Short-term variability and uncertainties of trace gases in the boreal summer UTLS from AirCore measurements during the OSTRICH campaign

Trace gas patterns in the upper troposphere and lower stratosphere (UTLS) can provide valuable insights into the mechanisms and the interplay of processes controlling the distribution of these species. We use vertical greenhouse gas profiles derived from measurements with the balloon-based AirCore technique to obtain detailed information on the distribution of carbon dioxide (CO₂), carbon monoxide (CO) and methane (CH₄) in and around the polar UTLS region during the boreal summer. The analysis is based on new data from the ATMO-ACCESS campaign called OSTRICH (Observations of Stratospheric TRace gases Influencing Climate using High-altitude platforms), which took place in the summer of 2023 in Sodankylä, Finland. More than 30 vertical profile measurements over a ten-day period allow for study of short-term changes in composition, with the balloons covering an altitude range from the ground to >30 km. In addition, the results of the comparison between the six participating international AirCore groups (Universities of Groningen (The Netherlands), Bern (Switzerland), Frankfurt (Germany), Finnish Meteorological Institute, Forschungszentrum Jülich (Germany) and the National Oceanic and Atmospheric Administration (NOAA, US)) enable an evaluation of the measurements themselves. Due to the simultaneous sampling with different AirCores during each flight these results contribute to the assessment of AirCore data quality in general and the determination of uncertainties. Moreover, future technical improvements and adaptations in processing algorithms can be derived from this intercomparison. The measurements mostly agree quite well, confirming the quality of vertical trace gas distributions derived from the AirCore technique. It is however an essential task to pinpoint the reasons behind the deviations that were observed in some cases. Next to these differences within one flight we discuss the variability in the profiles between individual flights. Only at altitudes above the 80 hPa level do expected and remaining similar behaviours of CO₂ and CH₄ mole fractions appear. The measurements in lower parts of the atmosphere show large deviations from day-to-day during the campaign phase. These clear (short term) variations in trace gas composition show that a single vertical profile in many different layers of the atmosphere, such as the UTLS, is not necessarily representative. Balloon-borne sensors with higher spatial and temporal resolution can therefore help to better constrain trace gas variability across various altitude ranges.