CHEMICAL AND SIZE EFFECTS OF HYGROSCOPIC AEROSOLS ON LIGHT-SCATTERINGCOEFFICIENTS

The extensive thermodynamic and optical properties recently reported [ Tang and Munkelwitz, 1994a] for sulfate and nitrate solution droplets are incorporated into a visibility model for computing light scatterin g by hygroscopic aerosols. The following aerosol systems are considere d: NH4HSO4, (NH4)(2)SO4, (NH4)(3)H(SO4), NaHSO4, Na2SO4, NH4NO3, and N aNO3. In addition, H2SO4 and NaCl are included to represent freshly fo rmed sulfate and background sea-salt aerosols, respectively. Scatterin g coefficients, based on 1 mu g dry salt per cubic meter of air, are c alculated as a function of relative humidity for aerosols of various c hemical compositions and lognormal size distributions. For a given siz e distribution the light scattered by aerosol particles per unit dry-s alt mass concentration is only weakly dependent on chemical constituen ts of the hygroscopic sulfate and nitrate aerosols. Sulfuric acid and sodium chloride aerosols, however, are exceptions and scatter light mo re efficiently than all other inorganic salt aerosols considered in th is study. Both internal and external mixtures exhibit similar light-sc attering properties. Thus for common sulfate and nitrate aerosols, sin ce the chemical effect is outweighed by the size effect, it follows th at observed light scattering by the ambient aerosol can be approximate d, within practical measurement uncertainties, by assuming the aerosol being an external mixture. This has a definite advantage for either v isibility degradation or climatic impact modeling calculations, becaus e relevant data are now available for external mixtures but only very scarce for internal mixtures.