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The Arctic winter 2015/2016 was characterized by extremely low temperatures in the stratosphere and by a very strong polar vortex, accompanied by extended fields of Polar Stratospheric Clouds. During this winter, aircraft-based measurements were carried out with the research aircraft HALO (High Altitude and Long-Range Research Aircraft) from Kiruna/Sweden and Oberpfaffenhofen/Germany.Total reactive nitrogen and its distribution between the gas and particle phases are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. Tracer-tracer correlations were applied to study the vertical redistribution of gas-phase total reactive nitrogen. The extended observation period from December to March provided the opportunity to study the changing distribution of reactive nitrogen in the lowermost Arctic stratosphere during the course of the winter. In early winter, during December, the lowermost Arctic stratosphere did not show any indications for disturbed conditions.The situation changed during the observational period in January and February. Tracer-tracer correlations showed elevated levels of total reactive nitrogen of up to 6 nmol/mol. These observations could be interpreted by evaporation of polar stratospheric particles falling down from the stratosphere above and leading to a nitrification of the lowermost stratosphere. During some periods up to 60 % of the observed total reactive nitrogen can be attributed to evaporating particles. The observation of gas phase nitrification was accompanied by the occurrence of particulate nitrate in extended regions at altitudes between about 10 and 14 km. Usually, the occurrence of particulate nitrate is rare at such altitudes. The diameter of these particles was estimated to range between about 9 and 18 µm.During the late-winter observation period, no indications for polar stratospheric cloud particles at flight altitude were found. However, extended regions with elevated gas-phase concentrations of total reactive nitrogen were still observed. In late winter, the subsidence of air masses from the polar vortex became increasingly important for the distribution of total reactive nitrogen in the lowermost stratosphere. Air masses with substantial denitrification of up to 5 nmol/mol were observed. In these cases, up to 50 % of the undisturbed total reactive nitrogen was missing. Concurrently lower ozone concentrations were observed, indicating destruction of ozone at higher altitudes.Nitrification and denitrification of the lowermost stratosphere during the course of the winter are linked by heterogeneous processes in the above-lying stratosphere. Simulations with the CLaMS model confirm and complement the findings of the in-situ observations. They also suggest that the observations have been representative of the vortex-wide redistribution of total reactive nitrogen. The aircraft-based in-situ measurements provided a comprehensive picture of the temporal evolution of the distribution of total reactive nitrogen over the entire winter period 2015/2016.

Climate change is happening in a transient manner – with continuously increasing greenhouse gases in the atmosphere, humans have started a radiative imbalance that leads to rising near-surface temperatures. However, there are good reasons why it makes sense to look at quasi-equilibrium climate change simulations. In such simulations, we approximate climate change by “fixing” the amount of long-lived greenhouse gases and use recurring boundary conditions that are representative of a particular year - past, present or future. With such a setup any climate model should simulate a stable climate (after a spin-up phase) that reveals internal variability and does not show any trends. It is a necessary condition for the validity of the model - if no transience is provided in the boundary conditions – that the model does not drift. With such a model configuration, it is possible to estimate probability density functions, because each year of a multi-annual integration is an equally valid realisation for the meteorology of the pre-selected year. Using such a time-slice approach, sensitivities to well-specified individual changes can be assessed. Here, we provide a range of examples using the ICON-ART modelling system to investigate (idealised) climate change scenarios with respect to different threshold temperatures, jet variability and the climatic impact of the ozone hole. We illustrate how such integrations allow the unambiguous attribution of certain climate change effects, e.g. the change of jet stream variability under global warming or the contribution of the ozone hole to regional surface warming. However, we caution against a strict causality chain of processes in explaining the response, because given the nature of the quasi-equilibrium modelled, consistency might not always imply causality.

During winter 2015/2016 the Arctic stratosphere was characterized by extraordinarily low temperatures in connection with the occurrence of extensive polar stratospheric clouds. From mid of December 2015 until mid of March 2016 the German research aircraft HALO (High Altitude and Long–Range Research Aircraft) was deployed to probe the lowermost stratosphere in the Arctic region within the POLSTRACC (Polar Stratosphere in a Changing Climate) mission. More than twenty flights have been conducted out of Kiruna/Sweden and Oberpfaffenhofen/Germany, covering the whole winter period. Besides total reactive nitrogen (NOy), observations of nitrous oxide, nitric acid, ozone and water were used for this study. Total reactive nitrogen and its partitioning between gas- and particle phase are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. The redistribution of total reactive nitrogen was evaluated by using tracer–tracer correlations. In January air masses with extensive nitrification were encountered at altitudes between 12 and 15 km. The excess NOy amounted up to about 6 ppb. During several flights, along with gas–phase nitrification, indications for extensive occurrence of nitric acid containing particles at flight altitude were found. These observations support the assumption of sedimentation and subsequent evaporation of nitric acid containing particles leading to redistribution of total reactive nitrogen. Remnants of nitrified air masses have been observed until mid of March. Between end of February and mid of March also de-nitrified air masses have been observed in connection with high potential temperatures. Using tracer–tracer correlations, missing total reactive nitrogen was estimated to amount up to 6 ppb. This indicates the downward transport of air masses that have been denitrified during the earlier winter phase. Observations within POLSTRACC, at the bottom of the vortex, reflect heterogeneous processes from the overlying Arctic winter stratosphere. The comparison of the observations with CLaMS model simulations confirm and complete the picture arising from the present measurements. The simulations confirm, that the ensemble of all observations is representative for the vortex–wide vertical NOy-redistribution.

