The stratosphere is often considered to be dynamically stable with limited vertical exchange; however, episodic deep convection can even transport tropospheric air masses into the upper troposphere (UT) and even across the tropopause into lower stratosphere (LS). We deployed a newly developed airborne single particle mass spectrometer, Particle Analysis by Laser Mass Spectrometry – Next Generation (PALMS-NG), aboard a NASA ER-2 stratospheric aircraft to characterize aerosol particles in the UTLS during the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. Here, we present observations revealing substantial perturbations of the stratospheric aerosol layer during an active convection and wildfire season in 2022.

We show that carbonaceous–sulfate particles of tropospheric origin account for up to 90% of stratospheric particles with physical diameters between 0.1 and 1.5 µm within an approximately 4 km layer above the tropopause. Approximately 43% of these stratospheric carbonaceous–sulfate particles are directly attributed to biomass burning. The injected particles are chemically complex and organic-rich, and some exhibit internally mixed signatures containing both tropospheric and stratospheric components.

Our observations further demonstrate that biomass-burning-related aerosols do not remain chemically unchanged following injections into the stratosphere. Instead, they undergo chemical mixing with stratospheric components, indicating a pronounced perturbation of the stratospheric aerosol layer driven by convective transport.

These results highlight the coupling between dynamics and chemistry in modulating UTLS aerosol populations. As wildfire frequency and intensity increase alongside enhanced deep convection in a warming climate, convective delivery of biomass-burning products to the stratosphere may become increasingly important, with implications for ozone chemistry and radiative forcing.

How to cite: Shen, X., Jacquot, J., Li, Y., Sharpe, S., Dykema, J., Schill, G., Bowman, K., Homeyer, C., Fraund, M., Moffet, R., Olayemi, T., Pittman, J., Rivera-Adorno, F., Murphy, D., Smith, J., Laskin, A., Keutsch, F., and Cziczo, D.: Stratospheric aerosol perturbation by tropospheric biomass burning and deep convection, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3004, https://doi.org/10.5194/egusphere-egu26-3004, 2026.

State-of-the-art Chemistry Climate Models (CCMs) exhibit a large model spread in their simulated chemical composition of the Upper Troposphere / Lower Stratosphere (UTLS). As the surface climate is highly sensitive to differences in greenhouse gas composition in the UTLS region, understanding and reducing this model spread is required for more confidence in climate projections. One large source of uncertainty in atmospheric simulations is the representation of convection. While heavily parameterised, convection is crucial for the water distribution in the air, the formation of clouds, and their radiative effect, as well as for the fast vertical transport in the troposphere. In addition to its direct effect on the UTLS composition, it may also have an indirect impact on transport into the stratosphere. By affecting the wind and temperature fields, convection influences the creation, propagation, and dissipation of Rossby waves that drive the Brewer-Dobson circulation in the stratosphere.

To investigate the effect of the convection parameterisation on the simulated UTLS composition, we performed sensitivity simulations with the CCM ECHAM/MESSy Atmospheric Chemistry (EMAC). Two simulations were performed under present climate conditions. The simulations are identical except for the applied convection parameterisation. The simulations were repeated, but with projected future climate boundary conditions, to investigate the effect of different convection parameterisations on the simulated UTLS composition under climate change.

Our results show that the simulated UTLS composition is highly sensitive to the applied convection parameterisation. The convective transport strength and outflow altitude vary strongly between different parameterisations, affecting the distribution of short-lived tracers in the upper troposphere as well as their transport into the tropical lower stratosphere. Significant differences in cloud and water distribution lead to changes in chemical reaction rates, particularly in the polar lower stratospheric ozone chemistry. Despite these differences, the effects of climate change on convective transport are in close agreement between the sensitivity simulations. Nevertheless, the strong coupling between temperature, water, and ozone creates large differences in the projected changes of the UTLS composition.

We found that the choice of the convection parameterisation influences the composition and the transport in the entire atmosphere, far beyond its direct effect in the troposphere.

How to cite: Menken, J. M., Jöckel, P., Tost, H., Garny, H., Jeske, A., and Schmidt, A.: The influence of the convection parameterisation on simulated present and future UTLS greenhouse gas distributions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6815, https://doi.org/10.5194/egusphere-egu26-6815, 2026.

The atmospheric distribution and variability of CO₂ result from the interplay of different processes and mechanisms. Although these trace gas patterns contain valuable information on mixing and transport at different timescales, the information is difficult to extract from observed or simulated mole fractions, particularly in the upper troposphere and lower stratosphere (UTLS), due to the combination of long-term increase and the seasonal cycle of CO₂.

