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Anthropogenic trace gases often exhibit interhemispheric gradients because of larger emissions in the northern hemisphere. Depending on a tracer's emission pattern and sink processes, trace gas observations can thus be used to investigate interhemispheric transport in the atmosphere. Vice versa, understanding interhemispheric transport is important for interpreting spatial tracer distributions and for inferring emissions. We combine several datasets from the upper troposphere (UT) to investigate the interhemispheric gradient of sulfur hexafluoride (SF6) covering latitudes from ~80° N to~60° S: canister sampling based measurements from the IAGOS-CARIBIC infrastructure and data from the in-flight gas chromatography instruments GhOST and UCATS. The interhemispheric gradient of SF6 in the UT is found to be weaker than near the surface. Using the concept of a lag time removes the increasing trend from the time series. At the most southern latitudes, a lag time of over 1 year with respect to the northern mid-latitude surface is derived, and lag times decrease over the period 2006–2020 in the tropics and in the southern hemisphere. Observations are compared to results from the two-dimensional AGAGE 12-box model. Based on EDGAR 7 emissions, fair agreement of lag times is obtained for the northern hemisphere, but southern hemispheric air appears too 'old'. This is consistent with earlier findings that transport from the northern extra-tropics into the tropics is too slow in many models. The influence of the emission scenario and the model transport scheme are evaluated in sensitivity runs. It is found that EDGAR 7 underestimates emissions of SF6 globally and in the southern hemisphere, whereas northern extra-tropical emissions seem overestimated. Faster southward transport from the northern extra-tropics would be needed in the model, but transport from the southern tropics into the southern extra-tropics appears too fast. [ABSTRACT FROM AUTHOR]

Age of stratospheric air is a concept commonly used to evaluate transport timescales in atmospheric models. The mean age can be derived from observations of a single long-lived trace gas species with a known tropospheric trend. Commonly, deriving mean age is based on the assumption that all air enters the stratosphere through the tropical (TR) tropopause. However, in the lowermost stratosphere (LMS) close to the extra-tropical (exTR) tropopause cross tropopause transport needs to be taken into account. We introduce the new exTR-TR method, which considers exTR input into the stratosphere in addition to TR input. We apply the exTR-TR method to in situ SF6 measurements from three aircraft campaigns (PGS, WISE and SouthTRAC) and compare results to those from the conventional TR-only method. Using the TR-only method, negative mean age values are derived in the LMS close to the tropopause during the WISE campaign in northern hemispheric (NH) fall 2017. Using the new exTR-TR method instead, the number and extent of negative mean age values is reduced. With our new exTR-TR method we are thus able to derive more realistic values of typical transport times in the LMS from in situ SF6 measurements. Absolute differences between both methods range from 0.3 to 0.4 years among the three campaigns. Interhemispheric differences in mean age are found when comparing seasonally overlapping campaign phases from the PGS and the SouthTRAC campaigns. On average, within the lowest 65 K potential temperature above the tropopause the NH LMS is 0.5 years ± 0.3 years older around March 2016 than the southern hemispheric (SH) LMS around September 2019. The derived differences between results from the exTR-TR method and the TR-only method, as well as interhemispheric differences are higher than the sensitivities of the exTR-TR method to parameter uncertainties, which are estimated to be below 0.22 years for all three campaigns. [ABSTRACT FROM AUTHOR]

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 mid to high latitudes. A clear CH2Br2 maximum is observed in the NH during autumn with a less pronounced similar feature in the Southern Hemisphere (SH). 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. However, the SH database is insufficient to quantify this difference. We further compare the observations to model estimates of TOMCAT and CAM-Chem, both using the same emission inventory. 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 are 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 available observations. Our study emphasises the need for more extensive 20 observations in the SH to fully understand the impact of CH2Br2and CHBr3 on lowermost stratospheric ozone loss and to help constraining emissions. [ABSTRACT FROM AUTHOR]