Bare black carbon (BC) can be coated by other nonabsorbing components, inducing light absorption enhancement (Eabs) via the so-called "lensing effect." The coating components/processes, however, are complex in terms of both chemical composition and microphysical properties; therefore, Eabs is highly uncertain and inconsistent among different observations. Particle heterogeneity in composition is critical to the accurate estimation of Eabs; here, we quantified the impacts of various coating components on Eabs880 (Eabs at 880 nm, thus no impact from light-absorbing organics) by using observations from three megacities in China (Beijing, Nanjing, and Shanghai). Under all three scenarios with contrastingly different atmospheric conditions investigated here, low-volatility/highly oxygenated secondary organic aerosol (SOA) species and sulfate were consistently important contributors to Eabs880, while OA from both biomass burning and traffic appeared to be insignificant. The effects of semivolatile/less-oxygenated SOA species and nitrate varied largely, being highly dependent upon the meteorology and location. Specially, an industry-related OA resolved in Nanjing exhibited a notable contribution to Eabs880 thus its chemical and physical characteristics warrant future attention. Overall, our findings regarding the roles of specific sources in Eabs880 provide a direct and practically feasible guidance to effectively reduce BC pollution and its positive climate forcing.
Cui et al. (Thu,) studied this question.