Using a compilation of vertical trace gas profiles derived from measurements with the balloon-based AirCore technique together with ECHAM/MESSy Atmospheric Chemistry (EMAC) model data, we investigate how the seasonality of CO₂ in the troposphere propagates into the lowermost stratosphere. Simulating an artificial, deseasonalised CO₂ tracer enables us to separate and study the seasonal cycle in a unique way in remote areas and on a global scale. Our results show that the tropospheric CO₂ seasonal cycle is strongly modulated in the extratropical UTLS region, characterised by a substantial change in amplitude, a phase shift of several months and a tilt in the shape of the seasonal cycle, which can be associated with the transport barrier related to the strength of the subtropical jet. In the stratosphere, we identified both a vertical and a horizontal “tape recorder” of the CO₂ seasonal cycle. Originating in the tropical tropopause region this imprint is linked to the upwelling and the shallow branch of the Brewer-Dobson circulation.

To validate these model-based findings we developed a strategy to isolate the seasonal signal in observational data as well. This requires CO₂-independent Age of Air (AoA) information to disentangle seasonality from the combined effect of transport and long-term trend. To achieve this, we choose an approach using a normalised methane vs. mean age correlation based on independent observational data. We present average vertical profiles of the isolated CO₂ seasonal signal for latitude bands with sufficient AirCore measurement coverage. Statistical analyses are then used to assess the robustness and representativeness of these results and to determine whether AirCore observations can be used to constrain the CO₂ seasonality in the UTLS.

How to cite: Degen, J., Baier, B. C., Jöckel, P., Lachnitt, H.-C., Menken, J. M., Schuck, T. J., Sweeney, C., and Engel, A.: The CO2 seasonal signal as a transport diagnostic in the UTLS , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11979, https://doi.org/10.5194/egusphere-egu26-11979, 2026.

Vertical transport processes, such as for example associated with Warm Conveyor Belt airstreams (WCBs) which is defined as a coherent strongly ascending airstream associated with extratropical cyclones, play a critical role in determining the distribution of greenhouse gases within the Upper Troposphere and Lower Stratosphere (UTLS). This study evaluates the performance of two global numerical weather prediction models, ICON-ART (ICOsahedral Nonhydrostatic model with Aerosol and Reactive Trace gases) and IFS (Integrated Forecasting System), in simulating CO₂ mixing ratios during the winter of 2022. Model outputs with different resolutions are compared against in situ measurements from the IAGOS (The In-service Aircraft for a Global Observing System, https://iagos.aeris-data.fr/) infrastructure during transatlantic flights during a period characterised by strong latitudinal CO₂ gradients and vigorous synoptic activity.

The analysis focuses on specific flight campaigns where measured CO₂ mixing ratios exhibited distinct enhancements of 4–6 ppm above background levels in the mid-Atlantic UTLS region. To attribute these anomalies to specific meteorological features, a multi-diagnostic approach is employed. A machine learning algorithm to detect footprints of WCB inflow, ascent and outflow regions (ELIAS 2.0; Quinting et al., 2022) is utilised alongside HYSPLIT Lagrangian backward trajectories initialised from flight coordinates to characterise air mass origin relative to cyclone evolution.

Results reveal persistent model–data discrepancies during January–February 2022, with both ICON-ART and IFS underestimating observed CO₂ spikes by 1–5 ppm. Our analyses show a spatial proximity between WCB activity and elevated CO₂ anomalies suggesting vertical transport of air with distinct chemical signatures from the boundary layer into the upper troposphere through the WCB air stream. Specifically, we find co-located high WCB ascent probabilities (0.4 – 0.8). Moreover, trajectory origins over eastern North America confirm that surface-influenced air masses are lifted via the WCB airstream. We hypothesise that systematic biases in simulated CO₂ distributions stem from model misrepresentation of vertical transport processes and/or uncertainties in emission inventories and natural fluxes, as well as missing chemical production of CO₂ in both modelling frameworks.

These findings highlight the value of combining machine learning-based flow identification with in situ observations to diagnose transport errors in atmospheric models. As WCB activity is projected to intensify under climate change scenarios, improved representation of both synoptic-scale ascent and parametrised turbulent mixing is critical for reducing uncertainties in modelled CO₂ distributions and constraining the global carbon budget.

How to cite: Qor-el-aine, A., Versick, S., Oertel, A., and Agusti-Panareda, A.: Diagnosing CO2 Transport in the North Atlantic Upper Troposphere: Evaluation of ICON-ART and IFS using IAGOS Observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2278, https://doi.org/10.5194/egusphere-egu26-2278, 2026.

