Apportioning light extinction coefficients to chemical species in atmospheric aerosol

Abstract

A 1978 Denver aerosol data set has been analyzed by a variety of methods for determining light scattering and extinction coefficients per unit mass concentration for specific chemical species in fine particles (< 2.5 μm diameter). Multiple regression of light scattering coefficient versus chemical species concentrations was used to determine scattering to mass concentration ratios and their related uncertainties for specific chemical species, and the results are compared with those reported by previous investigators. Differences in the scattering to mass concentration ratios for all the major species in the aerosol except ammonium sulfate were found to be statistically insignificant. Light scattering and absorption coefficients were apportioned by chemical species for an external mixture using a published equation and for an internal mixture using equations that we derived. It was found that while the type of mixture greatly affected the contribution of elemental carbon to light scattering, it had relatively little effect on the contributions of non-absorbing species. Light absorption coefficients computed for external and internal mixtures were found to be highly dependent on the type of mixture. For light extinction coefficients, the contributions computed for chemical species were found to be nearly independent of mixture type. The extinction coefficients apportioned to specific chemical species using various models agreed with each other within a factor of two.