Absorption and scattering by particles can be simultaneously important, dependent upon the physical-chemical makeup of the particle. The measure of the relative importance of scattering to absorption is the single scatter albedo, S70, defined as:
For purely scattering particles, S70 = 1, and for purely absorbing particles, sj0 = 0. Neither extreme will be encountered in the ambient atmosphere where values generally range from 0.55 to 0.95. The single scatter albedo (®0) is related to the amount of absorption that is occurring, and is dependent upon particle size, chemical composition, and relative humidity (Kiehl and Rodhe, 1995). Reported values of co0 have been primarily restricted to visible and infrared wavelength regions because of the significance to climate change (Hansen et al., 1979; Charlock et al., 1993; Pueschel, 1993). For the UV-B wavelength region, ®0 is believed to be close to 1.0 for sulfate and marine aerosols and range from 0.5 to 0.7 for desert dust and soot aerosols (Lenoble, 1993; Lacis and Mishchenko, 1995). Madronich (1993) reports a typical ®0 value at UV wavelengths of 0.8, with a dependence on the levels of chemical impurities in the particles. The amount of moisture available for growth regulates the size of hygroscopic aerosols which in turn affects the scattering efficiency of the aerosol (Charlson et al., 1984). For relative humidities greater than 70%, Waggoner et al. (1981) observed strongly increasing aerosol scattering. When water surrounds an aerosol particle, the particle will not absorb as much, particularly at UV wavelengths since water does not absorb strongly in the UV. Wenny et al. (1998) also found that relative humidity is a controlling factor in the resulting value of co0. A larger database encompassing all environmental conditions experienced throughout a year is needed to discern if any seasonal trend exists in co0. Based on a short-term study across the Midwest, Ogren and Sheridan (1996) concluded that the dry aerosol co, for broadband visible radiation is relatively constant in the troposphere both vertically and horizontally, across the U.S. The evidence presented by Wenny et al. (1998) indicates that typically moist climates (such as in the Southeast or Northwest of the U.S) should display larger variations of co0 than arid climates (such as in the Midwest and Southwest of the U.S). The magnitude of co0 for the dryer atmospheric conditions implies that a fairly substantial portion of aerosol attenuation in the UV is due to absorption. Relative humidities are generally high in the eastern U.S. (typically > 80%), so it is probable that comparatively lower ®0 values exist in arid climates and will contribute a significant absorption component to aerosol attenuation of UV-B.
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