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 alerts its radiative effect, has been still poorly constrained. This study investigates photobleaching rates of laboratory-synthesized secondary BrC (aq-BrC), biomass burning-derived BrC (b-BrC), and ambient PM_2.5-derived BrC (p-BrC). Our results reveal a source dependence in BrC photobleaching rates. The highest photobleaching rate constant (k_BrC) 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. 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 k_BrC 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 k_BrC. 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.