Halogens deplete tropospheric and stratospheric ozone, but the role of iodine remains elusive. Nevertheless, recent research has demonstrated iodine’s wide-ranging impact on tropospheric photochemistry. We report airborne measurements of atmospheric iodine oxide (IO) concentrations up to 15 km altitude from two flights of the WISE (Wave-driven ISentropic Exchange) campaign over the mid-Atlantic in September and October 2017. IO was retrieved from limb-scattered skylight in the upper troposphere (UT) using the airborne mini-DOAS instrument onboard the German High Altitude and Long Range research aircraft (HALO). Up to sixfold elevated IO mixing ratios (up to 0.6 ± 0.1 ppt) above background levels were observed in the UT in air masses transported by the category 5 hurricanes Irma and Maria, as indicated by CLaMS back-trajectory analyses. Atmospheric iodine predominantly originates from marine inorganic (I₂, HOI) and organic (CH₃I, CH₂I₂, CH₂IBr, and CH₂ICl) emissions. Our findings suggest that enhanced IO mixing ratios are likely driven by enhanced marine iodine emissions associated with high surface wind speeds in the vicinity of hurricanes, photochemical conversion of source gases into reactive iodine and efficient vertical transport of these iodine-rich air masses by tropical cyclones. Furthermore, our observations imply a potentially significant role of iodine-driven chemistry in air masses affected by tropical storms.

How to cite: Voss, K., Vogel, B., Diederich, T., Engel, A., Grooß, J.-U., Keber, T., Kluge, F., Rotermund, M. K., Schuck, T., Weyland, B., Butz, A., and Pfeilsticker, K.: Tropical cyclones drive enhanced inorganic iodine in the mid-latitude upper troposphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7037, https://doi.org/10.5194/egusphere-egu26-7037, 2026.

The upper troposphere and lower stratosphere (UTLS) serves as the transition region between the two atmospheric layers. Chemical constituents, aerosols, and water are transported in coherent airstreams or mixed from their tropospheric source regions into the UTLS. Therefore, the vertical and geographical distribution of all these constituents in the UTLS strongly depends on the transport or mixing pathways and on their (tropospheric) sources. Furthermore, during the transport constituent concentrations can be modified due to many microphysical and chemical processes. A detailed understanding about the constituent concentrations in the UTLS is needed because of their dynamic-radiative-chemical coupling, which affects atmospheric tracer distributions, the radiative budget and the dynamics of the tropopause.  

We present a 10-year climatology of selected aerosol (dust, sea salt, sulphate), tracer (CO) and greenhouse gas (CO2 and CH4) concentrations at the dynamical tropopause (2-pvu isosurface).  We make use of the Copernicus Atmosphere Monitoring Service reanalysis datasets CAMSRA and CAMS GHG and combine these Eulerian climatologies with a Lagrangian troposphere-to-stratosphere transport (TST) climatology to determine and characterize the pathways from the constituent sources to the UTLS. This way, we analyse anomalies of aerosols and greenhouse gases at the tropopause that arise from the TST, and we compare them to seasonal atmospheric composition climatology.

We show that the TST trajectories that originated from the boundary layer (deep TST) lead to stronger anomalies at the dynamical tropopause. The concentration patterns at the dynamical tropopause for different species depend on the emissions at the surface, the exact dynamical pathway from the surface to the UTLS, and the sinks and sources along the way. For example, we demonstrate that dust concentrations at the dynamical tropopause over Asia are mostly from local dust source regions but they can be enhanced with dust advection from other regions.

How to cite: Müürsepp, T., Joos, H., Wernli, H., and Sprenger, M.: Aerosols and greenhouse gases at the dynamical tropopause: Lagrangian transport and climatology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8259, https://doi.org/10.5194/egusphere-egu26-8259, 2026.

Volcanic injections into the upper troposphere-lower stratosphere (UTLS) affect climate by altering Earth's radiation budget and atmospheric chemistry. However, the pathways by which mid-latitude eruptions spread globally remain poorly understood. We combine nighttime Compact Optical Backscatter Aerosol Detector (COBALD) profiles over Lhasa with ERA5-driven Chemical Lagrangian Model of the Stratosphere (CLaMS) backward trajectories and global three-dimensional sulfur dioxide (SO₂)-based tracer simulations. With this integrated framework, we track the Raikoke plume (21-22 June 2019; VEI 4) as it evolved within the mature Asian Summer Monsoon Anticyclone (ASMA). Balloon-borne measurements capture the plume’s arrival, vertical spreading, and dilution by ASMA-interior air. Trajectories reveal two principal pathways from distinct Raikoke plumes: (i) an upper-level branch within the summertime stratospheric easterly flow (460-490 K) carrying the trailing filament of the vorticized volcanic plume (VVP), and (ii) a lower-level branch within the subtropical westerly jet (390-430 K) carrying the main plume. Although the ASMA can act as a transport barrier at certain potential-temperature levels, it admits in-mixing along jet-aligned filaments and redistributes aerosols internally. SO₂-based tracer simulations are sensitive to how parameterized small-scale mixing is represented in CLaMS, underscoring the need to adjust subgrid-scale mixing parameterizations when model resolution changes (here, from ERA-Interim to ERA5 reanalyses). Portable Optical Particle Spectrometer (POPS) profiles over Boulder (USA) confirm the plume’s timing and altitude, providing an independent evaluation away from the ASMA region.

