Understanding divergent brown carbon photobleaching rates from molecular perspective

Abstract. The global radiative effect of brown carbon (BrC) remains highly uncertain. BrC's photobleaching, which significantly affects its radiative effect, has been still poorly constrained. This study investigates and compares photobleaching rates of laboratory-synthesized secondary BrC (aq-BrC), biomass burning-derived BrC (b-BrC), and ambient PM2.5-derived BrC (p-BrC) to illustrate the variability in BrC photobleaching kinetics from different types of BrC reported in prior studies. Our results reveal a source dependence in BrC photobleaching rates. The highest photobleaching rate constant (kBrC) is observed for aq-BrC (1.13 ± 0.08 h−1), followed by p-BrC (0.12 ± 0.02 h−1) and b-BrC (0.05 ± 0.01 h−1), indicating the stable light-absorption capacity of b-BrC in the atmosphere. Integration of these rate constants with data derived from previous studies highlighted the differences in photobleaching behaviors among BrC from different sources. The OH oxidation of imidazole-2-carboxaldehyde (2-IC) and methylglyoxal oligomers, nitrophenols (including phenols), and lignin derivatives governs the photobleaching of aq-BrC, p-BrC, and b-BrC, respectively. The high kBrC of aq-BrC is attributed to the high reactivity of the chain structures in 2-IC and methylglyoxal oligomers. In contrast, the highly conjugated structures of lignin derivatives in b-BrC impart stability against OH oxidation, resulting in a low kBrC. Our findings reveal the significant differences in the photobleaching behavior of BrC originated from different sources, underscoring the crucial need to account for source differences in assessments of BrC's global radiative forcing effect.