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The reduction of carbon emissions is required to limit global warming. Thus, measurement-based methods to monitor the progress in reducing emissions are essential. A significant part of the global emissions is produced in urban areas where also industry is located. Here, we investigate the urban area of Thessaloniki to better understand the distribution of local greenhouse gas sources during October 2021 and the summer of 2022.We present results of a measurement campaign using a pair of Fourier-Transform Infrared (FTIR) Spectrometers of the type EM27/SUN developed by Bruker and KIT.The measurement campaign took part in the framework of the Collaborative Carbon Column Observing Network (COCCON). During the campaign the spectrometers were used in an up- downwind setup. One spectrometer was located in a central position of the city while the second instrument was transported to various locations at the boundaries of the city, selected according to the prevailing wind direction. Additionally, measurements to characterize the advected background variability were performed. Here, the spectrometers were arranged orthogonal to the prevailing wind direction to estimate the variability of the background concentrations of carbon dioxide and methane.In total, 30 days of measurements were collected, giving a comprehensive dataset to study the emissions of the city area. The measurements are interpreted using a box model to estimate the averaged emission area fluxes. We present results for both, carbon dioxide and methane, and discuss the temporal and spatial variability we encountered.In a next step, we aim to confront these measurements with simulation results with idealized tracer emission patterns using the ICON-ART modeling framework, a numerical weather forecast model developed and used operationally by the German Weather Service (DWD). The results from the box model introduced above will be used as a starting point to combine measurements and simulations. First simulations produced for this purpose will be shown.

The ICOsahedral Non-hydrostatic (ICON) modelling system was originally developed by DWD and MPI-M for a range of weather (forecast) and climate applications. An Aerosols and Reactive Tracers (ART) module was added by KIT to enable a comprehensive assessment of composition interactions within the atmospheric domain. Recognising that atmospheric processes happen on a multitude of temporal and spatial scales, flexible horizontal and vertical grid options are a key element of versatile model configurations in use. Here, we present a selection of results from different ICON-ART configurations that explore (stratospheric) ozone-climate interactions and stratosphere-troposphere coupling – e.g. regional climatic impacts of the ozone hole (and ozone losses in other regions) and global warming induced changes in jet-streams – in different types of integrations. In addition, we explore the potential to forecast “chemical weather” with ICON-ART, including environmental (UV) indices.Starting with time-slice experiments, we provide a range of examples using the ICON-ART modelling system to investigate (idealised) climate change scenarios with respect to different threshold temperatures (reached under global warming) and the climatic impact of the ozone hole (and ozone losses in other regions). For the latter, halogen induced depletion of (stratospheric) ozone can be switched on and off in our modelling world. We illustrate how such integrations allow the unambiguous attribution of certain climate change effects, e.g. the contribution of the ozone hole (and other regional ozone losses) to regional surface warming in Antarctica and changes to regional and global “effective radiative forcing”, and the change of jet stream variability under global warming. Moving on, we explore the capability of ICON-ART to work with regionally nested grids to capture accurately smaller spatial scales and to provide “meaningful” forecasts of environmental (UV) indices, thus, demonstrating comprehensively the seamless philosophy regarding processes, scales and applications with the flexible ICON-ART modelling system.

Stratospheric ozone (O3) absorbs biologically harmful solar ultraviolet radiation, mainly in the UV_B and UV_C spectral range. When reaching the surface, such UV radiation poses a well documented hazard to human health. In order to quantify this amount of UV radiation and to make it generally understandable, the World Health Organization has defined an UV Index[1]. It is calculated by weighting the incoming solar irradiance at surface level between 250 and 400 nanometers with their ”harmfulness” to the skin and scaling the results to values that normally range between 1 and 10, surpassing 10 for excessive UV exposure.Implementing UV Index forecasts in numerical weather prediction (NWP) models allows to alert the public in time if special care for sun protection needsto be taken. The German Weather Service (DWD) uses its NWP model ICON (ICOsahedral Nonhydrostatic Model)[2] to offer such a forecast for Germany[3]using external data such as ozone forecasts by the Royal Dutch Weather Service (KNMI) and radiation lookup tables[4].In our project we extend the capability of ICON to provide a configuration of self-consistent UV Index forecasts that do not require external data. For this, we use ICON-ART[5],[6] with a linearized ozone scheme (LINOZ)[7] and couple the prognostic ozone to the atmospheric radiation scheme Solar-J[8].Here we present the current state of our UV Index forecast system and compare our results to available reference data.References:[1] World Health Organization, World Meteorological Organization, United Nations Environment Programme, and International Commission on Non-Ionizing Radiation Protection. Global solar uv index : a practical guide,2002.[2] Günther Zängl et al.. The icon (icosahedral non-hydrostatic) modelling framework of dwd and mpi-m:Description of the non-hydrostatic dynamical core. Quarterly Journal of the Royal Meteorological Society, 2015.[3] https://kunden.dwd.de/uvi/index.jsp.[4] Henning Staiger and Peter Koepke. Uv index forecasting on a global scale. Meteorologische Zeitschrift, 2005.[5] D. Rieger et al.. Icon–art 1.0 – a new online-coupled model system from the global to regional scale. Geoscientific Model Development, 2015.[6] J. Schröter et al.. Icon-art 2.1: a flexible tracer framework and its application for composition studies in numerical weather forecasting and climate simulations. Geoscientific Model Development, 2018.[7] C. A. McLinden et al. Stratospheric ozone in 3-d models: A simple chemistry and the cross-tropopause flux. Journal of Geophysical Research: Atmospheres, 2000[8] J. Hsu, M. J. Prather et al.. A radiative transfer module for calculating photolysis rates and solar heating in climate models: Solar-j v7.5. Geoscientific Model Development, 2017.

