Role of atomic chlorine in atmospheric volatile organic compound oxidation and secondary organic aerosol formation: a review

Abstract

As a highly reactive atmospheric oxidant, chlorine (Cl) atoms significantly contribute to the oxidation of volatile organic compounds (VOCs) and the formation of secondary organic aerosol (SOA) in coastal and industrial environments. To assess the environmental impacts of SOA generated from Cl-initiated oxidation, elucidating its chemical composition, formation mechanisms, and physicochemical properties under varying atmospheric conditions is of paramount importance. This review summarizes recent research advances on atmospheric chlorine chemistry. We first outline the sources and generation mechanisms of Cl atoms, followed by an analysis of the kinetic characteristics, oxidation mechanisms, and SOA formation potential of Cl-initiated VOC oxidation. Compared to hydroxyl (OH) radicals, Cl atoms exhibit faster reaction rates and reaction pathways that preferentially generate low-volatility products, significantly enhancing SOA formation and demonstrating higher SOA yields. Given the complexity of SOA formation and its strong dependence on environmental conditions, we further discuss the responses of gas-phase chemistry as well as SOA mass yields and composition to the [Cl₂/VOC]₀ ratios, Cl exposure, NO_x levels, and relative humidity. Finally, we outline key experimental challenges and future research priorities.