The influence of photochemical aging on light absorption of atmospheric black carbon and aerosol single-scattering albedo

Coating enhancement of black carbon (BC) light absorption (E-abs) is a large uncertainty in modelling direct radiative forcing (DRF) by BC. Reported E-abs values after atmospheric aging vary widely and the mechanisms responsible for enhancing BC absorption remain elusive. Here, we report on the direct field measurement of size-resolved mixing state, E-abs, and aerosol single-scattering albedo (SSA) at lambda = 532 nm at a rural site in east China from June to July 2016. Strong diurnal variability of E-abs, SSA, and O-x (O-x = NO2 + O-3, a proxy for atmospheric photochemical aging) was observed. A method that combined E-abs and SSA was developed to retrieve the fraction contribution of BC absorption (f(BC)), lensing-driven enhancement (f(Lens)), as well as the fractional contribution of coating absorption (fraction absorption contribution (f(Shell)), the coated shell diameter (D-Shell) and the imaginary part of the complex refractive index (CRI) of the shell (k(Shell))). Parameterization of E-abs and SSA captures much of the influence of BC coating and the particle absorption. In our measurements at this site, the results showed that the absorption amplification depended on the coating thickness and the absorption of coating materials, and photochemistry plays a role in modifying the absorption of BC-containing particles. The lensing-driven enhancement was reduced by light absorption of the shell. One implication of these findings is that the contribution of light-absorbing organic compounds (brown carbon, BrC) at a longer aging time should be included in climate models.