The oxidation capacity or self-cleaning in the troposphere is mainly controlled by the existence of the OH radical. The photolysis of ozone into O(1D) and the subsequent reaction with H2O is the primary OH production, which is thus tightly related to the local solar UV actinic flux and hence the overhead ozone column. Globally, the main destruction of OH occurs by the reaction with the greenhouse methane, which lifetime itself is controlled by the concentration of the OH radical. In order to improve our understanding of the effects of anthropogenic changes of stratospheric ozone on the oxidation of the greenhouse gas methane, we perform calculations within the ICON-ART framework and report results of long-term simulations with two model configurations concerning stratospheric ozone: a) without interactive ozone, and b) with linearized interactive ozone schemes. The simulations also include a simplified OH chemistry scheme and the CloudJ scheme for the calculation of photolysis rates. With this chemical configurations of ICON-ART two long-term simulations are performed, one AMIP type simulation and one with increased temperatures in the troposphere by 4 K seen by the chemistry. This set of simulations allows to investigate whether the main influence of stratospheric ozone changes on tropospheric oxidation capacity and hence on the lifetime of CH4 is due to changes in the actinic UV flux reaching the troposphere or to tropospheric warming.

Ozone (O3) in the stratosphere absorbs the biologically harmful ultraviolet radiation from the sun (most of the UV-B radiation) and prevents it from reaching the earth's surface. The high-energy UV radiation can destroy genetic material in the cells of plants and animals, as well as humans. Without the stratospheric ozone layer, life on Earth as we know it would not be possible.The German Weather Service (DWD) provides UV index maps to inform and warn the population about high UV exposure [1]. For this purpose, data from the golobal forecast model ICON (ICOsahedral Non-hydrostatic model) [2], external ozone data and an own UV model are used to generate a forecast of the UV index, which is visualized e.g. on the DWD website as a forecast.In this project, a self-consistent framework is developed in cooperation with the DWD to generate UV index maps entirely in ICON. For this purpose, a linearized ozone scheme (LINOZ) [3] is optimized for daily ozone forecasts (including a procedure to create an initial ozone distribution). This is done as an extension of the ICON-ART structure [4] [5] (ART: Aerosols and Reactive Trace gases). A radiative transfer model for solar radiation (Cloud-J) [6] was implemented and adapted for the calculation of UV radiation fluxes and indices. Since the entire system is to be provided to the DWD as an efficient solution for UV index predictions, special emphasis is placed on comprehensive functionality at very low computational cost. An important part of the work is therefore also the validation and optimization of the procedures and workflows in order to produce reliable and high-quality forecasts.We present preliminary results of the UV radiative flux through the atmosphere modeled by ICON-ART on a global scale and over selected areas, its diurnal variation, and the influence of clouds on UV intensity.Comment:This project is funded by the German Weather Service under Extramural Research with the following number: 4819EMF03.References:[1] https://kunden.dwd.de/uvi/index.jsp[2] Zängl, G., et al., The ICON (ICOsahedral Non-hydrostatic) modelling framework of DWD MPI-M: Description of the non-hydrostatic dynamical core. Q.J.R. Meteorol. Soc., 141(687), 563-579 (2014)[3] McLinden, C. A., et al., Stratospheric ozone in 3-D models: A simple chemistry and the cross-tropopause flux, Journal of Geophysical Research: Atmospheres, 105(D11), 14653-14665 (2000)[4] Rieger, D., et al., ICON-ART - A new online-coupled model system from the global to regional scale, Geosci. Model Dev., 8(6), 1659-1676 (2015)[5] Schröter, et al., ICON-ART 2.1: a flexible tracer framework and its application for composition studies in numerical weather forecasting and climate simulations. Geosci. Model Dev., 11(10), 4043-4068 (2018)[6] Prather, M.J., Photolysis rates in correlated overlapping cloud fields: Cloud-J 7.3c. Geosci. Model Dev., 8(8), 2587-2595 (2015)

