Identifying Gas- and Particle-Phase Tracers to Quantify the Extent of Chlorine Oxidation in the Remote Atmosphere
SEAMUS FREY, Hannah Kenagy, Yaowei Li, Frank Keutsch, Jesse Kroll, MIT
Abstract Number: 355
Working Group: Aerosol Chemistry
Abstract
Chlorine radical (Cl)-initiated oxidation of volatile organic compounds (VOCs) is thought to be an important process in the marine atmosphere, but its extent and product yields remain uncertain. Estimates of the global importance of Cl-initiated oxidation vary by over an order of magnitude; this prevents an accurate understanding of methane’s (CH4) Cl sink, the efficacy of proposed methane abatement strategies via increased Cl formation from iron-salt aerosols (ISA), and the role of Cl in secondary organic aerosol (SOA) formation. Here, we conduct laboratory studies aimed at improving estimates of the extent of Cl-initiated VOC oxidation, via the identification and quantification of chemical tracers of Cl oxidation. Oxidation experiments are conducted in a temperature-controlled, 7.5 m3 Teflon chamber using 340 nm UVA lights to initiate photochemistry. Gas- and particle-phase composition is monitored using an Ammonia Chemical Ionization Mass Spectrometer (NH4+-CIMS) and a High-Resolution Aerosol Mass Spectrometer (HR-AMS), respectively. Experimental conditions are controlled such that 1) only one of the first two oxidation steps is Cl-initiated, with the other being OH-initiated, and 2) no NOx is added, so the peroxy radical (RO2) fate is dominated by reaction with the hydroperoxyl (HO2) radical. We begin identifying possible Cl-tracers from the reactions of Cl with isoprene and dimethylsulfide (DMS), two model VOCs. Promising gas-phase Cl-tracers are identified and quantified using NH4+-CIMS, and the HR-AMS is used to identify particle-phase Cl-tracers. Tracer and SOA yields are compared with previous studies’ results of Cl-initiated oxidation carried out under different reaction conditions. These results are discussed in the context of ambient tracer measurements to assess the extent of Cl-initiated oxidation in the real atmosphere, both in the present-day and in future scenarios involving enhanced Cl production by ISA.