“…The imaginary part of the refractive index of pure (NH4)2SO 4 is •< 10 -7 in the visible [Toon et al, 1976], and the soluble fraction of the atmospheric aerosol is often dominated It is also likely that some, if not all, of the absorption reported above for soluble aerosol is spurious, an artifact of absorption measurements on scattering media, and some of the work cited above has been criticized for such an oversight [Bergstrom, 1973;Egan and Hilgeman, 1979]. Accordingly, we will treat the soluble material in the composite particles as nonabsorbing sulfate compounds.…”