Hemispheric differences in ozone across the stratosphere-troposphere exchange region

Abstract. This study examined the ozone mixing ratios in the upper troposphere-lower stratosphere (UTLS) to determine whether the ozone levels in the lowermost stratosphere are lower in the Southern (SH) than in the Northern Hemisphere (NH), as suggested by total column ozone observations. We utilized unique in situ measurements from the High Altitude and Long Range research aircraft deployed over the southernmost region of South America (45–60° S) in September–November 2019 as a part of the Southern Hemisphere Transport, Dynamics, and Chemistry (SouthTRAC) research campaign. In addition, the mission period enabled us to examine the impact of an early stratospheric sudden warming (SSW) event, which triggered a breakdown of the southern polar vortex in September 2019. In situ measurements from IAGOS commercial aircraft (45–60° N), ozonesondes (Patagonia) and CAMS reanalysis data were considered for comparative analysis. Stratospheric air was identified using relative humidity (<20 %) and carbon monoxide (<50 nmol mol^-1) thresholds. Our results show that air masses of stratospheric origin had higher ozone abundances in the NH UTLS than in the SH (between 300–200 hPa and 45–60° latitude). In high ozone depletion years in the stratospheric vortex, the SH ozone median (170 nmol mol^-1) was only 51 % of that in the NH (330 nmol mol^-1), while in low depletion years, SH ozone median (210 nmol mol^-1) reached 57 % of the NH values (370 nmol mol^-1). The SSW event increased SH UTLS by 37 % (46 nmol mol^-1) during the SouthTRAC mission compared to high depletion years.