How to cite: Yang, Z.: Transport of volcanic aerosol from the Raikoke eruption in 2019 through the Northern Hemisphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11329, https://doi.org/10.5194/egusphere-egu26-11329, 2026.

For many important trace gases, precise observations in the upper troposphere and lower stratosphere (UTLS) are spatially and/or temporally sparse and/or inhomogeneous. This is particularly problematic since the UTLS features a highly variable transition region between the troposphere and stratosphere. We here present a new dataset obtained from weather balloon-based sensors from the Juelich Modular Balloon Observatory (JUMBO) across the summer and autumn of 2025. Between 27th May and 5th November, 46 balloons were launched on an almost weekly basis from near Jülich, Germany, typically reaching altitudes of around 30 km.  Of those, 37 flights focused on trace gas composition, including 18 near-simultaneous double launches quantifying the vertical distribution of water vapour and ozone as well as multiple halogenated species. The JUMBO campaign was planned, among other objectives, to infer the impact of the Asian summer monsoon on the UTLS over Germany during the full monsoon period. The observations thus enabling access to the temporal evolution of many key UTLS components over a period of about 5 months. Focusing on chlorinated very short-lived substances (Cl-VSLSs) and water vapor in combination with a model-based regional tracer approach, we also investigate the influence of the Asian Summer Monsoon as the respective anticyclone increasingly exports air masses into the global northern hemispheric UTLS.

How to cite: Rolf, C., Laube, J., Geldenhuys, M., Mainika, A., Vogel, B., and Hegglin, M. I.: Balloon-based observations of trace gas composition over Europe/Germany in summer 2025, impacted by the Asian Summer Monsoon, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10333, https://doi.org/10.5194/egusphere-egu26-10333, 2026.

The Asian Summer Monsoon (ASM) plays a major role in lifting surface pollutants into the upper troposphere, influencing air quality and climate at regional and global scales. We use 17 years (2007-2023) of carbon monoxide (CO) satellite observations from the Infrared Atmospheric Sounding Interferometer (IASI) to study variability in the ASM region. Seasonal cycles, long-term changes, and dynamical processes such as eddy shedding are analyzed to understand how pollution is transported within and beyond the ASM anticyclone.

IASI measurements show good agreement with aircraft observations from the 2022 Asian Summer Monsoon Chemical and CLimate Impact Project (ACCLIP) campaign, confirming the reliability of satellite data for assessing pollution in the upper troposphere. Climatological CO patterns reveal persistent enhancements associated with ASM circulation features, demonstrating IASI’s ability to capture monsoon dynamics over extended periods. Two case studies using IASI observations illustrate upper-tropospheric CO transport from the ASM: the first, supported by GEOS-FP (Goddard Earth Observing System – Forward Processing) simulations, shows consistent spatial structures over the Western Pacific during quiet and eddy shedding periods, while the second highlights how eddy shedding drives long-range transport of ASM-sourced CO across the Pacific towards North America.

These findings emphasize the value of long-term satellite observations for tracking upper-tropospheric pollution and understanding its regional and global impacts.

How to cite: Boynard, A., Viatte, C., Pan, L., Honomichl, S., Luque-Lazaro, A., Sinnathamby, S., Hadji-Lazaro, J., Smith, W., Liang, Q., D'Amato, F., Viciani, S., Campos, T., and Clerbaux, C.: Upper-Tropospheric pollution transport by the Asian Summer Monsoon from IASI observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12268, https://doi.org/10.5194/egusphere-egu26-12268, 2026.

Tropical cyclones (TCs) have received notable attention for their damaging hazards and impacts on the climate system. One under-investigated climate impact is stratosphere-troposphere exchange (STE) from TCs and accompanying upper troposphere and lower stratosphere (UTLS) composition change. STE irreversibly modifies the distribution and concentration of greenhouse gases such as water vapor and ozone in the UTLS, which is important for the radiation budget and climate. Prior case studies have individually identified multiple STE processes that can occur in TCs, however it remains unclear how these individual processes contribute to total STE in TCs, and how contributions vary by TC intensity and deep-layer shear. This study uses multiple idealized simulations conducted with Cloud Model 1 (CM1) to provide a more thorough understanding of STE in TCs, including the role of various STE process and how they vary based on TC intensity and deep-layer shear. UTLS composition change and STE is assessed using water vapor concentrations and a suite of custom passive tracers. Simulations suggest substantial hydration of the lower stratosphere occurs within the TC inner core, reaching up to 20 ppmv (4x stratospheric background) at 18.5 km (2 km above the tropopause). This hydration is spatially limited to the inner core of the TC due to surrounding cold temperatures near the tropopause. Overshooting tops within the TC and its inner core are shown to lead to substantial two-way transport. Downward transport of stratospheric air occurs (a) via subsidence within the eye and (b) in the upper portion of the near-tropopause outflow. Additional simulations reveal that TC intensity and deep-layer environmental wind shear are also important for TC STE.

