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

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 (Cl-2), and nitryl chloride (ClNO2) radical precursors under high and low NOx 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 Cl-2 experiments resulted in bicyclic peroxide and methylbenzoquinone species, respectively. When Cl-2 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.