Predictions of factors affecting extinction efficiencies of internally mixed aerosols using the Reist-Wilson-MARS program

A program that combines the MARS code with a code for calculating optical properties of aerosols is used to predict scattering and absorbing efficiencies of secondary aerosols as a function of chemical composition, relative humidity, temperature, and size distribution parameters of the aerosols. The predictions are compared with empirical data from the literature. Scattering and absorbing efficiencies are mixture-specific and not species-specific; thus they can be used for determining visibility budgets only in a specific geographical location and under specific meteorological conditions. Estimation of scattering and absorbing efficiencies obtained from regression analysis and deterministic methods do not differ significantly. The relative abundances of sulfates and nitrates do not affect scattering efficiencies much. The presence of an absorbing species causes even non-absorbing species such as sulfates and nitrates to have an absorption efficiency. The real part of the refractive index does not affect the absorption efficiency; however, the imaginary part of the refractive index does affect the scattering efficiency of the bulk aerosol.