Since March 2003, measurements of surface ozone have been made at the British Antarctic Survey Clean Air Sector Laboratory (CASLab) at Halley station in coastal Antarctica. Detailed measurements of boundary layer meteorology, as well as standard meteorological parameters, are also measured at the CASLab. Combining these data allows us to probe the transport pathway of air masses during ozone depletion events (ODEs). ODEs were observed at Halley on several occasions during Antarctic spring 2003. On some occasions, extremely rapid loss of ozone was observed (loss of 16 ppbv in 1 min on one occasion), which was associated with regional‐scale transport. For each such event during 2003, the air mass originated in the southern Weddell Sea, an area of vigorous sea‐ice production. On other occasions the development of the event and its recovery were strongly associated with the build‐up and decline of a stable boundary layer. In these cases, air masses had had recent contact with a nearby open water lead where sea‐ice production is known to occur. The data presented here are entirely consistent with the idea that halogens responsible for ozone loss are derived during new sea‐ice formation from an associated surface such as brine slush or frost flowers.

Abstract. The majority of tropospheric ozone depletion event (ODE) studies have focussed on time-series measurements, with comparatively few studies of the vertical component. Those that exist have almost exclusively used freeflying balloon-borne ozonesondes and almost all have been conducted in the Arctic. Here we use measurements from two separate Antarctic field experiments to examine the vertical profile of ozone during Antarctic ODEs. We use tethersonde data to probe details in the lowest few hundred meters and find considerable structure in the profiles associated with complex atmospheric layering. The profiles were all measured at wind speeds less than 7 ms −1 , and on each occasion the lowest inversion height lay between 10 m and 40 m. We also use data from a free-flying ozonesonde study to select events where ozone depletion was recorded at altitudes >1 km above ground level. Using ERA-40 meteorological charts, we find that on every occasion the high altitude depletion was preceded by an atmospheric low pressure system. An examination of limited published ozonesonde data from other Antarctic stations shows this to be a consistent feature. Given the link between BrO and ODEs, we also examine ground-based and satellite BrO measurements and find a strong association between atmospheric low pressure systems and enhanced BrO that must arise in the troposphere. The results suggest that, in Antarctica, such depressions are responsible for driving high altitude ODEs and for generating the large-scale BrO clouds observed from satellites. In the Arctic, the prevailing meteorology differs from Correspondence to: A. E. Jones (aejo@bas.ac.uk) that in Antarctica, but, while a less common effect, major low pressure systems in the Arctic can also generate BrO clouds. Such depressions thus appear to be fundamental when considering the broader influence of ODEs, certainly in Antarctica, such as halogen export and the radiative influence of ozone-depleted air masses.

Capulse Summary The Year of Polar Prediction in the Southern Hemisphere had a Special Observing Period (SOP) during the 2018-2019 austral summer. Activities during and resulting from the Antarctic SOP are described.

Abstract. The majority of tropospheric ozone depletion event (ODE) studies have focussed on time-series measurements, with comparatively few studies of the vertical component. Those that exist have almost exclusively used free-flying balloon-borne ozonesondes and almost all have been conducted in the Arctic. Here we use measurements from two separate Antarctic field experiments to examine the vertical profile of ozone during Antarctic ODEs. We use tethersonde data to probe details in the lowest few hundred meters and find considerable structure in the profiles associated with complex atmospheric layering. The profiles were all measured at wind speeds less than 7 ms−1, and on each occasion the lowest inversion height lay between 10 m and 40 m. We also use data from a free-flying ozonesonde study to select events where ozone depletion was recorded at altitudes >1 km above ground level. Using ERA-40 meteorological charts, we find that on every occasion the high altitude depletion was preceded by an atmospheric low pressure system. An examination of limited published ozonesonde data from other Antarctic stations shows this to be a consistent feature. Given the link between BrO and ODEs, we also examine ground-based and satellite BrO measurements, and find a strong association between enhanced BrO and atmospheric low pressure systems. The results suggest that, in Antarctica, such depressions are responsible for driving high altitude ODEs and for generating the large-scale BrO clouds observed from satellites. In the Arctic, the prevailing meteorology differs from that in Antarctica, but we show that major low pressure systems in the Arctic, when they occur, can also generate BrO clouds. Such depressions thus appear to be fundamental when considering the broader influence of ODEs, particularly in Antarctica, such as halogen export and the radiative influence of ozone-depleted air masses.