Different chlorine and hydroxyl radical environments impact m-xylene oxidation products

Abstract

Airborne emissions of aromatic hydrocarbons including benzene, ethylbenzene, toluene, and xylenes are associated with anthropogenic activities (e.g. transportation) and form secondary organic aerosol (SOA) when oxidized. While hydroxyl radicals (OH) dominate oxidation, chlorine radicals (Cl) react with alkyl substituted aromatics more rapidly and favor a different oxidative pathway. High concentrations of reactive chlorine species have been observed in continental and coastal regions, where mixed Cl/OH chemistry is expected to influence regional SOA formation and composition. This study uses environmental chamber experiments to assess SOA formation and composition from the oxidation of m-xylene in mixed Cl/OH oxidation environments. Experiments were conducted with hydrogen peroxide (H₂O₂), chlorine (Cl₂), and nitryl chloride (ClNO₂) radical precursors under high and low NO_x conditions. Data was collected with an Aerosol Chemical Speciation Monitor (ACSM), Scanning Electrical Mobility System (SEMS) and time of flight chemical ionization mass spectrometer with filter desorption (FIGAERO-CIMS) utilizing H₃O⁺ and I⁻ reagent ions. Different oxidative pathways in H₂O₂ and Cl₂ experiments resulted in bicyclic peroxide and methylbenzoquinone species, respectively. When Cl₂ was the sole radical precursor, SOA was more highly oxidized and less fragmented. ClNO₂ experiments formed substantial amounts of bicyclic peroxide and minimal methylbenzoquinone in the gas phase and less oxidized SOA with a lower fraction of organochlorides. These differences are related to secondary OH formation and slower ClNO₂ photolysis driving lower Cl radical concentrations. This study provides evidence that gas and particle-phase products vary depending on the oxidative environment and underlines the importance of studying novel oxidants and oxidative pathways.