Accelerated Methane Photo-oxidation at the Air–Water Interface

Methane (CH 4 ) is a potent greenhouse gas, and its atmospheric lifetime is primarily regulated by oxidative depletion. While hydroxyl radical (•OH)-initiated oxidation of CH 4 in bulk-phase systems has been thoroughly characterized, the photo-oxidation of CH 4 at the air–water interface remains poorly understood. Here, we report that photo-oxidation of CH 4 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 (CH 3 OOH, CH 3 OH, CH 2 O, HCOOH, and CO 2 ) in microdroplet-mediated processes relative to gas-phase counterparts under UV irradiation. We propose that this accelerated CH 4 oxidation is fueled by the reaction of interfacially accumulated •OH with CH 4, resulting in the formation of •CH 3 through a low-barrier mechanism. Additionally, we uncover a new reaction path in which •CH 3 reacts with the interfacial superoxide radical (O 2 •– ) via a barrierless exothermic process, leading to a higher yield of CH 3 OOH than in bulk-phase reactions. This discovery provides important insights into the atmospheric CH 4 depletion process and may help resolve existing uncertainties in the estimation of atmospheric CH 4 depletion rates and the greenhouse gas budget.