ A contribution of x-amount of pollutants in a clean area will have a much greater impact than the same amount in a dirtier area.⁴ The regional haze problem is truly regional, with atmospheric conditions and other factors separating the eastern and western United States. For example, on average, the trend between 1988 and 1997 was that the worst day in the West was only slightly worse than the best day in the East.⁵ During that same time period, the West realized "steady visibility improvement," with total light extinction decreasing between 11% and 14%.⁶ Additionally, it is important to note that while sulfate is responsible for almost two-thirds of the visibility problems in the East, the contributors in the West are more varied.⁷ Sulfate accounts for about one-third of visibility impairment in the West, while organic carbon, crustal materials, and nitrates play a much more significant role in western regional haze than in the East.⁸