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

摘要

Climate change assessment, especially model evaluation, requires a betterunderstanding of complex refractive indices (CRIs) of atmospheric aerosols –separately for both fine and coarse modes. However, the widely used aerosolCRI obtained by the global Aerosol Robotic Network (AERONET) corresponds tototal-column aerosol particles without separation for fine and coarse modes.This paper establishes a method to separate CRIs of fine and coarse particlesbased on AERONET volume particle size distribution (VPSD), aerosol opticaldepth (AOD) and absorbing AOD (AAOD). The method consists of two steps. Firsta multimodal log-normal distribution that best approximates the AERONET VPSDis found. Then the fine and coarse mode CRIs are found by iterative fittingof AERONET AODs to Mie calculations. The numerical experiment shows goodperformance for typical water-soluble, biomass burning and dust aerosoltypes, and the estimated uncertainties on the retrieved sub-mode CRIs areabout 0.11 (real part) and 78 % (imaginary part). The 1-year measurementsat the AERONET Beijing site are processed, and we obtain CRIs of1.48–0.010 (imaginary part at 440 nm is 0.012) for fine mode particlesand 1.49–0.004 (imaginary part at 440 nm is 0.007) for coarse modeparticles, for the period of 2014–2015. Our results also suggest that bothfine and coarse aerosol mode CRIs have distinct seasonal characteristics; inparticular, CRIs of fine particles in winter season are significantly higherthan summer due to possible anthropogenic influences.