Measurements of stable isotopes and isotope effects in atmospheric methane

Measurements of stable carbon and hydrogen isotope ratios in atmospheric methane provide valuable information for constraining budget estimates and understanding temporal and spatial variability in methane abundance. The development of a new analytical technique, continuous flow gas chromatography isotope ratio mass spectrometry, for the measurement of 13C/ 12C and D/H ratios in atmospheric methane is described. This technique provides for measurement on a 60 ml aliquot of ambient air compared with the more conventional technique of offline preparation followed by dual inlet isotope ratio mass spectrometry which requires at least three orders of magnitude more air. Additionally, isotope analysis can be performed much more rapidly while retaining a comparable level of precision. Using this and other analytical techniques, measurements of methane mixing and isotope ratios are reported from air collected at several remote clean air sites: Montana de Oro, California, Niwot Ridge, Colorado, and from recent Pacific Ocean transects. D/H ratios in background atmospheric methane are the first to be reported from this laboratory. Observed seasonal behavior in the D/H ratios is roughly anti-correlated with observed methane mixing ratios and is in large part sink-driven. These observations contrast with that of 13C/12C ratios which appear more source-driven in their seasonal behavior. Measurements of 13C/12C and D/H ratios are also reported from whole air samples collected from the upper troposphere and lower stratosphere aboard the NASA ER2 and comprise the most extensive high precision data set to date. These measurements show significant enrichment in the heavy isotopomers with decreasing methane mixing ratio as a result of kinetic isotope effects in methane chemical loss processes.