Intended and Unintended Consequences of Atmospheric Methane Oxidation Enhancement

Abstract. Atmospheric oxidation enhancement (AOE) of methane via either tropospheric hydroxyl radicals (OH) or chlorine (Cl) radicals is being considered as a method to decrease greenhouse gas concentrations. The chemistry involved is coupled; is nonlinear; and affects air quality, other greenhouse gases, and ozone-depleting substances. Here I perform a suite of experiments in a three-dimensional (3D) atmospheric chemistry model representing different OH- and Cl-based atmospheric oxidation enhancement methods, to estimate the effectiveness of each at decreasing greenhouse gases and the impacts on air quality and stratospheric ozone. I find that iron salt aerosol may not be effective at reducing methane on a global scale, depending on the reaction mechanism employed. More work is needed to understand the kinetics of chlorine release from iron salt aerosol and the potential for bromine co-release, which further decreases effectiveness. Hydrogen peroxide–based approaches can decrease global methane, but the hydrogen peroxide emissions required may be too large to be feasible. I find that limiting emissions to daytime for hydrogen peroxide–based scenarios has negligible effects. All methods increase surface particulate matter (PM) pollution and in some regions lead to exceedances of annual air quality standards. Cl-based methods decrease ozone air pollution, but OH-based methods increase ozone air pollution in populated areas. While Cl-based methods can increase ozone-depleting substances, I predict minimal changes in stratospheric ozone after 1 year of deployment. The overall impacts of atmospheric oxidation enhancement methods on climate and human health involve multiple competing factors.