Ozon (O3) in der Stratosphäre absorbiert die biologisch schädliche ultraviolette Strahlung der Sonne (den größten Teil der UV-B-Strahlung) und verhindert, dass sie die Erdoberfläche erreicht. Die energiereiche UV-Strahlung kann das genetische Material in den Zellen von Pflanzen und Tieren, sowie von Menschen zerstören. Ohne die stratosphärische Ozonschicht wäre das Leben auf der Erde, wie wir es kennen, nicht möglich. Der Deutsche Wetterdienst (DWD) stellt UV-Indexkarten zur Verfügung, um die Bevölkerung bezgl. hoher UV-Belastungen zu informieren und zu warnen [1]. Dazu werden Daten aus dem golobalen Vorhersagemodell ICON (ICOsahedral Non-hydrostatic model) [2], externe Ozondaten und ein eigenes UV-Modell verwendet, um eine Vorhersage des UV-Index zu erstellen, der z.B. auf der DWD-Webseite als Vorhersage visualisiert wird. In diesem Projekt wird in Zusammenarbeit mit dem DWD ein selbstkonsistentes System entwickelt, um UV-Indexkarten vollständig mittels ICON zu generieren. Zu diesem Zweck wird ein linearisiertes Ozonschema (LINOZ) [3] für tägliche Ozonvorhersagen optimiert. Dies geschieht als Erweiterung der ICON-ART Struktur [4] [5] (ART: Aerosols and Reactive Trace gases). Für die Berechnung von UV-Strahlungsflüssen und -indizes wurde ein Strahlungstransportmodell für Sonnenstrahlung (Cloud-J) [6] implementiert und angepasst. Da das gesamte System als effiziente Lösung für UV-Indexvorhersagen dem DWD zur Verfügung gestellt werden soll, wird besonders Wert auf eine umfassende Funktionalität bei sehr geringem Rechenaufwand gelegt. Ein wichtiger Teil der Arbeit ist daher auch die Validierung und Optimierung der Verfahren und Abläufe, um zuverlässige und qualitativ hochwertige Vorhersagen zu erstellen. Wir präsentieren erste Ergebnisse des von ICON-ART modellierten UV-Strahlungsflusses durch die Atmosphäre auf globaler Skala und über ausgewählten Gebieten, dessen tageszeitliche Variation, sowie den Einfluss von Wolken auf die UV-Intensität. Anmerkung: Dieses Projekt wird durch den Deutschen Wetterdienst im Rahmen der Extramuralen Forschung mit folgender Nummer gefördert: 4819EMF03. Referenzen: [1] https://kunden.dwd.de/uvi/index.jsp [2] Zängl, G., et al., The ICON (ICOsahedral Non-hydrostatic) modelling framework of DWD MPI-M: Description of the non-hydrostatic dynamical core. Q.J.R. Meteorol. Soc., 141(687), 563-579 (2014) [3] McLinden, C. A., et al., Stratospheric ozone in 3-D models: A simple chemistry and the cross-tropopause flux, Journal of Geophysical Research: Atmospheres, 105(D11), 14653-14665 (2000) [4] Rieger, D., et al., ICON-ART - A new online-coupled model system from the global to regional scale, Geosci. Model Dev., 8(6), 1659-1676 (2015) [5] Schröter, et al., ICON-ART 2.1: a flexible tracer framework and its application for composition studies in numerical weather forecasting and climate simulations. Geosci. Model Dev., 11(10), 4043-4068 (2018) [6] Prather, M.J., Photolysis rates in correlated overlapping cloud fields: Cloud-J 7.3c. Geosci. Model Dev., 8(8), 2587-2595 (2015)

Large cities and urban areas are major contributors to methane and carbon-dioxide emissions. Official emission inventories are mostly based on a bottom-up approach where the emissions are derived from activity data, e.g. fossil fuel burning. Differences in the reporting processes in different countries lead to high uncertainties in the official inventories.The COllaborative Carbon Column Network (COCCON) measures greenhouse gases using ground-based remote sensing. In particular, portable Fourier transform infrared spectrometers EM27/SUN are used, which were developed at KIT in cooperation with Bruker. Both stationary measurements over longer periods of time as well as time-limited measurement campaigns with several instruments are performed. In the measurement campaigns, the emissions of selected cities in Europe have been determined in a top-down approach in order to compare them with the results of the official inventories.In the presented project, results of a preliminary campaign in Thessaloniki in October 2021 are shown, which will be complemented by a campaign lasting several months in the same location planned for summer 2022.In order to be able to draw more precise conclusions on emissions from the measured volume mixing ratio data, the distribution and dispersion of greenhouse gases will be simulated with the state-of-the-art weather forecast model ICON and the ART extension for aerosols and reactive trace gases developed at KIT. The aim of the project presented here is to improve the evaluation of the measurement campaigns by linking them more directly with simulations of emissions and transport.