How to cite: Gordon, A. and Homeyer, C.: Simulated Tropical Cyclone Impacts on Upper Troposphere Lower Stratosphere Composition , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14483, https://doi.org/10.5194/egusphere-egu26-14483, 2026.

The Arctic Springtime Chemistry Climate Investigations (ASCCI) aircraft campaign studied processes in the Arctic upper troposphere and lower stratosphere and their impact on midlatitudes in a changing climate. It was conducted between February and early April 2025 as a coordinated research effort by several German Universities and research institutes.

For the ASCCI mission, the German High Altitude and Long-Range Research Aircraft HALO was equipped with a payload consisting of a mixture of in-situ and remote sensing instruments, allowing for a detailed chemical and dynamical characterization of the lowermost polar stratosphere during late winter to early spring 2025. The campaign was especially designed to complement information from the POLSTRACC campaign carried out with HALO during the Arctic winter 2015 to 2016, and the SouthTRAC campaign in 2019 which was aimed at studying the Antarctic lower polar stratosphere during late winter and early spring. Our main aims were to study (i) the inter-annual variability of Arctic lower stratospheric ozone depletion in comparison to POLSTRACC, (ii) high latitude stratosphere-troposphere exchange and the structure of the high latitude tropopause and (iii) the impact of short-lived climate pollutants (ozone, aerosols) and their precursors on the Arctic upper troposphere.

The Arctic winter 2024/2025 was characterized by a record-cold mid-winter period, followed by an early stratospheric warming from which the polar vortex only partly recovered. Despite this warming, we were able to observe signs of heterogeneous redistribution of nitrogen species and of chemical ozone depletion. We will present an overview of the flights carried out during ASCCI and first results of the observations.

The ASCCI team

University of Mainz, Institute for Physics of the Atmosphere

Franziska Weyland, Peter Hoor, Vera Bense, Heiko Bozem, Jonas Blumenroth, Hans-Christoph Lachnitt,

Forschungszentrum Jülich (FZJ)

Jens-Uwe Grooß, Michaela Hegglin, Marc von Hobe, Tom Neubert, Felix Ploeger, Markus Retzlaff,     Christian Rolf, Georg Schardt, Nicole Spelten, Martin Riese, Sebastian Rhode, Joern Ungermann.

University of Wuppertal

Michael Volk, Valentin Lauther, Johannes Strobel, Ronja von Luijt.

Deutsches Zentrum für Luft- und Raumfahrt (DLR)

Georgios Dekoutsidis, Andreas Fix, Silke Groß, Konstantin Krüger, Andreas Schäfler, Martin Wirth, Stefan Kaufmann, Mara Montag, Elisabeth Horst, Laura Tomsche, Christiane Voigt, Helmut Ziereis.

Karlsruhe Institute of Technology (KIT)

Bastian Kirsch, Simone Scheer, Florian Obersteiner, Andreas Zahn, Felix Friedl-Vallon, Michael Höpfner, Wolfgang Woiwode, Erik Kretschmer, Georg, Wetzel, Anne Kleinert, Guid Maucher, Hans Nordmeyer, Christog Piesch, Franziska Trinkl.

University of Heidelberg

Benjamin Weyland, Karolin Voss, Maximilain Albrecht, Andre Butz, Klaus Pfeilsticker

How to cite: Engel, A. and Sinnhuber, B.-M.: The Arctic Springtime Chemistry Climate Investigations – ASCCI aircraft campaign – an overview, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17846, https://doi.org/10.5194/egusphere-egu26-17846, 2026.

Transport and mixing strongly determine the trace gas composition of the Arctic upper troposphere / lower stratosphere (UTLS), but spatial and temporal variability of the relevant processes are still not well quantified. The Arctic lowermost stratosphere (LMS) is fed via, and thus controlled by, various transport paths from regions with very differing chemical composition - young tropospheric air from the subtropics or even directly from the boundary layer, to photochemically very old air descending slowly from the mid- and high-latitude stratosphere. The HALO aircraft mission ASCCI conducted from Kiruna, Sweden in late winter and early spring 2025 aimed at investigating transport and mixing processes and time scales in the Arctic UTLS region, especially stratosphere-troposphere exchange, and the role of chemistry and tropospheric pollution for ozone in the Arctic UTLS. For answering these questions, measuring a wide range of trace gases with different source regions and chemical lifetimes ranging from days to many years is crucial.

