Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou

Black carbon (BC) aerosols have been widely recognized as a vital climate forcer in the atmosphere. Amplification of light absorption can occur due to coatings on BC during atmospheric aging, an effect that remains uncertain in accessing the radiative forcing of BC. Existing studies on the absorption enhancement factor (E-abs) have poor coverage on both seasonal and diurnal scales. In this study, we applied a recently developed minimum R squared (MRS) method, which can cover both seasonal and diurnal scales, for E-abs quantification. Using field measurement data in Guangzhou, the aims of this study are to explore (1) the temporal dynamics of BC optical properties at seasonal (wet season, 31 July-10 September; dry season, 15 November 2017-15 January 2018) and did scales (1 h time resolution) in the typical urban environment and (2) the influencing factors on E-abs temporal variability. Mass absorption efficiency at 520 nm by primary aerosols (MAE(p520)) determined by the MRS method exhibited a strong seasonality (8.6 m(2) g(-1) in the wet season and 16.8 m(2) g(-1) in the dry season). E-abs520 was higher in the wet season (1.51 +/- 0.50) and lower in the dry season (1.29 +/- 0.28). Absorption Angstrom exponent (AAE(470-660)) in the dry season (1.46 +/- 0.12) was higher than that in the wet season (1.37 +/- 0.10). Collective evidence showed that the active biomass burning (BB) in the dry season effectively altered the optical properties of BC, leading to elevated MAE, MAE(p) and AAE in the dry season compared to those in the wet season. Diurnal E(abs520 )was positively correlated with AAE470-660 (R-2 = 0.71) and negatively correlated with the AE33 aerosol loading compensation parameter (k) (R-2 = 0.74) in the wet season, but these correlations were significantly weaker in the dry season, which may be related to the impact of BB. This result suggests that during the wet season, the lensing effect was more likely dominating the AAE diurnal variability rather than the contribution from brown carbon (BrC). Secondary processing can affect Eabs diurnal dynamics The E-abs520 exhibited a clear dependency on the ratio of secondary organic carbon to organic carbon (SOC/OC), confirming the contribution of secondary organic aerosols to E-abs; Ea(bs520) correlated well with nitrate and showed a clear dependence on temperature. This new finding implies that gas-particle partitioning of semivolatile compounds may potentially play an important role in steering the diurnal fluctuation of E-abs520. In the dry season, the diurnal variability in E-abs520 was associated with photochemical aging as evidenced by the good correlation (R-2 =0.69) between oxidant concentrations (O-x = O-3 + NO2) and E-abs520.