Free Radical Multiphase Chemistry on Tropospheric Photoreactive Particles

Abstract

Gas-phase and multiphase reactions of free radicals play pivotal roles in tropospheric chemistry. Recent studies indicate that photoinduced multiphase reactions serve as a potential source of free radicals rather than a traditional sink. To accurately evaluate their environmental impacts, this Review provides a comprehensive overview of radical multiphase chemistry on key photoactive components. By systematically reviewing the multiphase generation of photoinduced free radicals, we first provide insights into the underlying multiphase reaction mechanisms and kinetics. Subsequently, the synergistic reactions among photoreactive components and the mutual feedback mechanism between short-lived and persistent free radicals are illustrated, thereby revealing their complex interactions on environmental microinterfaces. Then, the effects of multiphase radical chemistry on primary particle aging and secondary particle formation and growth are described in detail. Finally, we critically examine existing challenges and inaccuracies in the exploration of radical multiphase chemistry, highlighting the urgent need for the comprehensive quantification of photoinduced surface oxidation capacity, along with the scientific evaluation of their health risks and climatic effects. The aim of this work is to identify the missing source of atmospheric radicals, thereby facilitating the evaluation of their environmental impacts and illuminating future directions for particle-emission reduction.