The release of greenhouse gases (GHG) like CH4 into the atmosphere plays a key role in driving the climate change. With the optimization of atmospheric chemistry climate models, the accuracy in assessing future scenarios is improved, which is an important factor in our efforts to mitigate climate change. Within this work we introduce the WALLACE workflow, a method for the quantification and adjustment of wrong or missing emissions in well-established GHG-inventories, which are used as input data in atmospheric chemistry transport or climate models. The overall goal of WALLACE is to highlight emission hotspots and it therefore includes spatiotemporal proxy data and a selection algorithm. For the North Sea as a show case region we apply WALLACE to quantify methane emission fluxes of oil and gas platforms. The adjusted emissions are implemented as pointsources into our model and idealized simulations are performed to derive their impact on the spatial distribution of methane and its global and regional budget. Additionally, we take a look at the anti-correlation between methane and its main sink in the atmosphere, the hydroxyl radical (OH), which is implemented as a simple OH-chemistry mechanism into the model. This work makes a new and innovative contribution to achieve an accurate quantification of environmentally harmful gases – in particular CH4 - that drive man-made climate change. In conjunction with WALLACE we use the global model ICON-ART (ICOsahedral Nonhydrostatic model - Aerosols and Reactive Trace gases). ART is an online-coupled model extension for ICON that includes chemical gases and aerosols. One aim of the model is the simulation of interactions between the trace substances and the state of the atmosphere by coupling the spatiotemporal evolution of tracers with atmospheric processes, thus testing the impact of WALLACE-adjusted emissions on the CH4 distribution in the atmosphere.

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.

We present the first observational dataset of vertically resolved global stratospheric BrONO2 distributions from July 2002 until April 2012, and compare them to results of the atmospheric chemical climate model EMAC. The retrieved distributions are based on space-borne measurements of infrared limb-emission spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat. The derived vertical profiles of BrONO2 volume mixing ratios represent 10° latitude bins and three-day means, separated into sunlit and observations in the dark. The estimated uncertainties are around 1–4 pptv caused by spectral noise for single profiles as well as for further parameter and systematic errors which may not improve by averaging. Vertical resolutions range from 3 to 8 km between 15 and 35 km altitude. All leading modes of spatial and temporal variability of stratospheric BrONO2 in the observations are well replicated by the model simulations: the large diurnal variability, the low values during polar winter as well as the maximum values at mid- and high latitudes during summer. Three major differences between observations and model results are observed: (1) a model underestimation of enhanced BrONO2 in the polar winter stratosphere above about 30 km of up to 15 pptv, (2) up to 8 pptv higher modelled values than observed globally in the lower stratosphere up to 25 km most obvious during night, and (3) up to 5 pptv lower modelled concentrations at tropical latitudes between 27 and 32 km during sunlit conditions. (1) is explained by the model missing enhanced NOx produced in the mesosphere and lower thermosphere subsiding at high latitudes in winter. This is the first time that observational evidence for enhancement of BrONO2 caused by mesospheric NOx production is reported. The other major inconsistencies (2,3) between EMAC model results and observations are studied by sensitivity runs with a 1d model. These tentatively hint to a model underestimation of heterogeneous loss of BrONO2 in the lower stratosphere, a too low simulated production of BrONO2 during day as well as strongly underestimated BrONO2 volume mixing ratios when loss via reaction with O(3P) is considered additionally to photolysis. However, considering the uncertainty ranges of model parameters and of measurements, an unambiguous identification of the causes for the differences remains difficult. The observations have also been used to derive the total stratospheric bromine content relative to years of stratospheric entry between 1997 and 2007. With an average value of 21.2 ± 1.4 pptv of Bry at mid-latitudes where the modelled adjustment from BrONO2 to Bry is lowest, the MIPAS data agree with estimates of Bry derived from observations of BrO as well as from MIPAS-Balloon measurements of BrONO2.

Water vapour and ozone are important for the thermal and radiative balance of the upper troposphere (UT) and lowermost stratosphere (LMS). Both species are modulated by transport processes. Chemical and microphysical processes affect them differently. Thus, representing the different processes and their interactions is a challenging task for dynamical cores, chemical modules and microphysical parameterisations of state-of-the-art atmospheric model components. To test and improve the models, high resolution measurements of the UT/LMS are required. Here, we use measurements taken in a challenging case study by the GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) instrument on HALO. The German research aircraft HALO (High Altitude and LOng range research aircraft) performed a research flight on 26 February 2016, which covered deeply subsided air masses of the aged 2015/16 Arctic vortex, high-latitude LMS air masses, a highly textured troposphere-to-stratosphere exchange mixing region, and high-altitude cirrus clouds. Therefore, it provides a multifaceted case study for comparing GLORIA observations with state-of-the-art atmospheric model simulations in a complex UT/LMS region at a late stage of the Arctic winter 2015/16. Using GLORIA observations in this manifold scenario, we test the ability of the numerical weather prediction (NWP)-model ICON (ICOsahedral Nonhydrostatic) with the extension ART (Aerosols and Reactive Trace gases) and the chemistry-climate model (CCM) EMAC (ECHAM5/MESSy Atmospheric Chemistry) to model the UT/LMS composition of water vapour (H2O), ozone (O3), nitric acid (HNO3) and clouds. Within the scales resolved by the respective model, we find good overall agreement of both models with GLORIA. The applied high-resolution ICON-ART setup involving a R2B7 nest (local grid refinement with a horizontal resolution of about 20 km), covering the HALO flight region, reproduces mesoscale dynamical structures well. An observed troposphere-to-stratosphere exchange connected to an occluded Icelandic low is clearly reproduced by the model. Given the lower resolution (T106) of the nudged simulation of the EMAC model, we find that this model also reproduces these features well. Overall, trace gas mixing ratios simulated by both models are in a realistic range, and major cloud systems observed by GLORIA are mostly reproduced. However, we find both models to be affected by a well-known systematic moist-bias in the LMS. Further biases are diagnosed in the ICON-ART O3, EMAC H2O and EMAC HNO3 distributions. Finally, we use sensitivity simulations to investigate (i) short-term cirrus cloud impacts on the H2O distribution (ICON-ART), (ii) the overall impact of polar winter chemistry and microphysical processing on O3 and HNO3 (ICON-ART/EMAC), (iii) the impact of the model resolution on simulated parameters (EMAC), and (iv) consequences of scavenging processes by cloud particles (EMAC). We find that changing of the horizontal model resolution results in notable systematic changes for all species in the LMS, while scavenging processes play only a role in case of HNO3. We need to understand the representativeness of our results. However, this is a unique opportunity to characterise model biases that potentially affect forecasts and projection (adversely), and to discover deficits and define paths for further model improvements.

