Measurements of CH4 Mixing Ratio and D/H and 13C/12C Isotope Ratios in Atmospheric Samples from Continental United States

We report δD and δ13C measurements of atmospheric CH₄ from air samples collected bi-weekly from fixed surface sites in the United States. Our fixed surface sites are located at the mid-continental site Niwot Ridge, CO (41°N, 105°W) and a Pacific coastal northern hemispheric site receiving strong westerlies, Montaña de Oro, CA (35°N, 121°W). Data from multiyear bi-weekly sampling provide information relating seasonal cycling of CH₄ sources and sinks in background air, provide a record of long term trends in CH₄ mixing and isotope ratio related to atmospheric CH₄ loading, and may reveal information on regional sources of CH₄. Measurements of δD-CH₄ average -93+/-3‰ (versus V-SMOW) at Niwot Ridge from 1999 through 2001 and -97+/-4‰ at Montaña de Oro from 2000 through 2001. Annual seasonal cycles observed at both sites are 6-7‰ , exhibiting strong anti-correlation with CH₄ mixing ratio seasonality that appears to be largely sink-driven (i.e. through reaction with OH). More extensive time series in δ13C-CH₄ average -47.2+/-0.1‰ (versus V-PDB) at Niwot Ridge from 1995 through 2001 and -47.3+/-0.2‰ at Montaña de Oro from 1996 through 2001. In contrast to δD-CH₄, seasonality in δ13C-CH₄ has a relatively poor anti-correlation with seasonality in CH₄ mixing ratio, indicating significant source-driven influence. We discuss the implications of these measurements on the atmospheric CH₄ budget and detail the precision and accuracy of our data with respect international reference standards. It is our intention to have these data archived and available for modeling and advanced calculations by other atmospheric researchers in the near future.