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Abstract. Climate models predict that the Brewer–Dobson circulation (BDC) will accelerate due to tropospheric warming, leading to a redistribution of trace gases and, consequently, to a change of the radiative properties of the atmosphere. Changes in the BDC are diagnosed by the so-called “age of air”, that is, the time since air in the stratosphere exited the troposphere. These changes can be derived from a long-term observation-based record of long-lived trace gases with increasing concentration in the troposphere, such as sulfur hexafluoride (SF6). The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) provides the longest available continuous time series of vertically resolved SF6 measurements, spanning 2004 to the present. In this study, a new age-of-air product is derived from the ACE-FTS SF6 dataset. The ACE-FTS product is in good agreement with other observation-based age-of-air datasets and shows the expected global distribution of age-of-air values. Age of air from a chemistry–climate model is evaluated, and the linear trend of the observation-based age of air is calculated in 12 regions within the lower stratospheric midlatitudes (14–20 km, 40–70°) in each hemisphere. In 8 of 12 regions, there was not a statistically significant trend. The trends in the other regions, specifically 50–60 and 60–70° S at 17–20 km and 40–50° N at 14–17 and 17–20 km, are negative and significant to 2 standard deviations. This is therefore the first observation-based age-of-air trend study to suggest an acceleration of the shallow branch of the BDC, which transports air poleward in the lower stratosphere, in regions within both hemispheres. 

Abstract. Bromine released from the decomposition of short-lived brominated source gases contributes as a sink of ozone in the lower stratosphere.The two major contributors are CH2Br2 and CHBr3.In this study, we investigate the global seasonal distribution of these two substances, based on four High Altitude and Long Range Research Aircraft (HALO) missions, the HIAPER Pole-to-Pole Observations (HIPPO) mission, and the Atmospheric Tomography (ATom) mission.Observations of CH2Br2 in the free and upper troposphere indicate a pronounced seasonality in both hemispheres, with slightly larger mixing ratios in the Northern Hemisphere (NH).Compared to CH2Br2, CHBr3 in these regions shows larger variability and less clear seasonality, presenting larger mixing ratios in winter and autumn in NH midlatitudes to high latitudes.The lowermost stratosphere of SH and NH shows a very similar distribution of CH2Br2 in hemispheric spring with differences well below 0.1 ppt, while the differences in hemispheric autumn are much larger with substantially smaller values in the SH than in the NH.This suggests that transport processes may be different in both hemispheric autumn seasons, which implies that the influx of tropospheric air (“flushing”) into the NH lowermost stratosphere is more efficient than in the SH.The observations of CHBr3 support the suggestion, with a steeper vertical gradient in the upper troposphere and lower stratosphere in SH autumn than in NH autumn.However, the SH database is insufficient to quantify this difference.We further compare the observations to model estimates of TOMCAT (Toulouse Off-line Model of Chemistry And Transport) and CAM-Chem (Community Atmosphere Model with Chemistry, version 4), both using the same emission inventory of Ordóñez et al. (2012).The pronounced tropospheric seasonality of CH2Br2 in the SH is not reproduced by the models,presumably due to erroneous seasonal emissions or atmospheric photochemical decomposition efficiencies.In contrast, model simulations of CHBr3 show a pronounced seasonality in both hemispheres, which is not confirmed by observations.The distributions of both species in the lowermost stratosphere of the Northern and Southern hemispheres are overall well captured by the models with the exception of southern hemispheric autumn,where both models present a bias that maximizes in the lowest 40 K above the tropopause, with considerably lower mixing ratios in the observations.Thus, both models reproduce equivalent flushing in both hemispheres, which is not confirmed by the limited available observations.Our study emphasizes the need for more extensive observations in the SH to fully understand the impact of CH2Br2 and CHBr3 on lowermost-stratospheric ozone loss and to help constrain emissions. 

Abstract The southern part of South America and the Antarctic peninsula are known as the world’s strongest hotspot region of stratospheric gravity wave (GW) activity. Large tropospheric winds are deflected by the Andes and the Antarctic Peninsula and excite GWs that might propagate into the upper mesosphere. Satellite observations show large stratospheric GW activity above the mountains, the Drake Passage, and in a belt centered along 60°S. This scientifically highly interesting region for studying GW dynamics was the focus of the Southern Hemisphere Transport, Dynamics, and Chemistry–Gravity Waves (SOUTHTRAC-GW) mission. The German High Altitude and Long Range Research Aircraft (HALO) was deployed to Rio Grande at the southern tip of Argentina in September 2019. Seven dedicated research flights with a typical length of 7,000 km were conducted to collect GW observations with the novel Airborne Lidar for Middle Atmosphere research (ALIMA) instrument and the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) limb sounder. While ALIMA measures temperatures in the altitude range from 20 to 90 km, GLORIA observations allow characterization of temperatures and trace gas mixing ratios from 5 to 15 km. Wave perturbations are derived by subtracting suitable mean profiles. This paper summarizes the motivations and objectives of the SOUTHTRAC-GW mission. The evolution of the atmospheric conditions is documented including the effect of the extraordinary Southern Hemisphere sudden stratospheric warming (SSW) that occurred in early September 2019. Moreover, outstanding initial results of the GW observation and plans for future work are presented.