We present a survey of first results of in situ tracer measurements made with the High Altitude Gas AnalyseR - five channel version (HAGAR-V) instrument. HAGAR-V measured a suite of more than 30 trace gases including very short-lived NMHCs (e.g. Benzene, C₂H₂, C₄H₁₀), halogenated VOC (e.g. CH₂Cl₂, CHCl₃, C₂Cl₄, CH₂Br₂), as well as longer-lived halocarbons (e.g. CH₃Cl, CH₃Br, CCl₄, Halons, HCFCs, and HFCs) every 120 s using in-flight gas chromatography and mass spectrometry. Further very long-lived species, including the age-of-air tracer SF₆, were measured every 40 s (CFC-12, SF₆) and every 80 s (CFC-11, CFC-113, H1211) using electron capture detection. Additionally, very precise CO₂ measurements by a NDIR analyser were conducted at high time resolution (5 s).

Using tracer-tracer relations of short-lived pollutants with long-lived tracers, we can distinguish between different transport and mixing processes in the Arctic UTLS. Besides observing mixing of fresh tropospheric air with older stratospheric air at the extratropical tropopause, we also identified rather young (several months) air transported from the tropical tropopause layer (TTL) to the high latitude stratosphere and mixing with old polar air at potential temperatures about 380 K. We also observed pollution by short-lived chlorinated substances such as CH₂Cl₂, CHCl₃, C₂Cl₄ and 1,2-dichloroethanein the Arctic, likely from both regional and remote sources. Besides the analysis of transport processes, we also derived the mean age of air both from SF₆ and CO₂. Using both species independently increases the reliability of the calculated ages significantly.

How to cite: Strobel, J., van Luijt, R., Lauther, V., Weyland, F., Bozem, H., Kessler, S., Hoor, P., and Volk, C. M.: Transport and mixing of pollutants into the Arctic LMS derived from HAGAR-V in situ observations of a wide range of trace gases during the HALO ASCCI mission, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20116, https://doi.org/10.5194/egusphere-egu26-20116, 2026.

The tropopause represents a central feature of the vertical structure of the atmosphere, marking the transition between the troposphere and stratosphere. While common definitions such as the thermal tropopause (TTP) defined by the WMO primarily rely on quantitates that  are conserved under adiabatic processes, diabatic effects resulting from radiation, cloud processes, or turbulence are also decisive for the tropopause structure.

We propose a new definition of the tropopause based on the vertical gradient of the relative humidity with respect to ice (RHi), named the RHi Gradient Tropopause (RHi-GT). The RHi-GT is determined using a simple, robust gradient method. We demonstrate that the RHi-GT definition is generally consistent with the TTP but often provides a clearer characterization. In individual profiles, the RHi-GT coincides more closely with regions that mark a clear transition in atmospheric structure, such as sharp gradients in absolute humidity or increases in static stability. Furthermore, when examining mean profiles over the 10-year period relative to the RHi-GT, both RHi and static stability show a more coherent and distinct transition between the moist troposphere and the very dry stratosphere compared to when referenced to the TTP.

How to cite: Reutter, P. and Spichtinger, P.: The frosty frontier: redefining the mid-latitude tropopause using the relative humidity over ice, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1775, https://doi.org/10.5194/egusphere-egu26-1775, 2026.

Convection represents a major pathway of moisture and trace species into the upper troposphere. However, the role of convection for cross-tropopause transport is still under discussion, e.g., the processes of moisture entry into the lower stratosphere. Deep convection highly perturbs the local tropopause structure, which is partly reversible. It thus can either penetrate the tropopause, injecting trace species (including water) into the stratosphere, but it can also lead to a lifting of the tropopause as the thermals from below and their adiabatic cooling reach to higher altitude.

In this study, we analyse model results on the kilometer scale of idealised deep convective events and analyse the modifications of the tropopause above convection, using both thermal as well as dynamical tropopause definitions. Furthermore, we depict how the thermal structure of the atmosphere is modified after the convective event, effectively changing the tropopause altitude. Additionally, we determine the amount of water vapour transported to elevated altitudes above the original tropopause and how much water irreversibly enters the stratosphere. The exchange also encompasses downward transport of air masses with stratospheric characteristics into the troposphere. We analyse, which factors (e.g., vertical wind shear, CAPE and the strength of the initial disturbance) show the strongest influence on the tropopause modifications and, therefore, assess the effects of deep convection for the UTLS.

How to cite: Tost, H. and Hoor, P.: Convective local tropopause modifications and entrance into the stratosphere - a modelling perspective, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6812, https://doi.org/10.5194/egusphere-egu26-6812, 2026.

Long-term global trace gas observations can be used to define chemical tropopauses, which are not limited by the availability or resolution of vertical profiles or reanalysis data sets. Tracers that can be used for these definitions need to show clearly defined differences in stratospheric vs. tropospheric characteristics with notable examples being O₃, N₂O, CO or H₂O.

