Kinetic Isotope Effects in Atmospheric Sinks of Methane

This project aims to improve the understanding of atmospheric methane oxidation by hydroxyl and chlorine radicals. A new, state-of-the-art laboratory setup will provide high accuracy measurements. A wide range of experimental conditions will be employed, including using different radical precursors and varying temperature, pressure, and reactant concentrations in ways not previously studied. This work is motivated by the fact that methane is highly important in global atmospheric chemistry, and the uncertainties associated with atmospheric methane oxidation currently limit the capability to use methane isotopic measurements to study the global methane budget. The new measurements from this study are expected to contribute to the understanding of societally important issues of air pollution and radiative forcing. Training and supporting early career scientists are significant components of this project, and outreach activities are planned. The project will involve fabricating a laboratory photochemical reactor that uses a laser source for radical generation. Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS) is the primary analytical technique to be used for high-precision measurements of methane mole fraction and isotopic composition, while FTIR and UV Absorption Spectroscopy will be used for measurements of methane and other key chemical species inside the reactor. The photochemical reactor system will enable studying the reactions of OH and Cl with 12CH4, 13CH4, and 12CH3D to improve understanding of the Kinetic Isotope Effects. This will be the first study to cover the full range of temperatures and pressures relevant to the troposphere. The results of the study are expected to substantially reduce current uncertainties for annual average global methane emissions from fossil fuel and microbial sources. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.