Besides chlorine, bromine is the major halogen species affecting stratospheric ozone with both anthropogenic and natural sources. Despite the significantly lower concentrations of bromine in the atmosphere, its potential for ozone depletion is similar to that of chlorine. An important prerequisite for the effectiveness of bromine ozone destruction cycles versus those of chlorine is the larger instability of bromine reservoir gases, especially the faster photolysis of bromine nitrate (BrONO2) compared to chlorine nitrate (ClONO2). With BrONO2 abundances in the stratosphere available from observations, (1) homogeneous, heterogeneous as well as photochemical processes involving bromine as implemented in atmospheric models can be assessed, and (2) independent information on the total stratospheric bromine content can be gained which is important, e.g. to analyse the amount of short-lived bromocarbons entering the stratosphere.The first detection of BrONO2 in the atmosphere had been achieved by analysis of infrared limb-emission spectra from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the Envisat satellite (doi: 10.5194/acp-9-1735-2009). On availability of improved infrared cross-sections, this was followed by the analysis of the behaviour of BrONO2 during sunrise and sunset through MIPAS balloon observations (doi: 10.5194/acp-17-14631-2017). Here we present a novel dataset of global stratospheric BrONO2 distributions based on the recently available MIPAS version 8 dataset of calibrated level-1b spectra. The altitude profiles of BrONO2 volume mixing ratios are zonally averaged in 10° latitude and 3-day bins, separated between day- and night-time observations, with a vertical resolution of 3-8 km between 15 and 35 km altitude for the whole MIPAS period from July 2002 until April 2012. The typical characteristics of this new dataset will be discussed. Furthermore, we will compare it to a multi-annual simulation of the chemistry climate model EMAC. Specific differences between observation and model simulation of BrONO2 will be highlighted and discussed by means of sensitivity 1-d model runs. Finally, a time series of the derived stratospheric Bry content normalized to the time of the entry into the stratosphere on basis of MIPAS age-of-air information will be discussed with regard to estimated uncertainties as well as independent observations.

Stratospheric Ozone (O3) absorbs biologically harmful solar ultraviolet radiation (most of the UV‑B radiation) and keeps it from reaching the surface. Such UV radiation is destructive of genetic cellular material in plants and animals, as well as human beings. Without the ozone layer, life on the surface of the Earth would not be possible as we know it.As part of its work the German Weather Service (DWD) provides UV index maps to warn the population in Germany of excessive UV exposure [[1]]. For this purpose, global ICON data, external ozone data and an external UV model is used.This study aims to create a self-consistent framework to generate UV index maps entirely from the non-hydrostatic global modelling system ICON [[2]]. For this purpose, a linearized ozone scheme (LINOZ) [[3]] will be optimized and the forecast functionality of ICON-ART [[4]][[5]] (ICOsahedral Non-hydrostatic – Aerosols and Reactive Trace gases) will be extended. For the derivation of UV radiation fluxes and indices a radiative transfer model for solar radiation (Cloud-J) [[6]] shall be implemented and extended. Since the entire framework is to be used at the DWD during ongoing operations, a functionality with very low computational effort is required. Here we present the first results of the UV radiation flux through the atmosphere and its diurnal variation. Furthermore, the influence of clouds on the UV radiation flux is considered. [[1]] https://kunden.dwd.de/uvi/index.jsp [[2]] Zängl, G., et al. (2014), The ICON (ICOsahedral Non-hydrostatic) modelling framework of DWD MPI-M: Description of the non-hydrostatic dynamical core. Q.J.R. Meteorol. Soc., doi:10.1002/qj.2378 [[3]] McLinden, C. A., et al. (2000), Stratospheric ozone in 3-D models: A simple chemistry and the cross-tropopause flux, Journal of Geophysical Research: Atmospheres, doi:10.1029/2000JD900124 [[4]] Rieger, D., et al. (2015), ICON-ART - A new online-coupled model system from the global to regional scale, Geosci. Model Dev., doi:10.5194/gmd-8-1659-2015 [[5]] Schröter, et al. (2018), ICON-ART 2.1: a flexible tracer framework and its application for composition studies in numerical weather forecasting and climate simulations. Geosci. Model Dev., doi:10.5194/gmd-11-4043-2018 [[6]] Prather, M.J. (2015), Photolysis rates in correlated overlapping cloud fields: Cloud-J 7.3c. Geosci. Model Dev., doi:10.5194/gmd-8-2587-2015

