Estimation of aerosol complex refractive indices for both fine and coarse modes simultaneously based on AERONET remote sensing products

摘要

Abstract. Climate change assessment, especially model evaluation, needs to know complex refractive indices (CRI) of atmospheric aerosols, separately for both fine and coarse modes. However, the widely used aerosol CRI obtained by the global Aerosol Robotic Network (AERONET), correspond to total-column aerosol particles without separation for fine and coarse modes. This paper establishes a method to separate CRIs of fine and coarse particles based on AERONET aerosol products, including volume particle size distribution (VPSD), aerosol optical depth (AOD) and absorbing AOD. The method consists of VPSD breaking-down and sub-mode CRI separation parts and yields spectral CRIs for both fine and coarse modes simultaneously. Numerical experiment shows good performance for typical water-soluble, biomass burning and dust aerosol types and the estimated uncertainties on the retrieved sub-mode CRIs are about 0.11 (real part) and 78 % (imaginary part), respectively. One year measurements at AERONET Beijing site are processed and we obtain CRIs of 1.48-0.010i (imaginary part at 440 nm is 0.012) for fine mode particles and 1.49-0.004i (imaginary part at 440 nm is 0.007) for coarse mode particles, for the period of 2014–2015. Our results also suggest that both aerosol fine and coarse mode CRIs have distinct seasonal characteristics, particularly CRIs of fine particles in winter season are significantly higher than summer, due to possible anthropogenic influences.