Co-photolysis of mixed chromophores affects atmospheric lifetimes of brown carbon

Brown carbon (BrC) affects radiation budget and thus global climate by absorbing light, during which photolysis can also occur and serve as an important sink of BrC. Yet, the interactive roles of mixed chromophores during BrC co-photolysis, which is anticipated in ambient aerosol particles, is seldom explored, making a model representation of atmospheric lifetimes of BrC highly uncertain. Herein, we investigate the photolysis of four typical atmospheric BrC chromophores (two methoxyphenols, MPs, and two nitrophenols, NPs), alone or mixed, with a wide range of concentrations as anticipated in atmospheric cloud/fog and aerosol droplets. The photo-decay rate constants (k) for the photolysis of single chromophores generally decrease as BrC concentration increases, but the effective quantum yields (Φ_e) are relatively constant in optically thin solutions; the Φ_e values increase by one order of magnitude in optically thick solutions for most BrC chromophores studied and even exceed 1 when BrC concentrations are high (e.g., 500 mM in isopropanol solutions), presumably due to secondary reactions involving their triplet states. During co-photolysis of two mixed chromophores, MPs increase the Φ_e values of NPs by factors of 4-26, but NPs affect the Φ_e values of MPs insignificantly, indicating a stronger sensitizing effect of MPs than NPs. In contrast, NPs mainly exert a shadowing effect on MP degradation, which only affects photo-decay rate constant but not effective quantum yield and is negligible for fine particles. Estimation of atmospheric lifetimes suggests that sensitizing by MPs can accelerate NP degradation by a factor of up to 30 while shadowing between them plays minor roles in fine particles but non-negligibly in coarse particles.