Methane (CH4) is the second most important greenhouse gas after CO2 affecting global warming. Various sources (e.g. fossil fuel production, agriculture and waste, biomass burning and natural wetlands) and sinks (the reaction with the OH-radical as the main sink contributes to tropospheric ozone production) determine the methane budget. Due to its long lifetime in the atmosphere methane can be transported over long distances.Disused and active offshore platforms can emit methane, the amount being difficult to quantify. In addition, explorations of the sea floor in the North Sea showed a release of methane near the boreholes of both, oil and gas producing platforms. The basis of this study is the established emission data base EDGAR (Emission Database for Global Atmospheric Research), an inventory that includes methane emission fluxes in the North Sea region. While methane emission fluxes in the EDGAR inventory and platform locations are matching for most of the oil platforms almost all of the gas platform sources are missing in the database. We develop a method for estimating the missing emission sources based on the EDGAR inventory and the known locations of gas platforms as additional point sources will be inserted in the model.In this study the global model ICON-ART (ICOsahedral Nonhydrostatic model - Aerosols and Reactive Trace gases) is used. ART is an online-coupled model extension for ICON that includes chemical gases and aerosols. One aim of the model is the simulation of interactions between the trace substances and the state of the atmosphere by coupling the spatiotemporal evolution of tracers with atmospheric processes. ICON-ART sensitivity simulations are performed with inserted and adjusted sources to access their influence on the methane and OH-radical distribution on regional (North Sea) and global scales.

Polar stratospheric clouds (PSCs) are a driver for ozone depletion in the lower polar stratosphere. They provide surfaces for heterogeneous reactions activating chlorine and bromine reservoir species during the polar night. PSCs are represented in current global chemistry-climate models, but one process is still a challenge: the representation of PSCs formed locally in conjunction with unresolved mountain waves. In this study, we present simulations with the ICOsahedral Nonhydrostatic modelling framework (ICON) with its extension for Aerosols and Reactive Trace gases (ART) that include local grid refinements (nesting) with two-way interaction. Here, the nesting is set up around the Antarctic Peninsula which is a well-known hot spot for the generation of mountain waves in the southern hemisphere. We compare our model results with satellite measurements from the Cloud-Aerosol LIdar with Orthogonal Polarisation (CALIOP) and the Atmospheric InfraRed Sounder (AIRS). We study a mountain wave event that took place from 19 to 29 July 2008 and find similar structures of PSCs as well as a fairly realistic development of the mountain wave in the Antarctic Peninsula nest. We compare a global simulation without nesting with the nested configuration to show the benefit. Although the mountain waves cannot be resolved adequately in the used global resolution (about 160 km), their effect from the nested regions (about 80 and 40 km) on the global domain is represented. Thus, we show in this study that by using the two-way nesting technique the gap between directly resolved mountain-wave induced PSCs and their representation and effect on chemistry in coarse global resolutions can be bridged by the ICON-ART model.

Polar Stratospheric Clouds (PSCs) play a key role in explaining ozone depletion on largescales as well as on regional scales. Mountain waves can be formed in the lee of a mountainin a stably stratified atmosphere. They can propagate upwards into the stratosphere andinduce temperature changes in the order of 10 to 15 K. Thus, large PSCs localised around themountain ridge can be formed, leading to increased chlorine activation and subsequently toa larger ozone depletion. It was estimated that 30 % of the southern hemispheric PSCs canbe explained by mountain waves. However, for the direct simulation of mountain-waveinduced PSCs, the mountains have to be represented adequately in global chemistry climatemodels which was a challenge in the past due to too low horizontal resolution.The ICOsahedral Nonhydrostatic (ICON) modelling framework with its extension for Aerosolsand Reactive Trace gases (ART) includes a PSC scheme coupled to the atmospheric chemistryin the model. The PSC scheme calculates the formation of all three PSC types independentlyresulting in the calculation of the heterogeneous reaction rates of chlorine and brominespecies on the surface of PSCs. ICON-ART provides the possibility of local grid refinementwith two-way interaction. With this, the grid around a mountain can be refined so thatmountain waves can be directly simulated in this region with a feedback to the coarserglobal resolution.In this study, we show the formation of mountain-wave induced PSCs with ICON-ART for theexample of a mountain wave event in July 2008 around the Antarctic Peninsula. It isevaluated with satellite measurements of AIRS and CALIOP and its impact on chlorine andbromine activation as well as on the ozone depletion in the southern hemisphere areanalysed. We demonstrate that the effect of mountain-wave induced PSCs can berepresented in the coarser global grid by using local grid refinement with two-wayinteraction. Thus, this study bridges the gap between directly simulated mountain-waveinduced PSCs and their representation in a global simulation.

