Isoprene-Chlorine Oxidation in the Presence of NOx and Implications for Urban Atmospheric Chemistry

Fine particulate matter (PM2.5) is a key indicator of urban air quality. Secondary organic aerosol (SOA) contributes substantially to the PM2.5 concentration. Discrepancies between modeling and field measurements of SOA indicate missing sources and formation mechanisms. Recent studies report elevated concentrations of reactive chlorine species in inland and urban regions, which increase the oxidative capacity of the atmosphere and serve as sources for SOA and particulate chlorides. Chlorine-initiated oxidation of isoprene, the most abundant nonmethane hydrocarbon, is known to produce SOA under pristine conditions, but the effects of anthropogenic influences in the form of nitrogen oxides (NOx) remain unexplored. Here, we investigate chlorine-isoprene reactions under low- and high-NOx conditions inside an environmental chamber. Organic chlorides including C5H11ClO3, C5H9ClO3, and C5H9ClO4 are observed as major gas- and particlephase products. Modeling and experimental results show that the secondary OH-isoprene chemistry is significantly enhanced under high-NOx conditions, accounting for up to 40% of all isoprene oxidized and leading to the suppression of organic chloride formation. Chlorine-initiated oxidation of isoprene could serve as a source for multifunctional (chlorinated) organic oxidation products and SOA in both pristine and anthropogenically influenced environments.

Automated summary

Fine particulate matter (PM2.5) is an important indicator of urban air quality. Recent studies have found elevated concentrations of reactive chlorine species in inland and urban regions, which contribute to the formation of secondary organic aerosols (SOA) and particulate chlorides. The oxidation of isoprene by chlorine has been found to produce SOA under pristine conditions, but the effects of anthropogenic influences, such as nitrogen oxides (NOx), have not been explored. This study investigates the reactions between chlorine and isoprene under low- and high-NOx conditions, and finds that the secondary OH-isoprene chemistry is significantly enhanced under high-NOx conditions, leading to the suppression of organic chloride formation.