Identifying and Quantifying Gas- and Particle-Phase Tracers for Chlorine Oxidation in the Remote Troposphere

Atmospheric methane (CH₄) abatement relies on good estimates of CH₄ chemical sinks in the atmosphere. While the hydroxyl radical (OH) is the dominant driver, a major source of uncertainty is oxidation by the chlorine radical (Cl). Estimates of global Cl-initiated oxidation (from isotopic measurements and global modeling) vary by over an order of magnitude; this prevents an accurate understanding of methane’s Cl sink and the efficacy of proposed CH₄ abatement strategies via Cl enhancement from iron-salt aerosols (ISA). Here we propose an alternative approach for quantifying ambient Cl concentrations (and hence the Cl sink of methane), the ambient measurement of long-lived products of the Cl-initiated oxidation of volatile organic compounds (VOCs). Here, we conduct laboratory studies aimed at identifying and quantifying such Cl-tracers, formed under chemical conditions representative of the remote atmosphere. 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 compounds are monitored using ammonium-ion Chemical Ionization Mass Spectrometry (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 NO_x is added, so the peroxy radical (RO₂) fate is dominated by reaction with the hydroperoxyl (HO₂) radical. We identify possible Cl-tracers from the reactions of Cl with isoprene and dimethylsulfide (DMS), two VOCs emitted in the remote marine atmosphere. 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 yields are compared to measurements made in previous studies , typically carried out under different reaction conditions.