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 λ = 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  = NO 2  + O 3 , a proxy for atmospheric photochemical aging) was observed. A three-stage absorption enhancement process for collapsed semispherical to fully compact spherical morphology BC with photochemical aging was suggested. For O x below 35 ppbv, E abs increased slowly with O x mixing ratio and ranged from 2.0 to 2.2 (with a growth rate of ~ 0.03 ppbv −1 ). Eabs was stable (E abs  = 2.26 ± 0.06) between 35 to 50 ppbv O x . Thirdly, for O x levels above 50 ppbv, Eabs grew rapidly from 2.3 to 2.8 (at a growth rate of ~ 0.18 ppbv −1 ). 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 Eabs and SSA captures much of the influence of BC coating and the particle absorption, and provides a plausible new method to better constrain the contribution of BC to the DRF. In our measurements at this site, the absorption amplification depended mainly on the coating thickness and the absorption of coating materials. The lensing driven enhancement was reduced by light absorption of the shell. Our observations highlight the crucial role of photochemical processes in modifying the absorption of BC-containing particles. One implication of these findings is that the contribution of light-absorbing organic compounds (Brown carbon, BrC) at longer aging time should be included in climate models.