Widespread reductions in haze across the United States from the early 1990s through 2011

Visibility has improved significantly at many remote areas across the United States since the early 1990s. Trends in visibility were calculated using ambient light extinction coefficients (bext) estimated from speciated particulate concentrations measured by the IMPROVE (Interagency Monitoring of Protected Visual Environments) network. The 20% haziest bext levels were computed for each year following Regional Haze Rule guidelines and aggregated over three major regions of the United States. Over the last two decades (1992–2011) the regional mean 20% haziest bext dropped by 52% (−2.6% yr−1, p < 0.01) in the eastern United States, and by 20% (−1.0% yr−1, p = 0.08) for sites along the West Coast. However, in the Intermountain/Southwest region, the trend was insignificant (−0.2% yr−1, p = 0.36). Over the last decade (2002–2011) the haziest bext in the Intermountain/Southwest region decreased by 15% (−1.5% yr−1, p = 0.09), compared to a decrease of 35% (−3.5% yr−1, p = 0.06) in the West Coast region and 50% (−5.0% yr−1, p = 0.02) in the East. A novel aspect to this study is the visualization of trends through the simulation of images of national parks and other remote areas for early and current haziest conditions. These images are an effective means for communicating trends and illustrate the dramatic improvement in visibility, especially in the East, where reductions in sulfate concentrations and sulfur dioxide emissions have had a positive impact on visibility degradation. However, while conditions are clearer for regions in the West, less improvement points to the need for understanding the influences on the trends in haziest conditions in those regions.