This problem statement was submitted as part of a research funding application which was awarded by Spark.

Any approach to methane (CH₄) removal via Atmospheric Oxidation Enhancement (AOE) relies critically on our understanding of present-day CH₄ sinks. Loss of atmospheric CH₄ is dominated by oxidation by the hydroxyl radical (OH), which accounts for ~90% of the global CH₄ sink [1]. Other sinks include stratospheric loss, uptake to soils, and tropospheric oxidation by the chlorine radical (Cl). This last one is small but highly uncertain. Current estimates of global CH₄ loss by Cl, from global chemistry-transport models (CTMs) or isotopic measurements of CH₄ and its oxidation products (13CH₄, 13CO) [2-7], span over an order of magnitude, from 1 to 35 Tg CH₄ /yr (0.16% to 5.6% of the total atmospheric sink) [1]. Such a wide range poses a challenge to Cl-based AOE methods (e.g., the addition of iron salt aerosols), since the current rate of CH₄ oxidation by Cl determines the amount of Cl that would need to be added via AOE to make a meaningful change to CH₄ levels [8]

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There is thus a critical need for an improved understanding of global CH₄ loss by tropospheric Cl. Current approaches have inherent limitations: isotopic measurements are indirect and sparse, and CTMs may miss key processes (e.g., newly-identified Cl sources [9]). An alternative, complementary approach involves the measurement of molecular species that serve as markers of Cl chemistry. The objective of this project is thus the identification and quantification of such Cl-containing tracers, via laboratory studies of the Cl oxidation of volatile organic compounds (VOCs), under conditions relevant to the remote troposphere. We will also measure yields of secondary organic aerosol (SOA) and other products, enabling an improved understanding of the atmospheric impacts of Cl-based AOE. The primary metric of success will be the quantitative identification of unique molecular tracers that are sufficiently long-lived and detectable to serve as effective marker species for ambient measurements.

If successful, this project will improve our ability to quantify tropospheric Cl oxidation and Cl-based CH₄ abatement via:

• identification of key marker compounds, with yields, for reactions of Cl with major organic species, to serve as targets for ambient studies;
• offering a new approach for constraining Cl sources and chemistry, via model-measurement
• comparisons of these tracer compounds, which will in turn improve constraints on CH₄ oxidation by Cl;
• providing a new means to assess the efficacy (degree of enhancement of Cl oxidation) of AOE methods;
• improving the descriptions of SOA and other Cl oxidation products in CTMs, enabling more accurate predictions of the atmospheric-chemistry and air-quality impacts of Cl-based AOE approaches.