Approximated Expression of the Mass Extinction Efficiency for Polydispersed Aerosols

Aerosol mass extinction efficiency (MEE) is one of the most important parameters in understanding aerosol optical properties among many aerosol physical and optical parameters.

MEE should be expressed as a function of the refractive index and size distribution of aerosol particles. Theoretically, the physical relationship between light extinction and atmospheric particulate constituents can be established if both the particulate concentration and the size distribution of each chemical species are known. Under the given mass concentration, the resulting aerosol MEE is defined as the ratio of aerosol extinction coefficient to aerosol mass concentration in a unit volume of air.

However, MEE is often regarded as a constant parameter and assumed to be depend on chemical compositions of aerosol only. For example, one of the widely used expression is the reconstructured equation proposed by Malm et al. (1994), which express extinction coefficient as a function of aerosol mass concentration and MEE.

In this study, mass extinction, scattering and absorption efficiencies of polydispersed aerosol particles were estimated analytically. The entire size in the range of geometric mean diameter 0.01–2.5um were considered assuming lognormal size distribution for sulfate, nitrate and NaCl aerosol species. An analytical approach to the approximated formula for the MEE of each aerosol chemical composition was estimated by fitting the formula results to the results from Mie. Throughout this study, obtained results are compared with the results from the Mie-theory-based calculations and showed comparable results.