Intended and unintended consequences of atmospheric methane oxidation enhancement

Abstract

Atmospheric oxidation enhancement (AOE) of methane via tropospheric hydroxyl radicals (OH) or chlorine (Cl) radicals is being considered as a method to decrease greenhouse gas concentrations. The chemistry involved is coupled and nonlinear, affecting air quality, other greenhouse gases, and ozone-depleting substances. Here I simulate different OH- and Cl-based AOE methods in a 3D atmospheric chemistry model to estimate their effectiveness and impacts on air quality and climate forcers. I find that larger emissions of iron salt aerosol are required relative to previous work to reduce methane on a global scale by at least a few percent (≥ 565 Tg yr−1), reflecting uncertainty in the reaction mechanism and modeling framework employed. More work is needed to understand the kinetics of chlorine release from iron salt aerosol and the potential for bromine co-release, which decreases effectiveness. Hydrogen peroxide–based approaches can decrease global methane, but feasibility is limited by the large emissions required. Limiting emissions to daytime for hydrogen peroxide–based scenarios has negligible effects. All methods increase surface particulate matter pollution and in some regions lead to exceedances of annual air quality standards. Ozone air pollution decreases under Cl-based methods, but increases in populated areas under OH-based methods. While Cl-based methods can increase ozone-depleting substances, the 1-year timeframe of this study is insufficient to predict impacts on stratospheric ozone. The overall impacts of AOE methods on climate and human health involve not only their effectiveness at decreasing methane, but competing or complementary effects on other greenhouse gases, aerosol, and surface air pollution.