Using laser absorption spectroscopy for the measurement of d13C, d18O and D17O of atmospheric CO2

A new method to do stable isotope composition measurements on atmospheric CO2 is presented, using laser absorption spectroscopy (LAS). Traditionally measurements are conducted on pure CO2 extracted from air with isotope ratio mass spectrometry (IRMS). With LAS measurements can be conducted directly on dry air, thereby reducing the sample preparation time dramatically and eliminating any inaccuracies related with the CO2 extraction. The 12C16O2, 13C16O2, 12C16O18O and 12C16O17O molecules are measured directly, so no mass interferences occur and this enables us to
measure the δ17O (next to the δ13C and δ18O), something that is still very complex using IRMS. With the δ17O and δ18O, the Δ17O can be determined, a promising new proxy to quantify the gross primary production of the biosphere.

We focus on minimizing the combined uncertainty of the measurements by comparing various calibration methods and study sample stability. The oxygen composition of CO2-in-air stored in glass sample flasks tends to drift over time and we present new insights in the mechanisms behind this process and do recommendations on how to ensure a better stability. Uncertainties of 0.03 ‰ for δ13C and δ18O and 0.05 ‰ for δ17O and Δ17O are reached with the LAS method. Measurement results from the two coastal atmospheric measurement stations over the period 2017-2022 are presented. Results show that LAS measurements capture the seasonal cycles of δ13C and δ18O well. For the Δ17O measurements seasonal cycles cannot be identified, but a significant change of Δ17O values over the years is observed.