The relative importance of atmospheric sulfates and nitrates in visibility reduction

Abstract Atmospheric visibility is largely determined by the concentration, size distribution and chemical composition of the ambient aerosol. In the present paper, total extinction coefficients were calculated for mixed NH 4 NO 3 -(NH 4 ) 2 SO 4 aerosols of two different compositions under various humidity conditions. Pertinent thermodynamic and optical properties were measured at 25° C and incorporated into a computer model for droplet growth and Mie calculations. Log-normal particle size distributions with mass median diameters characteristic of the Los Angeles aerosol were assumed throughout the study. The results show that the extent of light extinction by aerosol particles depends on whether the aerosol is present as a multicomponent solution droplets of identical composition (internal mixture) or as a mixture of single-salt aerosols (external mixture|. The extinction coefficients calculated for externally mixed sulfate — nitrate aerosols compare remarkably well with published statistical correlations derived from the Los Angeles visibility data. In addition, it is shown that, for externally mixed sulfate-nitrate aerosols, the sulfate indeed contributes overwhelmingly to visibility reduction, a finding in agreement with general observations for atmospheric aerosols.