Long-term Trends in Reconstructed Haze at Remote Sites across the United States (1990 – 2018).

Abstract:

The enactment of the Clean Air Act amendments of 1990 resulted in reduced anthropogenic emissions of the precursors to particulate haze, such as sulfur dioxide (SO₂) and nitrogen oxides (NO_x). 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. Light extinction (b_ext) was derived from speciated particulate concentrations measured by the IMPROVE network at nearly 150 sites following the Environmental Protection Agency’s Regional Haze Rule guidelines. Trend analyses of the annual mean b_ext indicate strong reductions in haze associated with reduced SO₂ and NO_x emissions, especially in the eastern United States where the b_ext values historically were dominated by sulfate particles. Less improvement in visibility conditions occurred in the West, where sulfate contributions were lower. The widespread improvement in visibility across the United States, especially in the East, demonstrates the success of combined regulatory activities that have reduced anthropogenic emissions over the last three decades of the Clean Air Act Amendments. These results also point to the importance of long-term networks in providing data used to assess the impacts of emission reductions on particulate haze. As emissions from regulated sources continue to decline, the contributions to haze from unregulated sources, such as biomass burning, dust, oil and gas extraction, and international sources, are likely to increase. Reducing haze from these sources will require additional mitigation strategies and resource management plans.