Isoprene-Chlorine Oxidation in the Presence of NO x and Implications for Urban Atmospheric Chemistry.

Fine particulate matter (PM 2.5 ) is a key indicator of urban air quality. Secondary organic aerosol (SOA) contributes substantially to the PM 2.5 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 (NO x ) remain unexplored. Here, we investigate chlorine-isoprene reactions under low- and high-NO x conditions inside an environmental chamber. Organic chlorides including C 5 H 11 ClO 3 , C 5 H 9 ClO 3 , and C 5 H 9 ClO 4 are observed as major gas- and particle-phase products. Modeling and experimental results show that the secondary OH-isoprene chemistry is significantly enhanced under high-NO x 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.