We focus on two approaches: (1) the definition of an O₃-based tropopause using a clearly defined climatology expressed as mixing ratios relative to the tropopause and (2) the filtering of stratospheric data by applying an iterative baseline filter on N₂O measurements. Our objective is to provide clear, globally applicable definitions of these chemical tropopauses, that can be easily applied to new measurement data and provide a representative distance to the tropopause.

We analyse globally distributed ozone sonde measurements, as well as aircraft and balloon measurements of N₂O, in combination with meteorological parameters from interpolated ERA5 reanalyses. By evaluating profiles for each month and geographical region, a representative distance to the tropopause can be assigned to any measurement of O₃ or N₂O. We further investigate the sensitivity of these assignments to spatial and temporal factors and apply these to separate measurement data sets.

Tropopause-relative coordinates are beneficial for trace gas analysis in regions close to the tropopause. However, this effect diminishes with greater distances. We examine for which lower and upper boundaries tropopause-relative coordinates remain beneficial.

How to cite: Bauchinger, S., Engel, A., Zahn, A., Bönisch, H., Lachnitt, H.-C., Krysztofiak, G., and Schuck, T.: Climatology-based chemical tropopauses from global O3 and N2O observations since 1980, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10707, https://doi.org/10.5194/egusphere-egu26-10707, 2026.

The tropopause is an important boundary (or transition layer) for many studies in the atmospheric sciences and is an indicator of global climate change. Numerous characteristics of the tropopause have been shown to exhibit significant long-term change over the most recent 40+ year period, including height, temperature, and the occurrence of multiple tropopauses. However, prior observational studies rely mostly upon suitable records of radiosondes, which are only available over land and vary in coverage globally. Observation-based model reanalyses have been used in many studies to provide global coverage but resulting assessments of tropopause characteristics are not always consistent with observational analyses. In recent decades, the emergence of global navigational satellite system (GNSS) radio occultation (RO) atmospheric profiles provides an observational record with global coverage and fine vertical resolution necessary for tropopause analysis. The GNSS-RO data record is now approaching the period length necessary for robust assessment of long-term changes (trends). In this study, we leverage a continuous record of nearly 25 years of GNSS-RO data and apply two universal tropopause definitions, the WMO temperature lapse-rate tropopause (LRT) and the potential temperature gradient tropopause (PTGT), to evaluate global tropopause characteristics and their long-term changes. We find widespread increases in multiple tropopause frequency in the midlatitudes, consistent with several recent radiosonde and reanalysis studies. We also find regionally varying changes in tropopause height and temperature, which in some cases imply changes in the width of the tropics. Results are generally insensitive to the choice of LRT or PTGT definition. Implications of the diagnosed changes and their relationships to drivers of climate variability will be discussed.

How to cite: Homeyer, C. and Tinney, E.: Long-term Changes in Global Tropopause Characteristics from GNSS-RO Observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14224, https://doi.org/10.5194/egusphere-egu26-14224, 2026.

Methane is a potent greenhouse gas with an increasing trend in the atmosphere due to rising emissions. Aside from its climate impacts, it is important to monitor methane because its long lifetime of about ten years makes it a useful tracer of atmospheric transport. As a result, modelled methane fields can therefore be compared with observations to evaluate transport in atmospheric models. Several methods have been proposed for assessing the strength of the subtropical mixing barrier and the polar vortex edge using long-lived tracers, but most require high data density. In addition, it is difficult to separate the effects of mixing from those of chemical production and loss or from other aspects of atmospheric transport. In this study, we explore various methods of using methane probability density functions and time series to quantify the strength of the subtropical mixing barrier and the polar vortex edge, based on comparisons with relatively sparse satellite measurements from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). ACE-FTS is a solar occultation instrument with near-global coverage and 3–4 km vertical resolution, spanning the upper troposphere to the lower mesosphere. The focus of the comparisons is on a specified dynamics run of the Canadian Middle Atmosphere Model (CMAM39-SD) for the 2004-2018 period. In general, we find that the modelled subtropical mixing barrier is too weak in the lower stratosphere and too strong in the upper stratosphere. In contrast, CMAM39-SD reproduces methane variability near the polar vortex edge very well. To provide context, we also compare ACE-FTS with the air quality model GEM-MACH, the Earth system model MRI-ESM2, and the chemical transport model GEOS-Chem.

How to cite: Saunders, L., Walker, K., Plummer, D., Pendlebury, D., Whaley, C., Oshima, N., Sheese, P., Chien, R.-Y., Fu, J., Manney, G., and Millán, L.: Using ACE-FTS to assess mixing barrier strength in nudged chemistry-climate models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14363, https://doi.org/10.5194/egusphere-egu26-14363, 2026.

