Chemical and size effects of hygroscopic aerosols on light scattering coefficients

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 scattering by hygroscopic aerosols. The following aerosol systems are considered: NH 4 HSO 4 , (NH 4 ) 2 SO 4 , (NH 4 ) 3 H(SO 4 ), NaHSO 4 , Na 2 SO 4 , NH 4 NO 3 , and NaNO 3 . In addition, H 2 SO 4 and NaCl are included to represent freshly formed sulfate and background sea‐salt aerosols, respectively. Scattering coefficients, based on 1 μg dry salt per cubic meter of air, are calculated as a function of relative humidity for aerosols of various chemical compositions and lognormal size distributions. For a given size distribution the light scattered by aerosol particles per unit dry‐salt mass concentration is only weakly dependent on chemical constituents of the hygroscopic sulfate and nitrate aerosols. Sulfuric acid and sodium chloride aerosols, however, are exceptions and scatter light more efficiently than all other inorganic salt aerosols considered in this study. Both internal and external mixtures exhibit similar light‐scattering properties. Thus for common sulfate and nitrate aerosols, since the chemical effect is outweighed by the size effect, it follows that observed light scattering by the ambient aerosol can be approximated, within practical measurement uncertainties, by assuming the aerosol being an external mixture. This has a definite advantage for either visibility degradation or climatic impact modeling calculations, because relevant data are now available for external mixtures but only very scarce for internal mixtures.