Cl Oxidation in Chamber and Flow Reactors: SOA Formation, Composition, and Gas Phase Chemistry

NIRVAN BHATTACHARYYA, Pearl Abue, Leif Jahn, Kristi McPherson, Eunha Kang, William Brune, Pawel K. Misztal, Anita Avery, Andrew Lambe, Lea Hildebrandt Ruiz, University of Texas at Austin

     Abstract Number: 217
     Working Group: Aerosol Chemistry

Abstract
Chlorine radicals (Cl) are an important atmospheric oxidant in marine, arctic, and polluted urban and industrial regions. Secondary organic aerosol (SOA) formation from VOC + Cl oxidation is often assessed in environmental chamber (EC) experiments, but the chlorine oxidative flow reactor (Cl-OFR) provides an alternative with lower operation costs and increased portability. Due to reduced residence times and higher oxidant concentrations in the OFR, the oxidation chemistry and microphysics may differ from ECs.

In this study, we investigated SOA formation and gas phase products from Cl oxidation of m-xylene, ethylbenzene, limonene, isoprene, and decamethylcyclopentasiloxane in parallel and sequential EC and Cl-OFR experiments. At the beginning and end of EC experiments, chamber air was introduced into the OFR and oxidized over Cl exposures ranging from 10⁹ – 10¹¹ molecules cm^-3 s. Oxidation products were analyzed with an Aerosol Chemical Speciation Monitor, a Scanning Electrical Mobility System, an iodide mode Chemical Ionization Mass Spectrometer and Filter Inlet for Gases and Aerosols, and a Vocus Proton Transfer Reaction Mass Spectrometer.

SOA yields were generally similar across EC and Cl-OFR experiments. Organic fractions at m/z 43 and 44 (f_43/44) suggest higher SOA oxidation in the Cl-OFR compared to the EC at similar Cl exposures. Sequential oxidation of limonene and isoprene derived EC SOA in the Cl-OFR generated additional SOA with a similar oxidation state to SOA from the EC alone. This sequential EC/Cl-OFR SOA was less oxidized than SOA only oxidized in the Cl-OFR, suggesting that continued oxidation of the reaction product mixture generates different products than fresh multigenerational oxidation. Gas phase product distributions show reduced Cl-OFR formation of early reaction products in m-xylene experiments related to differences in radical exposures and multigenerational chemistry. While EC and Cl-OFR SOA yields are largely similar, there is evidence for substantial differences in chemistry.