The Arctic winter 2015/16 was characterized by exceptionally cold stratospheric temperatures, favouring the formation of polar stratospheric clouds (PSCs) from mid-December until the end of February down to low stratospheric altitudes. Observations by GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) on HALO (High Altitude and LOng range research aircraft) during the PGS (POLSTRACC/GW-LCYLCE II/SALSA) campaign from December 2015 to March 2016 allow an investigation of the influence of denitrification on the lowermost stratosphere (LMS) with a high spatial resolution. For the first time vertical cross-sections of nitric acid (HNO3) along the flight track and tracer-tracer correlations derived from the GLORIA observations document detailed pictures of wide-spread nitrification of the Arctic LMS during the course of an entire winter. GLORIA observations show large-scale structures and local fine structures with strongly enhanced absolute HNO3 volume mixing ratios reaching up to 11 ppbv at altitudes of 11 km in January and nitrified filaments persisting until the middle of March. Narrow streaks of enhanced HNO3, observed in mid-January, are interpreted as regions recently nitrified by sublimating HNO3-containing particles. Overall, a nitrification of the LMS between 5.0 ppbv and 7.0 ppbv at potential temperature levels between 350 and 380 K is estimated. This extent of nitrification has never been observed before in the Arctic lowermost stratosphere. The GLORIA observations are compared with CLaMS (Chemical Lagrangian Model of the Stratosphere) simulations. The fundamental structures observed by GLORIA are well reproduced, but differences in the fine structures are diagnosed. Further, CLaMS predominantly underestimates the spatial extent of maximum HNO3 mixing ratios derived from the GLORIA observations as well as the enhancement at lower altitudes. Sensitivity simulations with CLaMS including (i) enhanced sedimentation rates in case of ice supersaturation (to resemble ice nucleation on NAT), (ii) a global temperature offset, (iii) modified growth rates (to resemble aspherical particles with larger surfaces) and (iv) temperature fluctuations (to resemble the impact of small-scale mountain waves) mostly improve the agreement with the GLORIA observations. The sensitivity simulations suggest that details of particle microphysics play a significant role for simulated LMS nitrification in January, while air subsidence, transport and mixing become increasingly important towards the end of the winter.

The Arctic winter 2015/16 was characterized by cold stratospheric temperatures. Here we present a comprehensive view of the temporal evolution of chlorine in the lowermost stratosphere over the course of the studied winter. We utilize two-dimensional vertical cross sections of ozone (O3) and chlorine nitrate (ClONO2), measured by the airborne limb imager GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) during the POLSTRACC/GW-LCYCLE II/GWEX/SALSA campaigns, to investigate the tropopause region in detail. Observations from three long-distance flights in January, February, and March 2016 are discussed. ClONO2 volume mixing ratios up to 1100 pptv were measured at 380 K potential temperature in mesoscale structures. Similar mesoscale structures are also visible in O3 measurements. Both trace gas measurements are applied to evaluate simulation results from the chemistry transport model CLaMS (Chemical Lagrangian Model of the Stratosphere) and the chemistry–climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry). These comparisons show agreement within the expected performance of these models. Satellite measurements from Aura/MLS (Microwave Limb Sounder) and SCISAT/ACE-FTS (Atmospheric Chemistry Experiment – Fourier Transform Spectrometer) provide an overview over the whole winter and information about the stratospheric situation above the flight altitude. Time series of these satellite measurements reveal unusually low hydrochloric acid (HCl) and ClONO2 at 380 K from the beginning of January to the end of February 2016, while chlorine monoxide (ClO) is strongly enhanced. In March 2016, unusually rapid chlorine deactivation into HCl is observed instead of deactivation into ClONO2, the more typical pathway for deactivation in the Arctic. Chlorine deactivation observed in the satellite time series is well reproduced by CLaMS. Sensitivity simulations with CLaMS demonstrate the influence of low abundances of O3 and reactive nitrogen (NOy) due to ozone depletion and sedimentation of NOy-containing particles, respectively. On the basis of the different altitude and time ranges of these effects, we conclude that the substantial chlorine deactivation into HCl at 380 K arose as a result of very low ozone abundances together with low temperatures. Additionally, CLaMS estimates ozone depletion of at least 0.4 ppmv at 380 K and 1.75 ppmv at 490 K, which is comparable to other extremely cold Arctic winters. We have used CLaMS trajectories to analyze the history of enhanced ClONO2 measured by GLORIA. In February, most of the enhanced ClONO2 is traced back to chlorine deactivation that had occurred within the past few days prior to the GLORIA measurement. In March, after the final warming, air masses in which chlorine has previously been deactivated into ClONO2 have been transported in the remnants of the polar vortex towards the location of measurement for at least 11 d.