THE RELATIVE IMPORTANCE OF SOLUBLE AEROSOLS TO SPATIAL AND SEASONAL TRENDS OF IMPAIRED VISIBILITY IN THE UNITED-STATES

In the U.S.A., soluble aerosol species are recognized as major contrib utors 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 visibi lity 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 con sider the combined distribution can lead to serious underestimates of the average visibility impairment caused by soluble aerosols. The comb ined distribution of aerosols and relative humidity has been examined at 20 rural sites in the U.S.A. where aerosol and relative humidity mo nitors were operated concurrently. By doing so, empirical relations be tween the average relative humidity and average visibility impairment caused by soluble aerosols were derived. These relations are applied t o 16 additional aerosol monitoring sites where reliable estimates of a nnual and seasonal average relative humidities are available, but conc urrent relative humidity was not monitored. Thus, spatial and seasonal trends of visibility impairment caused by aerosols and an apportionme nt to individual species at 36 sites across the U.S.A., including Alas ka and Hawaii, are presented.