Complexities of Photosensitization in Atmospheric Particles

Brown carbons (BrCs) play a pivotal role in the light absorption by aerosol particulates by exerting a positive radiative forcing effect that contributes to global warming. Beyond impacts on radiative balance, some BrCs, as photosensitizers, can generate reactive triplet-state molecules toward various atmospheric molecules upon photoexcitation. The significance of photosensitization has been increasingly recognized, particularly in the context of escalated global wildfire incidents that emit substantial BrCs. We focus on the complex atmospheric photosensitization by discussing the current challenges, including (1) the diverse reactivities of the photosensitizer mixture in atmospheric particles, (2) the methodologies for investigating photosensitization processes, (3) the driving factors of photosensitization, and (4) the typical pathways and mechanisms of atmospheric photosensitized reactions. Lastly, we advise future research to focus on the refined parametrization of triplet and singlet oxygen concentrations, alongside their complex reactivities.

Atmospheric photosensitization, especially in light-absorbing particles, can drive secondary pollutant formation under light. This work comprehensively discusses the existing findings, remaining challenges, and future directions in this emerging research field.