Influence of soot mixing state on aerosol light absorption and single scattering albedo during air mass aging at a polluted regional site in northeastern China

An aerosol optical closure study was performed using the observed high time- and size-resolved soot mixing states determined by a Volatility Tandem Differential Mobility Analyzer (VTDMA) at a polluted regional site, Yufa, in the south of Beijing during the summer of 2006. Good agreement was found between the simulated and measured aerosol absorption (sigma(ap), R = 0.9) and scattering (sigma(sp), R >= 0.95). The soot mixing state at Yufa can be generally determined by VTDMA, in terms of properly predicting the sigma(ap) using a simple optical model combined with spherical homogeneous and core-shell coated Mie codes. The possible uncertainties in the modeled sap were discussed. Rapid soot aging was observed, which led to large variations in the fractional contributions tows sigma(ap) by externally mixed and coated soot. On average, about 37% of the sigma(ap) (similar to 10-60%) arose by the coated soot. The coating enhancement in sigma(ap) and sigma(sp) of the coated soot can reach up to a factor of 8-10 within several hours owing to the secondary processing during daytime. It was contributed not only by the increased thickness of coating shell, but also the transition of soot from externally mixed to coated one. Hence, assuming constant soot mixing state for the regional climate model is not realistic and may lead to uncertainties. In the highly polluted region in northeastern China, the aerosol single scattering albedo may increase very fast owing to the rapid secondary particle formation and condensation (up to 0.90-0.95). This increase took place although the concurrent coating processing enhanced the light absorption capability of soot.