Thirty years of the Clean Air Act Amendments: Impacts on haze in remote regions of the United States (1990-2018)

The Clean Air Act Amendments of 1990 were aimed at reducing major environmental threats in the United States, such as acid rain, urban air pollution, toxic air emissions, and regional haze. To this end, there have been major reductions in anthropogenic gaseous emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx). SO2 and NOx are also major contributors to particulate haze, which affects visibility in both urban and rural environments. The Interagency Monitoring of Protected Visual Environments (IMPROVE) network has tracked trends in haze in remote regions of the United States since the late 1980s by reconstructing total light extinction (b(ext)) from speciated particulate concentrations. Regional trends in b(ext) were examined using aggregated data from individual sites. Regional, annual mean short-term trends analyses (2002-2018) indicated strong reductions in b(ext) associated with reduced SO2 and NOx emissions, especially in the eastern United States (-4.3% yr(-1), p < 0.001) where haze historically was dominated by sulfate particles. Less improvement occurred in the Intermountain West/Southwest (-0.9% yr(-1), p = 0.03) and trends were negative but insignificant along the West Coast (-1.5% yr(-1), p = 0.19). On average, across the continental remote United States, b(ext) has decreased at a rate of -2.8% yr(-1) (p < 0.001) from 2002 through 2018 and -1.8% yr(-1) (p < 0.001) from 1992 through 2018. The composition of haze has shifted away from being sulfate-dominated to having higher contributions from carbonaceous and crustal aerosols. This shift points to the success of combined regulatory activities aimed at reducing anthropogenic emissions over the last three decades. As emissions from regulated sources of SO2 and NOx continue to decline, the contributions to haze from unregulated sources, both anthropogenic and natural, such as oil and gas extraction, international sources, biomass burning, and dust, have increased in importance. Reducing haze from these sources would require additional mitigation strategies and resource management plans.