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Visibility: Science and Regulation

INTRODUCTION The following is a discussion of the 2002 A&WMA Critical Review,1,2 which used the recent Regional Haze Rule3 as a context for examining poor atmospheric visibility, the history and physics of visibility issues, and the causes of today’s visibility problems. The Regional Haze Rule (henceforth termed “the Rule”) represents a 65-year effort to make progress toward natural visibility conditions at 156 U.S. national parks and wilderness areas in response to a provision in the Clean Air Act (CAA).4 Mandatory federal Class I areas are located in 36 states, most west of the Mississippi River, and have been determined by their federal land managers (FLMs) to have air quality-related values (AQRVs) that include visibility. Summarizing measurements from the Interagency Monitoring of Protected Visual Environments (IMPROVE) particle monitoring network, the review shows that light extinction (bext), the quantity of light removed from a sight path by gas and particle scattering and absorption, is highest in the eastern United States, where ammonium sulfate [(NH4)2SO4] concentrations and relative humidity (RH) are high. Along with sulfate (SO4 2–), nitrate (NO3 –), ammonium (NH4 +), organic carbon (OC), black carbon (BC, also referred to as elemental carbon, EC, or light-absorbing carbon), and dust are the major particulate matter (PM) contributors to bext. Although nitrogen dioxide (NO2) also absorbs light, its concentrations are low along sight paths distant from fresh emissions. In addition to light scattering by air molecules, natural sources of these materials also contribute to scattering and absorption. Most visibility-reducing materials result from man-made emissions, however; and these emissions are subject to management. Visibility: Science and Regulation