Chapter 14 - Stable isotope measurements of atmospheric CO2 and CH4

Abstract

Isotopic analyses of CO2 and CH4 provide key tools for better understanding global budgets of these trace gases. This chapter discusses two different methods for measuring carbon and oxygen isotopes in atmospheric CO2 and a method for measuring carbon isotopes in atmospheric CH4. The first method for CO2 uses traditional dual inlet Isotope Ratio Mass Spectrometry (IRMS) with cryogenic extraction on fairly large air samples. The second method for CO2 uses Gas Chromatography- Isotope Ratio Mass Spectrometry (GC-IRMS), which utilizes an order of magnitude smaller air samples. The GC-IRMS method is less precise than the dual inlet, but the elimination of liquid nitrogen use for CO2 extraction, and no requirement for the N20 correction. Carbon and oxygen stable isotope ratios of atmospheric carbon dioxide, δ13CO2 and δC18OO, provides independent information about carbon sources and sinks. Therefore, it is a method for high-precision, low volume, automated and relatively fast measurements which allowed to analyze air samples for δ13C of methane on a weekly basis from six sites around the globe. It is possible only by employing the GC-IRMS method and the NOAA global flask sampling network.