Analysis of multi-year near-surface ozone observations at the WMO/GAW "Concordia" station (75 degrees 06 ' S, 123 degrees 20 ' E, 3280 m a.s.l. - Antarctica)

This work focuses on the near-surface O-3 variability over the eastern Antarctic Plateau. In particular, eight years (2006-2013) of continuous observations at the WMO/GAW contributing station "Concordia" (Dome C-DMC: 75 degrees 06'S, 123 degrees 20'E, 3280 m) are presented, in the framework of the Italian Antarctic Research Programme (PNRA). First, the characterization of seasonal and diurnal O-3 variability at DMC is provided. Then, for the period of highest data coverage (2008-2013), we investigated the role of specific atmospheric processes in affecting near-surface summer O-3 variability, when O-3 enhancement events (OEEs) are systematically observed at DMC (average monthly frequency peaking up to 60% in December). As deduced by a statistical selection methodology, these OEEs are affected by a significant interannual variability, both in their average 03 values and in their frequency. To explain part of this variability, we analyzed OEEs as a function of specific atmospheric variables and processes: (i) total column of O-3 (TCO) and UV-A irradiance, (ii) long-range transport of air masses over the Antarctic Plateau (by Lagrangian back-trajectory analysis - LAGRANTO), (iii) occurrence of "deep" stratospheric intrusion events (by using the Lagrangian tool STLEFLUX). The overall near-surface O-3 variability at DMC is controlled by a day-to-day pattern, which strongly points towards a dominating influence of processes occurring at "synoptic" scales rather than "local" processes. Even if previous studies suggested an inverse relationship between OEEs and TCO, we found a slight tendency for the annual frequency of OEEs to be higher when TCO values are higher over DMC. The annual occurrence of OEEs at DMC seems related to the total time spent by air masses over the Antarctic plateau before their arrival to DMC, suggesting the accumulation of photochemically-produced O-3 during the transport, rather than a more efficient local production. Moreover, the identification of recent (i.e., 4-day old) stratospheric intrusion events by STEFLUX suggested only a minor influence (up to 3% of the period, in November) of "deep" events on the variability of near-surface summer O-3 at DMC.