Recent Progress in Impacts of Mixing State on Optical Properties of Black Carbon Aerosol

Abstract

Black carbon (BC) exerts profound impacts on air quality and climate because of its high-absorption cross section over a broad band of solar spectrum. Non-BC materials coated on BC could alter the mixing state of BC particles and can considerably enhance its mass absorption coefficient. Quantification of this absorption enhancement remains a challenge due to incomplete understanding of the complex physical and chemical properties related to mixing states. In this paper, we summarize the recent progress in measurement and modeling studies on the BC mixing state and their effects on optical properties. Laboratory and field-based observations have shown that the transformation of a mixing state from a highly fractal nature to a more compact shape exhibits a decrease in electric mobility diameter but an increase in fractal dimension and effective density. Meanwhile, the transition behavior is also obviously influenced by emission source which can determine the components of BC mixtures. Based on the empirically determined parameters, accurate numerical modeling shows great capability on calculating BC optical properties. However, considering the significant uncertainties related to BC microphysical properties, proper parameterization considering realistic BC aggregates and coating fraction can help to understand the progress from an externally to internally mixed state.