Advancing optical isotope ratio spectroscopy for carbon dioxide and methane The aim of this work package is to develop and metrologically characterise both laboratory based and field deployable spectroscopic methods and calibration approaches for isotope ratio measurements for carbon dioxide and methane. The work proposed will build on the foundations laid by the 16ENV06 SIRS project and will be guided by the GAW recommendations. Specifically, optical isotope ratio spectrometers (OIRS) have to be established with sufficient precision and instrument-specific correction algorithms developed to account for the variability in the amount fraction of carbon dioxide and methane, and in addition for spectral interferences of other trace gases or changes in line broadening due to differences in the matrix gas. In addition, calibration approaches have to be implemented to provide a link between amount fractions of isotopologues (analysed by OIRS) and the international isotope ratio scales (VPDB, VSMOW/SLAP). This work will be supported by the assessment and improvement of available isotope-specific spectral line data. Uncertainty contributions will be compiled in an uncertainty budget and the compatibility of OIRS techniques assessed towards WMO-GAW compatibility goals. In Task 3.1, activities will focus on carbon dioxide isotope ratios with a target precision of 0.05 ‰ for d13C-CO2 and d18O-CO2. Additionally, the precision of d17O-CO2 will be investigated, and the outcome will also feed into activities in WP1 on characterisation of 17O correction in IRMS d13C-CO2 measurements. Additionally, the feasibility of absolute spectroscopic isotope ratio measurements will be addressed in the task. Task 3.2 will cover activities on methane with target precisions of 0.2 ‰ for d13C-CH4 and 1 ‰ for d2H-CH4. In Tasks 3.1 and 3.2, the next generation of reference materials from WP1 and WP2 will be exploited. Task 3.3 will focus on field deployable instruments for carbon dioxide and methane isotope ratio measurements targeting the WMO-GAW compatibility goals of 0.05 ‰ for d13C-CO2 and for d18O-CO2, as well as 0.2 ‰ for d13C-CH4 and 1 ‰ for d2H-CH4, including field-measurement campaigns to be arranged with industrial and environmental research organisations.