The large-scale zonal-mean transport of tracers, such as ozone and water vapor, is governed by global circulations. Because of the radiative effects of tracers, an accurate representation of their transport in climate models is essential for reliable climate simulations. Small-scale processes such as gravity waves and turbulence can significantly influence the transport and distribution of tracers. As these processes are unresolved in most weather and climate models, their effects must be parameterized. We present the novel parameterization for the direct impact of gravity waves on tracer transport (Knop et al., 2026). Using multiple-scale analysis of the governing atmospheric equations, we derive expressions for gravity wave–induced tracer fluxes, enabling a physically based parameterization. The parameterization is thoroughly validated by comparing idealized simulations with parameterized waves to wave-resolving reference simulations. Finally, we aim to extend the theory to include turbulent effects.

How to cite: Steiger, I., Suresh, D., Dolaptchiev, S., and Achatz, U.: Impact of Small-Scale Gravity Waves on Tracer Transport, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10858, https://doi.org/10.5194/egusphere-egu26-10858, 2026.

Winds in the tropical lower stratosphere raise difficulties for numerical weather prediction models: without geostrophy, winds decouple from temperature and direct observations are scarce. The Strateole 2 project explores the tropical lower stratosphere using superpressure balloons that drift for up to three months between 18 and 21 km altitude. Wind measurements from the technological campaign (2019–2020) and the first scientific campaign (2021–2022) are used to assess errors in the ERA5 reanalysis for latitudes between 18° S and 10° N. The comparison reveals significant errors, with standard deviations of 3.76 m s−1 for zonal and 3.24 m s−1 for meridional wind. Relative to a previous comparison in 2010, only a modest decrease of 20 % and 10 % is found, revealing the persistent difficulty of modeling winds in the tropical lower stratosphere. 

Additionally, the errors in modelled balloon trajectories are also assessed, with a focus on the predictability of the trajectories. It is shown that the initial error in the wind gives a reliable indication on the skill of the subsequent forecast. Trajectory calculations have very variable skill, with median errors after 24 h of 260 km, but a tenth of the errors larger than 600 km. Factors leading to large errors, such as initial wind error and latitude are identified. 

Certain instruments onboard Strateole 2 balloons measure features below the balloons (temperature, thin cirrus, water vapour..). While the sampling of air at balloon flight level is quasi-Lagrangian, observations of features below the balloon describe both spatial and temporal variations. In order to disentangle these and facilitate the interpretation of observations made below the balloons, we document the dispersion of air below the balloons (altitudes between about 15 and 21 km). Trajectory dispersion of air below the balloon is very variable, depending on the initial shear. The persistent errors highlight the need for regular obsevations of winds in the tropical lower stratosphere. 

Overall, we emphasize the need for caution when using trajectory calculations for process studies.

How to cite: Plougonven, R., Cadiou, P., Podglajen, A., Hertzog, A., and Mac Farlane, A.: Strateole 2 superpressure balloons reveal persistent errors in reanalyzed winds in the tropical lower stratosphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14427, https://doi.org/10.5194/egusphere-egu26-14427, 2026.

Tropical upwelling transports air masses across the tropical tropopause into the lower stratosphere and constitutes the ascending branch of the global mean stratospheric circulation. The strength of the tropical upwelling influences the thermal characteristics and chemical composition of the lower stratosphere and the transition region between troposphere and stratosphere, the tropical tropopause layer (TTL). Given the lack of direct measurements and the small magnitude of vertical velocities, the variability and long-term changes of tropical upwelling are difficult to determine and poorly constrained in meteorological analysis data.

Here we use water vapor measurements from the MLS (Microwave Limb Sounder) instrument to determine interannual variations and long-term changes in tropical upwelling in the lower stratosphere for 2005-2023. Our upwelling estimates represent an effective vertical transport velocity and provide an estimate of the speed of the vertical branch of the stratospheric circulation. We show that interannual variations of the tropical upwelling are anti-correlated with TTL temperatures derived from Global Navigation Satellite System – Radio Occultation (GNSS-RO) measurements with warmer (colder) temperatures coinciding with years of less (more) upwelling. A regression analysis results in a negative upwelling trend of  consistent with positive temperature trends in the TTL. Upwelling is also found to be anti-correlated with independent time series of ozone and other gases in the lower stratosphere.

We compare the observational upwelling estimates to residual vertical velocity from four reanalysis and find very good agreement of the interannual variability between all data sets. The reanalysis eddy and momentum fluxes are used to investigate the impact of extratropical waves on tropical upwelling. Our analysis shows that a large fraction of the interannual variability in tropical upwelling is associated with waves propagating meridionally into the subtropical stratosphere.

How to cite: Tegtmeier, S., Abalos, M., and Randel, W.: Tropical upwelling in observations and reanalyses, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15393, https://doi.org/10.5194/egusphere-egu26-15393, 2026.