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Abstract The Pico-STRAT Bi Gaz spectrometer provides in situ mixing ratio measurements of water (H₂O) and methane (CH₄) [or carbon dioxide (CO₂)] under balloon. The instrument was flown in the tropical upper troposphere and lower stratosphere in 2019/20 and 2021/22 during the Strateole 2 campaigns for a total of five flights of 20–80 days between 18- and 20-km altitude. In this frame, in situ measurements of water vapor and methane were performed every 4–12 min in the equatorial tropopause layer. On several occasions, water vapor measurements of Pico-STRAT Bi Gaz have been compared with localized measurements from the Fluorescence Lyman-Alpha Stratospheric Hygrometer for Balloon (FLASH-B) Lyman-αhygrometer and vertical profiles of the NOAA Global Monitoring Laboratory (GML) frost point hygrometer over Hilo, Hawaii. Pico-STRAT Bi Gaz measurements agreed with the FLASH-B hygrometer to within 2.2% ± 5.3% between 18.2 and 18.7 km in 2021 and to within 1.3% ± 5.3% near 19 km in December 2019. Pico-STRAT Bi Gaz agreed with NOAA’s frost point hygrometer (FPH) hygrometer to within 1.2% ± 4.1% between 18 and 19 km on four occasions during the two campaigns. These are within both instruments’ uncertainties. Methane measurements from Pico-STRAT Bi Gaz have been compared with in situ measurements from the whole air sampler (WAS) instrument, flown aboard the NASA WB-57 aircraft during the Asian Summer Monsoon Chemical and Climate Impact Project (ACCLIP) 2022 campaign over South Korea, 8 months after the Pico-STRAT Bi Gaz overpass. The relative difference between both instruments is found to be −0.1% ± 0.9% within the altitude range from 17 to 19 km and within the Pico-STRAT measurement uncertainty. 

Deep convection in the Asian summer monsoon is a significant transport process for lifting pollutants from the planetary boundary layer to the tropopause level. This process enables efficient injection into the stratosphere of reactive species such as chlorinated very short-lived substances (Cl-VSLSs) that deplete ozone. Past studies of convective transport associated with the Asian summer monsoon have focused mostly on the south Asian summer monsoon. Airborne observations reported in this work identify the East Asian summer monsoon convection as an effective transport pathway that carried record-breaking levels of ozone-depleting Cl-VSLSs (mean organic chlorine from these VSLSs ~500 ppt) to the base of the stratosphere. These unique observations show total organic chlorine from VSLSs in the lower stratosphere over the Asian monsoon tropopause to be more than twice that previously reported over the tropical tropopause. Considering the recently observed increase in Cl-VSLS emissions and the ongoing strengthening of the East Asian summer monsoon under global warming, our results highlight that a reevaluation of the contribution of Cl-VSLS injection via the Asian monsoon to the total stratospheric chlorine budget is warranted. 

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 Tropospheric¹⁸O¹⁸O is an emerging proxy for past tropospheric ozone and free‐tropospheric temperatures. The basis of these applications is the idea that isotope‐exchange reactions in the atmosphere drive¹⁸O¹⁸O abundances toward isotopic equilibrium. However, previous work used an offline box‐model framework to explain the¹⁸O¹⁸O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the¹⁸O¹⁸O proxy, we developed a scheme to simulate atmospheric¹⁸O¹⁸O abundances (quantified as ∆₃₆values) online within the GEOS‐Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O₂isotopic equilibration scheme within GEOS‐Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆₃₆values are too high elsewhere. Investigations using a comprehensive model of the O‐O₂‐O₃isotopic photochemical system and proof‐of‐principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆₃₆values: the anomalously efficient titration of¹⁸O¹⁸O to form ozone. Incorporating these effects into the online ∆₃₆calculation scheme in GEOS‐Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of¹⁸O¹⁸O in O₂, the modeled change in the mean tropospheric ∆₃₆value since 1850 CE is only slightly altered; it is still quantitatively consistent with the ice‐core ∆₃₆record, implying that the tropospheric ozone burden increased less than 40% over the twentieth century. 

Abstract The Asian summer monsoon (ASM) as a chemical transport system is investigated using a suite of models in preparation for an airborne field campaign over the Western Pacific. Results show that the dynamical process of anticyclone eddy shedding in the upper troposphere rapidly transports convectively uplifted Asian boundary layer air masses to the upper troposphere and lower stratosphere over the Western Pacific. The models show that the transported air masses contain significantly enhanced aerosol loading and a complex chemical mixture of trace gases that are relevant to ozone chemistry. The chemical forecast models consistently predict the occurrence of the shedding events, but the predicted concentrations of transported trace gases and aerosols often differ between models. The airborne measurements to be obtained in the field campaign are expected to help reduce the model uncertainties. Furthermore, the large‐scale seasonal chemical structure of the monsoon system is obtained from modeled carbon monoxide, a tracer of the convective transport of pollutants, which provides a new perspective of the ASM circulation, complementing the dynamical characterization of the monsoon.