Accelerated Methane Photo-oxidation at the Air–Water Interface

Methane (CH4) is a potent greenhouse gas, and its atmospheric lifetime is primarily regulated by oxidative depletion. While hydroxyl radical (•OH)-initiated oxidation of CH4 in bulk-phase systems has been thoroughly characterized, the photo-oxidation of CH4 at the air-water interface remains poorly understood. Here, we report that photo-oxidation of CH4 at the air-water interface of microdroplets exhibits an order of magnitude rate enhancement compared to gas-phase reactions. Quantitative analysis reveals 5.0-30.4-fold enhancements in the production rates of key oxidation products (CH3OOH, CH3OH, CH2O, HCOOH, and CO2) in microdroplet-mediated processes relative to gas-phase counterparts under UV irradiation. We propose that this accelerated CH4 oxidation is fueled by the reaction of interfacially accumulated •OH with CH4, resulting in the formation of •CH3 through a low-barrier mechanism. Additionally, we uncover a new reaction path in which •CH3 reacts with the interfacial superoxide radical (O2•-) via a barrierless exothermic process, leading to a higher yield of CH3OOH than in bulk-phase reactions. This discovery provides important insights into the atmospheric CH4 depletion process and may help resolve existing uncertainties in the estimation of atmospheric CH4 depletion rates and the greenhouse gas budget.