Summary
Understanding and characterising the variability of ozone in the lower atmosphere is a key aspect of regional and global climate assessment. Tropospheric ozone influences climate by absorbing outgoing longwave terrestrial radiation, thereby acting as a greenhouse gas, and by affecting the lifetimes and hence concentrations of other important greenhouse gases such as methane and chlorofluorocarbons. The bulk of tropospheric ozone is produced photochemically from precursors, most notably nitrogen oxides, carbon monoxide, methane and non-methane hydrocarbons, all of which have variable biogenic and anthropogenic sources. Heterogenous and oxidation reactions produce ozone sinks. In this paper, we discuss tropospheric ozone variability at high southern latitudes through comparison of meteorological analyses with ozonesonde measurements obtained between 2003 and 2006 from Davis (69ºS) and Macquarie Island (54ºS), and surface ozone measurements during the Sea Ice Physics and Ecosystem eXperiment (SIPEX) of September-October 2007 from within the Antarctic pack-ice (~65ºS) and between Antarctica and Tasmania. The Davis and Macquarie Island ozonesonde measurements show a dominant annual cycle in near-surface ozone, with peak-to-peak amplitudes of approximately 20 parts per billion by volume (ppbv) and 10 ppbv respectively, on a background of 25 ppbv. The annual peak at the two sites occurs in early spring and is dictated by competition between meridional transport and photochemical loss [Klekociuk & Tully, 2007]. In the upper troposphere above Davis, distinct episodes of down-welling from the stratosphere associated with the passage of cut-off lows and the dissipation of the stratospheric ozone hole were observed. At Davis, ozone losses of up to 15 ppbv were observed episodically within the boundary layer during winter and spring. All of these events were accompanied by strong surface temperature inversions. Two of the strongest depletion events were associated with long and slow fetches over the sea-ice region, for which heterogeneous reactions are implicated. The remainder of the events appear to have been associated with long katabatic flows over the Antarctic plateau, for which we speculate that ozone was removed to the snow pack before reaching the station. In contrast to Davis, the near-surface ozone at Macquarie Island exhibited very weak vertical gradients which exhibited variability that was likely related to horizontal transport effects. Slow variations (on timescales of hours to 1-2 days) in surface ozone of up to 5 ppbv were observed in the pack ice during SIPEX, and the origins of these variations are discussed using back trajectory analyses. The SPIEX measurements provided an estimate of -0.2 ppbv for the meridional surface ozone gradient between latitudes of 65ºS and 45ºS. We discuss the SPIEX measurements in comparison with other surface ozone measurements obtained from Macquarie Island, the Southern Ocean and Cape Grim.