Precise measurements of the total concentration of atmospheric CO2 and C13O2∕C12O2 isotopic ratio using a lead-salt laser diode spectrometer

We have developed a tunable diode laser spectrometer, called SIMCO (spectrometer for isotopic measurements of CO2), for determining the concentrations of C12O2 and C13O2 in atmospheric air, from which the total concentration of CO2 and the isotopic composition (expressed in delta units) δC13O2 are calculated. The two concentrations are measured using a pair of lines around 2290.1cm−1, by fitting a line profile model, taking into account the confinement narrowing effect to achieve a better accuracy. Using the Allan variance, we have demonstrated (for an integration time of 25s) a precision of 0.1ppmv for the total CO2 concentration and of 0.3‰ for δC13O2. The performances on atmospheric air have been tested during a 3 days campaign by comparing the SIMCO instrument with a gas chromatograph (GC) for the measurement of the total CO2 concentration and with an isotopic ratio mass spectrometer (MS) for the isotopic composition. The CO2 concentration measurements of SIMCO are in very good agreement with the GC d...