The relative importance of soluble aerosols to spatial and seasonal trends of impaired visibility in the United States

Abstract

In the U.S.A., soluble aerosol species are recognized as major contributors to impaired visibility. Visibility impairment caused by soluble aerosols can be estimated using ambient aerosol and relative humidity data with physical optical models. Averaging over annual and seasonal time frames is necessary for an adequate description of average visibility conditions. When soluble aerosols are a large fraction of ambient aerosols, averaging over time requires consideration of the combined distribution of relative humidity and soluble aerosols. Failure to consider the combined distribution can lead to serious underestimates of the average visibility impairment caused by soluble aerosols. The combined distribution of aerosols and relative humidity has been examined at 20 rural sites in the U.S.A. where aerosol and relative humidity monitors were operated concurrently. By doing so, empirical relations between the average relative humidity and average visibility impairment caused by soluble aerosols were derived. These relations are applied to 16 additional aerosol monitoring sites where reliable estimates of annual and seasonal average relative humidities are available, but concurrent relative humidity was not monitored. Thus, spatial and seasonal trends of visibility impairment caused by aerosols and an apportionment to individual species at 36 sites across the U.S.A., including Alaska and Hawaii, are presented.