A statistical analysis of visibility-impairing particles in federal Class I areas

It is widely agreed that visibility conditions in many Class I areas are impaired to some extent. This paper provides an estimate of the degree of impairment in many of the Class I areas with respect to each of the six haze-forming aerosol classes as described in the supporting documents to the U.S. Environmental Protection Agency Regional Haze Rule. Analyses are performed comparing data from the Interagency Monitoring of Protected Visual Environments network to a modified version of the default natural conditions estimates from the Regional Haze Rule. Uncertainties in the measured annual mean concentrations and biases in the default natural condition estimates because of the effects of geography and meteorology are discussed. It is determined that all of the Class I areas in the contiguous 48 United States are significantly impaired with respect to sulfate aerosols, most of the Class I areas are significantly impaired with respect to nitrate and elemental carbon aerosols, and impairment with respect to organic mass, soil mass, and coarse mass is generally less discernable. No attempt is made to determine adverse impacts with respect to any specific source or group of sources.