The error in conventionally reported¹³C/¹²C ratios of atmospheric CO due to the presence of mass independent oxygen isotope enrichment

An inherent disadvantage of stable isotope mass spectrometry on CO₂ is that ¹³C- and ¹⁷O-substituted CO₂ are not resolved. Therefore, CO₂-based mass spectrometry relies on the widely applicable assumption of mass dependent fractionation in the oxygen isotopes, rendering the measurement of ¹⁷O superfluous. For atmospheric CO, however, recently published determinations of its oxygen isotopic composition reveal the presence of significant deviation from mass dependent fractionation. For instance, the observed ¹⁷O excess of tropospheric CO at high northern latitudes shows a prominent seasonal cycle peaking at 7.5‰ in late summer. This implies that the CO₂ based determination of the ¹³C content of CO introduces a systematical error that needs to be corrected for, and a correction formula is derived. Based on the available Δ¹⁷O data and the now identified sources of mass independent fractionation in CO, the extent of this correction for CO isotope analysis in clean tropospheric air is estimated.