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

Removing atmospheric methane now has outsized climate benefits by “flattening the curve” of global warming over the next few decades. A potential approach to methane removal uses chlorine radicals generated from iron-salt aerosols (ISA) (Oeste 2017), since methane reacts with chlorine radicals 16 times faster than with its dominant natural sink, hydroxyl radicals (Atkinson 2003). The feasibility of this approach has been indicated by observations of reduced methane over the North Atlantic by mineral dust sea spray aerosols, a natural analog of ISA (van Herpen 2023). To further develop the approach, a field test under the GeoRestoration Action Plan (GRAP) is scheduled for 2025 to release ferric chloride (FeCl3) aerosols (artificial ISA) via nucleation of heat-sublimated vapor over the subtropical ocean using aircraft (AMR AG 2024). While the concept of the test seems feasible, essential microphysical and chemical data needed for quantitative operation and climate assessment are lacking.

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The first problem is the unknown FeCl3 aerosol size distribution, which is governed by microphysical processes in the aircraft wake and determines the aerosol irradiation surface area, atmospheric lifetime, and thus efficacy of the ISA approach. The second problem is the poorly constrained chlorine production from FeCl3 aerosols, which is sensitive to aerosol properties and environmental conditions, due to acidity-dependent redox reactions (Wittmer 2017), and temperature-dependent chlorine degassing and background gas uptake (Seinfeld 2016). Our ultimate goal is to advance the understanding of atmospheric methane removal by FeCl3 aerosols, thereby informing the upcoming GRAP field test. Success will be measured by estimating aerosol size distributions from aircraft wakes and quantifying chlorine production under various tropospheric conditions. We will achieve this via four research tasks that combine laboratory experiments with aerosol microphysical modeling and Earth system modeling.

Without our proposed work, the development of the ISA approach would rely on “trial and error” rather than “insightful learning”, so the societal impact of our proposal cannot be overstated. Specifically, our experimentally constrained microphysical model will not only provide a predictive tool to optimize the FeCl3 aerosol generation process, but will also serve as an aerosol profile input to large-scale models to simulate aerosol lifetime, spatial distribution, and hence climate impact. Our chlorine production experiments will yield a quantitative understanding of the unknown influences of environmental conditions and trace gas uptake on the multiphase FeCl3 redox reactions, which are of interest to both the methane removal and broader atmospheric chemistry communities. Finally, our Earth system modeling will provide an initial estimate of chlorine concentrations and methane removal for the field test, motivating further climate assessment research by our and other research groups.