An estimation of the ¹⁸O / ¹⁶O ratio of UT/LMS ozone based on artefact CO in air sampled during CARIBIC flights

Abstract. An issue of O₃-driven artefact production of O₃ in the upper troposphere/lowermost stratosphere (UT/LMS) air analysed in the CARIBIC-1 project is being discussed. By confronting the CO mixing and isotope ratios obtained from different analytical instrumentation, we (i) reject natural/artificial sampling and mixing effects as possible culprits of the problem, (ii) ascertain the chemical nature and quantify the strength of the contamination, and (iii) demonstrate successful application of the isotope mass-balance calculations for inferring the isotope composition of the contamination source. The δ¹⁸O values of the latter indicate that the oxygen is very likely being inherited from O₃. The δ¹³C values hint at reactions of trace amounts of organics with stratospheric O₃ that could have yielded the artificial CO. While the exact contamination mechanism is not known, it is clear that the issue pertains only to the earlier (first) phase of the CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) project. Finally, estimated UT/LMS ozone δ¹⁸O values are lower than those observed in the stratosphere within the same temperature range, suggesting that higher pressures (240–270 hPa) imply lower isotope fractionation controlling the local δ¹⁸O(O₃) value.