Stratosphere to troposphere transport of ozone over South America during the SouthTRAC campaign

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Stratosphere-troposphere exchange (STE) plays a key role in the tropospheric ozone budget. In extratropical latitudes, ozone transport from the stratosphere occurs mainly through the development of cut-off lows and tropopause folds. However, chemical measurements in the free troposphere and between the upper troposphere and lower stratosphere (UTLS) are remarkably sparse in the southern hemisphere compared with the northern hemisphere. In this context, the Southern Hemisphere Transport, Dynamics, and Chemistry (SouthTRAC) campaign was conducted between September and November 2019. One of its key objectives was to investigate dynamical and chemical processes in the UTLS over South America, using the High Altitude and LOng Range (HALO) research aircraft for in situ and remote measurements. This study analyzes the impact of a cut-off low (7-13 September 2019) on free tropospheric ozone, which extended from subpolar to subtropical latitudes using ozone (O3), water vapor (H2O), methane (CH4), carbon monoxide (CO) and nitric acid (HNO3) measured by HALO instrumentation. These measurements were complemented with ozonesondes launched from Ushuaia, Argentina (54.8°S, 68.3°W) and Marambio, Antarctica (64.2°S, 56.6°W) and included surface ozone measured at Cerro Tololo, Chile (30.2°S, 70.8°W) at 2200 m above sea level.The observations revealed complex ozone structures during the evolution of the cut-off low between 65°S-30°S. The 9 September flight recorded O3 mixing ratios up to 100 nmol mol-1 above 8 km altitude over Río Grande, Argentina (53.78°S, 67.75°W). In contrast, the sonde in Ushuaia recorded 30 nmol mol-1 above 10 km. The difference observed between these sites was due to the formation of a tropopause fold, which transported O3-rich air into lower levels. The increases in O3 measured in the free troposphere were accompanied by consistent increases in HNO3 (a stratospheric tracer) and decreases in CO and H2O (tropospheric tracers). In addition, on 12 September, O3 levels at Cerro Tololo, where the diurnal O₃ cycle is nearly absent, experienced a sharp increase of approximately 13 nmol mol-1, sustained for nearly 24 hours. This O3 increase was accompanied by low relative humidity (

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