Laser spectroscopic measurement of the isotopic composition of trace gases in the atmosphere and in human breath

Laser spectroscopy is ideally suited to detect trace gases in atmospheric air since it combines high sensitivity and fast response. The objective of this work was the application of infrared laser spectroscopy for real-time isotopic composition measurements of atmospheric trace gases, e.g., methane. Of the various hydrocarbons in the atmosphere which cause the greenhouse effect, methane is the most important. Since different sources and sinks of atmospheric methane exhibit different super(12)CH sub(4)/ super(13)CH sub(4) ratios, the measurement of the abundance of these isotopomers provides important information about the global methane budget. Our measurements were carried out by means of cavity leak-out spectroscopy (CALOS) in the mid-infrared spectral region near 3 mu m. The gas sample flows through an absorption cell consisting of an optical high-finesse cavity (length 0.5 m) which provides an optical absorption pathlength of 3.6 km. A cw CO sideband laser was used to excite the optical cavity. After determination of the power decay rate of the cavity field the methane mixing ratio is calculated from the absorption coefficient achieved. We achieved a detection limit of 95 ppt methane in ambient air using an integration time of 20 s. This corresponds to a minimum detectable absorption of 1.7 x 10 super(-9)/cm. This unique sensitivity and temporal resolution enabled us to determine the super(13)C/ super(12)C isotopic ratio of methane in ambient air without sample preconcentration or gas processing. The present system requires only few minutes for a delta super(13)C measurement with a precision of 11 per mill. The accuracy of our spectroscopic delta super(13)C measurement depends on the statistical error arising from the limited signal-to-noise-ratio of the spectrometer and on systematic errors introduced by the conversion of the absorption coefficient obtained to the corresponding mixing ratio. The uncertainty of our present method is a factor of 4 smaller than previously reported spectroscopic measurements. A higher accuracy than that reported here can only be achieved at present through time-consuming sample processing and the use of a state-of-the-art mass spectrometer. The uncertainty of plus or minus 11 per mill does not permit tracking super(13) delta changes in the "clean" atmosphere far from methane sources which usually are below plus or minus 2 per mill; however it is sufficient to distinguish the isotopic signatures of different methane source