Accelerated Methane Photo-oxidation at the Air-Water Interface

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Physical Chemistry

Background:

• Methane (CH4) is a potent greenhouse gas with its atmospheric lifetime determined by oxidation.
• Oxidation of CH4 in bulk phases is well-understood, but reactions at the air-water interface are not.
• Understanding CH4 oxidation at interfaces is crucial for accurate atmospheric modeling.

Purpose of the Study:

• To investigate the photo-oxidation of methane (CH4) at the air-water interface.
• To quantify the rate enhancement of CH4 oxidation in microdroplets compared to gas-phase reactions.
• To elucidate the mechanisms driving accelerated CH4 oxidation at interfaces.

Main Methods:

• Utilized UV irradiation of microdroplets containing methane.
• Quantitatively analyzed the production rates of key oxidation products.
• Proposed reaction mechanisms based on experimental observations and theoretical considerations.

Main Results:

• Photo-oxidation of CH4 in microdroplets showed an order of magnitude rate enhancement versus gas-phase reactions.
• Production rates of CH4 oxidation products (e.g., CH3OOH, CH3OH, CO2) were 5.0-30.4 times higher in microdroplets.
• Identified interfacial hydroxyl radical (•OH) and superoxide radical (O2•−) as key species in accelerated CH4 oxidation.

Conclusions:

• Methane photo-oxidation is significantly enhanced at the air-water interface of microdroplets.
• A novel reaction pathway involving •CH3 and O2•− leads to higher CH3OOH yields.
• Findings offer critical insights into atmospheric CH4 depletion and greenhouse gas budget estimations.