Methane (CH4) is the second most important greenhouse gas after CO2 affecting global warming. Various sources (e.g. fossil fuel production, agriculture and waste, biomass burning and natural wetlands) and sinks (the reaction with the OH-radical as the main sink contributes to tropospheric ozone production) determine the methane budget. Due to its long lifetime in the atmosphere methane can be transported over long distances.Disused and active offshore platforms can emit methane, the amount being difficult to quantify. In addition, explorations of the sea floor in the North Sea showed a release of methane near the boreholes of both, oil and gas producing platforms. The basis of this study is the established emission data base EDGAR (Emission Database for Global Atmospheric Research), an inventory that includes methane emission fluxes in the North Sea region. While methane emission fluxes in the EDGAR inventory and platform locations are matching for most of the oil platforms almost all of the gas platform sources are missing in the database. We develop a method for estimating the missing sources based on the EDGAR emission inventory.In this study the global model ICON-ART (ICOsahedral Nonhydrostatic model - Aerosols and Reactive Trace gases) will be used. ART is an online-coupled model extension for ICON that includes chemical gases and aerosols. One aim of the model is the simulation of interactions between the trace substances and the state of the atmosphere by coupling the spatiotemporal evolution of tracers with atmospheric processes. ICON-ART sensitivity simulations are performed with inserted and adjusted sources to access their influence on the methane and OH-radical distribution on regional (North Sea) and global scales.

Three types of reference simulations, as recommended by the Chemistry–Climate Model Initiative (CCMI), have been performed with version 2.51 of the European Centre for Medium-Range Weather Forecasts – Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model: hindcast simulations (1950–2011), hindcast simulations with specified dynamics (1979–2013), i.e. nudged towards ERA-Interim reanalysis data, and combined hindcast and projection simulations (1950–2100). The manuscript summarizes the updates of the model system and details the different model set-ups used, including the on-line calculated diagnostics. Simulations have been performed with two different nudging set-ups, with and without interactive tropospheric aerosol, and with and without a coupled ocean model. Two different vertical resolutions have been applied. The on-line calculated sources and sinks of reactive species are quantified and a first evaluation of the simulation results from a global perspective is provided as a quality check of the data. The focus is on the intercomparison of the different model set-ups. The simulation data will become publicly available via CCMI and the Climate and Environmental Retrieval and Archive (CERA) database of the German Climate Computing Centre (DKRZ). This manuscript is intended to serve as an extensive reference for further analyses of the Earth System Chemistry integrated Modelling (ESCiMo) simulations.

Atmospheric composition studies on weather and climate time scales require flexible, scalable models. The ICOsahedral Nonhydrostatic model with Aerosols and Reactive Trace gases (ICON-ART) provides such an environment. Here, we introduce the most up-to-date version of the flexible tracer framework for ICON-ART and explain its application in one numerical weather forecast and one climate related case study. We demonstrate the implementation of idealised tracers and chemistry tendencies of different complexity using the ART infrastructure. Using different ICON physics configurations for weather and climate with ART, we perform integrations on different time scales, illustrating the model's performance. First, we present a hindcast experiment for the 2002 ozone hole split with two different ozone chemistry schemes using the numerical weather prediction physics configuration. We compare the hindcast with observations and discuss the confinement of the split-vortex using an idealised tracer diagnostic. Secondly, we study AMIP type integrations using a simplified chemistry scheme in conjunction with the climate physics configuration. We use two different simulations: The interactive simulation, where modelled ozone is coupled back to the radiation scheme and the non-interactive simulation that uses a default background climatology of ozone. Additionally, we introduce a chemical source term for water vapour for the interactive simulation. We discuss the impact of stratospheric ozone and water vapour variations in the interactive and non-interactive integrations on the water vapour tape recorder, as a measure of tropical upwelling changes. Additionally we explain the seasonal evolution and latitudinal distribution of the age of air. The age of air is measure of the strength of the meridional overturning circulation with young air in the tropical upwelling region and older air in polar winter downwelling regions. We conclude that our flexible tracer framework allows for tailor-made configurations of ICON-ART in weather and climate applications that are easy to configure and run well.

