Addressing discrepancies in 13CKIE and DKIE values for the CH₄-OH oxidation

Methane (CH₄) plays a critical role in the global carbon cycle, with its mole fraction currently 2.5 times higher than preindustrial levels. The increasing growth rate observed globally highlights the importance of accurate partitioning the atmospheric CH₄. Oxidation of CH₄ by hydroxyl radicals (OH) in the troposphere accounts for approximately 85% of the global CH₄ sink. The resulting isotopic fractionation of δ¹³C-CH₄ and δD-CH₄ provides a valuable tool for understanding the global CH₄ budget. However, discrepancies exist in the reported kinetic isotope effect (KIE) values for CH₄ destruction by OH with ^13CKIE ranging from 1.0036 to 1.010 and ^DKIE ranging from 1.25 to 1.31. These uncertainties significantly limit the precision of global CH₄ budget estimation.

 

This study aims to address these discrepancies by accurately characterizing the KIE values under varying temperature and pressure conditions. During the laboratory experiments, CH₄ is subjected to chemical reactions with OH, which is generated through the photolysis of vapor-phase hydrogen peroxide using a deep-UV light source (200-380 nm). To minimize interference from O(¹D) reactions, a coated glass filter is employed. The photochemical reactions take place in a 5-liter, triple-quartz-layered reactor, maintained at stable pressure and temperature, with by-products removed using a low-temperature trap. The reactor is coupled to two Isotope Ratio Mass Spectrometers (IRMS), enabling continuous measurements of δ¹³C, δD, and δ¹⁸O in remaining CH₄ and CO throughout the experiment. By enhancing our understanding of CH₄-OH reaction kinetics under controlled conditions, this study can improve the accuracy of global CH₄ budget assessments and refine the distribution between fossil and biogenic CH₄ sources.