JANUARY 2006: Regional visibility statistics in the United States: Natural and transboundary pollution influences, and implications for the Regional Haze Rule

The figure shows an endpoint of visibility conditions in wilderness areas either from U.S. EPA recommended default approach or from our model simulations. The EPA default values show mainly a separation between west and east. Our natural visibility endpoint estimates, however, show considerable site by site variation. The highest values are in the northwest due to fires. Transboundary pollution influences result in a higher background visibility endpoint, showing large contributions from Canada and Mexico. Recognizing the influence of uncontrollable transboundary pollution in the Regional Haze Rule would substantially decrease the schedule of emission reductions required in the 2004-2018 implementation phase. Meaningful application of the Rule in the future will require projections of future trends in foreign anthropogenic emissions, wildfire frequency, and climate variables. A full description is given in Park et al. [2005].