Analysis of multi-year near-surface ozone observations at the WMO/GAW “Concordia” station, Antarctica

In this work, eight years (2006–2013) of continuous measurements of near-surface ozone (O3) at the WMO/GAW contributing station “Concordia” (DMC, 75°06’S, 123°20’E, 3280 m a.s.l.) are presented, and the role of specific atmospheric processes in affecting O3 variability is investigated. In particular, during the period of highest data coverage (i.e., 2008–2013), O3 enhancement events (OEEs) were systematically observed at DMC, affecting 11.6% of the dataset. As deduced by a statistical selection methodology, the OEEs are affected by a significant interannual variability, both in the average and in the frequency of O3 values. To explain part of this variability, OEEs were analyzed as a function of: (i) total column of O3 and UV-A irradiance variability, (ii) long-range transport of air masses over the Antarctic plateau (by using LAGRANTO), and (iii) occurrence of “deep” stratospheric intrusion events (by using STEFLUX). The overall O3 concentrations are controlled by a day-to-day variability, which indicates the dominating influence of processes occurring at “synoptic” scales rather than “local” processes. Despite previous studies indicated an inverse relationship between OEEs and TCO, we found that the annual frequency of OEEs was higher when TCO values at DMC were higher than usual. The annual occurrence of OEEs at DMC was also related to the total time spent by air masses over the Antarctic plateau before their arrival at DMC, suggesting that the accumulation of photochemically-produced O3 during the transport dominated the local O3 production. Lastly, the influence of “deep” stratospheric intrusion events at DMC was analyzed, and it was observed that this contribution played only a marginal role (the highest frequency observed was 3% of the period, in November). This latter point, i.e., the frequency and seasonality of stratosphere-to-troposphere (STE) events, and the relative influence of specific transport mechanisms, as well as snow chemistry, are still under debate. These topics will be investigated in the STEAR (Stratosphere-to-Troposphere Exchange in the Antarctic Region) project, starting in 2020 and funded by the Italian Antarctic Research Program (PNRA). In particular, STEAR will provide an assessment of STE events in Antarctica, by using both continuous observations (e.g., O3 and Beryllium-7) at DMC, and modeling outputs. In addition to DMC measurements, simultaneous atmospheric composition datasets will be analyzed at Antarctic coastal observatories, i.e., the Mario Zucchelli (MZS) and Jang Bogo (JBS) stations.