Mixing Ratios of CO, CO2, CH4, and Isotope Ratios of Associated 13C, 18O, and 2H in Air Samples from Niwot Ridge, Colorado, and Montaña de Oro, California, USA (January 2004)
Stanley C. Tyler
Department of Earth System Science
University of California
Description and Methods
Air samples from Niwot Ridge, Colorado (41°N, 105°W) and Montaña de Oro, CA (35°N, 121°W) have been collected at approximately semi-monthly to monthly intervals since the mid 1990s. The beginning dates for each gas and isotope analyzed are as follows:
|Gas or isotope||Niwot Ridge||Montaña de Oro|
|CO||January 1995||January 1996|
|δ13C||November 1999||June 2000|
|CO2||February 1996||January 1996|
|δ13C||February 1996||January 1996|
|δ18O||February 1996||January 1996|
|CH4||January 1995||January 1996|
|δ13C||January 1995||January 1996|
|δ2H||August 1998||February 2000|
Such time series can provide information about: (1) seasonal cycling of CO, CO2, and CH4 sources and sinks in background air, (2) trends in atmospheric concentrations of CO2 and CH4 and their stable carbon, oxygen, and hydrogen isotopes, (3) the distribution of the hydroxyl (OH) radical in the atmosphere, and (4) the role of the terrestrial biosphere as a source or sink of atmospheric CO2. The data presented here are discussed in detail by Tyler et al., (1999).
Samples at Niwot Ridge were collected before noon in order to increase the likelihood of sampling when the prevailing wind was from the west, so as to represent well-mixed background air from over the western United States without * interference from regional contamination. Samples from Montaña de Oro were collected in late afternoon or early evening during periods when prevailing winds were from the west or northwest (i.e., from the Pacific Ocean).
Air from Niwot Ridge was collected into passivated aluminum high-pressure cylinders using a RIX model SA-3 compressor; air from Montaña de Oro was collected into electro-polished 32-liter stainless steel canisters using a portable battery- operated piston pump (Model 415CDC30/12B, Thomas Co., Sheboygan WI). Mixing ratios from canister and cylinder samples were measured at The University of California-Irvine, using a Hewlett Packard 5880A gas chromatograph with a flame ionization detector, for CO2 and CH4, and a Shimadzu Model 14A Gas Chromatograph with a Model RGD2 reduction gas analyzer detector (Trace Analytical, Menlo Park, California), for CO. Mixing-ratio working standards for CH4 and CO are based on the National Atmospheric and Oceanic Administration/Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) reference scale, and have been inter-compared with and calibrated to NOAA/CMDL reference standards (Lang et al., 1990, for CH4; Novelli et al., 1991, for CO; Thoning et al., 1987, for CO2). The precision level for measurements of CO2 is ~±3-4 ppm (1% uncertainty around 342 ppm); the precision level for measurements of CH4 is ~±5 to 10 ppb for values around a working-reference-gas value of 1903 ppb, and for measurements of CO, the precision level is ~±1.3 ppb for measurements around 133.5 ppb. Oxygen and carbon isotopes were measured on a Finnigan MAT Model 252 Isotope-Ratio Mass Spectrometer (IRMS). Precision of measurement on clean dry CO2 gas standards is ±0.01%. The reproducibility of δ13C measurements from ~200 liters each of replicate air samples, when all possible errors associated with differences in sample canisters, sample pumping, vacuum line processing, and isotope measurement are taken into account, is ±0.05% for δ13C of CH4, 0.20% for δ13C of CO and ±0.01% for δ13C of CO2.
The δD-CH4 analyses were performed using a cf-Gas-Chromatography/IRMS coupled to a custom-designed CH4 gas preconcentrator (Rice et al., 2001). A pyrolysis oven converts CH4 to H2 after its separation from the air stream and before its detection by the mass spectrometer. Precision of measurement is ±1.3% for CH4 processed from ~63 ml of whole air.
Additional information on instrumentation, calibration, analysis procedures, and reporting standards is given in the "readme.txt" file and references listed therein. In particular, Tyler et al. (1999) describe much of the experimental detail and sample collection.
- Lang, P.M., L.P. Steele, and R.C. Martin. 1990. Atmospheric methane data for the period 1986-1988 from the NOAA/CMDL global cooperative flask sampling network, in NOAA Technical Memorandum ERL CMDL-2, University of Colorado, Boulder CO, 1990.
- Novelli, P.C., J.W. Elkins and L.P. Steele. 1991. The development and evaluation of a gravimetric reference scale for measurements of atmospheric carbon monoxide. J. Geophys. Res. 96, 13109-13121.
- Rice, A.L. A.A. Gotoh, H.O. Ajie, and S.C. Tyler. 2001. High precision continuous flow measurement of δ13C and δD of Atmospheric CH4. Anal. Chem. 73, 4104-4110, 2001.
- Thoning, K.W., P. Tans, T.J., Conway, and L.S. Waterman. 1987. NOAA/GMCC calibrations of CO2-in-air reference gases: 1979-1985, NOAA Technical Memorandum ERL ARL-150, Environmental Research Laboratory, Boulder, CO, 63 pp.
- Tyler, S.C., H.O. Ajie, M.L. Gupta, R.J. Cicerone, D.R. Blake, and E.J. Dlugokencky. 1999. Carbon isotopic composition of atmospheric methane: A comparison of surface level and upper tropospheric air, J. Geophy. Res. 104. 13895-13910.