Chemical Composition Based Mass Extinction Efficiency for Polydispersed Aerosol Particles

Among many physical and optical parameters that characterize the atmospheric aerosol properties an aerosol mass extinction efficiency is important for better understanding of aerosol optical properties. In general, the aerosol mass extinction efficiency is represented as functions of refractive index and size distribution. However, the mass extinction efficiency is often regarded as a size independent parameter while it is assumed to be depended only on chemical composition (i.e., refractive index) because of its complexity. One of the main reasons that the mass extinction efficiency is considered as a constant is due to its simplicity in formulas. In order to integrate polydispersed effects of mass extinction efficiencies of aerosol particles, size dependent extinction of individual aerosol particle should be calculated every time, which makes applications to be restricted because of diversity of problems (e.g., different sizes and compositions). Thus, it is required to make simple but accurate parameterizations of mass extinction efficiencies of aerosols while considering polydispersity based on their compositions.

In this study, chemical composition based mass extinction efficiencies of polydispersed aerosol particles were calculated analytically and then parameterizations were developed. The polydispersity of lognormal size distribution and external mixture were assumed with size ranges of 0.3-2.5 μm in a geometric mean diameter. The analytic solution of the mass extinction efficiency of each chemical composition was computed by fitting the numerical solution of Mie theory. Based on these calculations, the optical properties of each chemical species were determined as functions of refractive index and size. Finally, the parameters of fitting curves are generalized for the polydispersed aerosols as functions of geometric mean diameter and geometric standard deviation. The developed parameterizations were compared with the original solutions (i.e., numerical solutions), which showed good agreement. The developed parameterizations will provide a conventional tool for estimating the mass extinction efficiency for polydispersed multi-composition aerosol particles.