GC44F-01 Major knowledge gaps in predicting the efficacy and impacts of atmospheric methane removal by iron salt aerosol injection

Abstract: One proposed technique for drawing down levels of atmospheric methane is the introduction of iron salt aerosols (ISA), a form of atmospheric oxidation enhancement. In this scheme, iron chloride-containing particles are injected into the marine troposphere. Photochemical reactions within these particles then lead to the release of gas-phase molecular chlorine (Cl2), which photolyzes to form Cl radicals that can react with methane. However, the detailed atmospheric chemistry underlying these processes is highly complex and uncertain, limiting our ability to accurately predict both the efficacy and the downstream impacts of ISA-driven methane removal. The chemistry involved in a proposed natural analog, chlorine activation followed by methane oxidation via the mineral dust-sea salt aerosol (MDSA) mechanism, is also poorly understood. Drawing on a critical review of the relevant atmospheric-chemistry literature, and discussions with experts in the different aspects of ISA and halogen chemistry, here we assess major knowledge gaps associated with ISA chemistry, as well as potential research efforts that could help close these gaps. Broad areas of uncertainty include: photochemistry and radical cycling within the ISA particles; emission rates of reactive chlorine species; chemical aging of the ISA particles; and the fate of the gas-phase Cl, including the regeneration of chloride to maintain catalytic activity of the ISA. This assessment is intended to serve to inform future research efforts, including mechanistic modeling, laboratory studies, field studies, and 3D modeling, aimed at assessing the efficacy and impacts of ISA-based methane removal.