Calibration uncertainty of optical isotope ratio spectroscopy measurements of methane and field comparison with mass spectrometry

The stable isotopic signatures of atmospheric methane (CH4) carbon 13C(CH4) and hydrogen 2H(CH4) are tracers that can help distinguish the relative contributions from different emissions sources. Optical isotope ratio spectrometers (OIRS) deployed at atmospheric monitoring stations have the capability for continuous measurements, providing time series data that can complement sampling campaigns using isotope ratio mass spectrometry (IRMS). OIRS instruments, however, require larger volumes of calibration gases than IRMS and the measurement is of the isotopologues directly (12CH4, 13CH4 and 12CH3D) rather than conversion to CO2 and H2. Here, we demonstrate the calibration method for Boreas, a preconcentrator-OIRS system deployed at an atmospheric monitoring station in the South of England and show that these measurements are compatible with those made by IRMS. Measurements with Boreas are referenced to a whole air working standard that is sampled in sequence with air, following the principle of identical treatment. We show the results of a field comparison to IRMS measurements of bag samples taken from the same air inlet simultaneously with the preconcentrator. The calibration method uses mixtures prepared gravimetrically at a range of amount fractions from a single high-purity CH4 parent that has been characterised for 13C and 2H by IRMS. This method is capable of calibration over a wide range of amount fraction and isotopic composition. A rigorously derived uncertainty budget shows that the major contributions are from the uncertainty in the assignment of 13C and 2H of the methane parent and the spectrometer, with minimal contribution from uncertainty in the amount fraction of the standards.