Light Absorption Enhancement of Black Carbon Aerosols Constrained by Chemical Components and Sources in Urban Air: A...

Bare black carbon (BC) can be coated by other nonabsorbing components, inducing light absorption enhancement (E_abs) via the so-called "lensing effect." The coating components/processes, however, are complex in terms of both chemical composition and microphysical properties; therefore, E_abs is highly uncertain and inconsistent among different observations. Particle heterogeneity in composition is critical to the accurate estimation of E_abs; here, we quantified the impacts of various coating components on $E_{\mathrm{abs}}^{880}$ (E_abs 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 $E_{\mathrm{abs}}^{880}$, 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 $E_{\mathrm{abs}}^{880}$ thus its chemical and physical characteristics warrant future attention. Overall, our findings regarding the roles of specific sources in $E_{\mathrm{abs}}^{880}$ provide a direct and practically feasible guidance to effectively reduce BC pollution and its positive climate forcing.