We present the new isotope enabled model ICON-ART-Iso. The physics of the global ICOsahedral Nonhydrostatic (ICON) modelling framework have been extended to simulate passive moisture tracers and the stable isotopologues HDO and H218O. The extension builds on the infrastructure provided by ICON-ART, which allows a high flexibility with respect to the number of related water tracers that are simulated. The physics of isotopologue fractionation follow the model COSMOiso. First, we present a detailed description of the physics of fractionation that have been implemented in the model. The model is then evaluated by comparing with measurements in precipitation and vapor representing a range of temporal scales. A multi annual simulation is compared to observations of the isotopologues in precipitation taken from the station network GNIP (Global Network for Isotopes in Precipitation). ICON-ART-Iso is able to reasonably simulate the seasonal cycles in δD and δ18O as observed at the GNIP stations. In a comparison with IASI satellite retrievals, the seasonal and daily cycles in the isotopologue content of vapor are examined for different regions in the free troposphere. On a small spatial and temporal scale, ICON-ART-Iso is used to simulate the period of two flights of the IAGOS-CARIBIC aircraft in September 2010, which sampled air in the tropopause level influenced by Hurricane Igor. The general features of this sample as well as all of tropical data available from IAGOS-CARIBIC are captured by the model. The study demonstrates that ICON-ART-Iso is a flexible tool to analyze the water cycle of ICON. It is capable of simulating tagged water as well as the isotopologues HDO and H218O.

The Arctic winter 2015/2016 was one of the coldest stratospheric winters in recent years. A stable vortex formed by early December and the early winter was exceptionally cold. Cold pool temperatures dropped below the Nitric Acid Trihydrate (NAT) existence temperature of about 195 K, thus allowing Polar Stratospheric Clouds (PSCs) to form. The low temperatures in the polar stratosphere persisted until early March allowing chlorine activation and catalytic ozone destruction. Satellite observations indicate that sedimentation of PSC particles led to denitrification as well as dehydration of stratospheric layers. Model simulations of the Arctic winter 2015/2016 nudged toward European Center for Medium-Range Weather Forecasts (ECMWF) analyses data were performed with the atmospheric chemistry–climate model ECHAM5/MESSy Atmospheric Chemistry (EMAC) for the Polar Stratosphere in a Changing Climate (POLSTRACC) campaign. POLSTRACC is a High Altitude and LOng Range Research Aircraft (HALO) mission aimed at the investigation of the structure, composition and evolution of the Arctic Upper Troposphere and Lower Stratosphere (UTLS). The chemical and physical processes involved in Arctic stratospheric ozone depletion, transport and mixing processes in the UTLS at high latitudes, polar stratospheric clouds as well as cirrus clouds are investigated. In this study an overview of the chemistry and dynamics of the Arctic winter 2015/2016 as simulated with EMAC is given. Further, chemical-dynamical processes such as denitrification, dehydration and ozone loss during the Arctic winter 2015/2016 are investigated. Comparisons to satellite observations by the Aura Microwave Limb Sounder (Aura/MLS) as well as to airborne measurements with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) performed on board of HALO during the POLSTRACC campaign show that the EMAC simulations are in fairly good agreement with observations. We derive a maximum polar stratospheric O3 loss of ~ 2 ppmv or 100 DU in terms of column in mid March. The stratosphere was denitrified by about 8 ppbv HNO3 and dehydrated by about 1 ppmv H2O in mid to end of February. While ozone loss was quite strong, but not as strong as in 2010/2011, denitrification and dehydration were so far the strongest observed in the Arctic stratosphere in the at least past 10 years.

The first stratospheric measurements of the diurnal variation of the inorganic bromine (Bry) reservoir species BrONO2 around sunrise and sunset are reported. Arctic flights of the balloon-borne Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) were carried out from Kiruna (68°N, Sweden) in January 2010 and March 2011 inside the stratospheric polar vortices where diurnal variations of BrONO2 around sunrise have been observed. High nighttime BrONO2 volume mixing ratios of up to 21 parts per trillion by volume (pptv) were detected in the late winter 2011 in the absence of polar stratospheric clouds (PSCs). In contrast, the amount of measured BrONO2 was significantly lower in January 2010 due to low available NO2 amounts (for the build-up of BrONO2), heterogeneous destruction of BrONO2 on PSC particles, and the gas-phase interaction of BrO (the source to form BrONO2) with ClO. A further balloon flight took place at mid-latitudes from Timmins (49°N, Canada) in September 2014. Mean BrONO2 mixing ratios of 22 pptv were observed after sunset in the altitude region between 21 and 29 km. Measurements are compared and discussed with the results of a multi-year simulation performed with the chemistry climate model ECHAM5/MESSy Atmospheric Chemistry (EMAC). The calculated temporal variation of BrONO2 is in principal agreement with the balloon-borne observations. Using the nighttime simulated ratio between BrONO2 and Bry, the amount of Bry observed by MIPAS-B was estimated to about 21–25 pptv in the lower stratosphere.

To evaluate future climate change in the middle atmosphere and the chemistry–climate interaction of stratospheric ozone, we performed a long-term simulation from 1960 to 2050 with boundary conditions from the Intergovernmental Panel on Climate Change A1B greenhouse gas scenario and the World Meteorological Organization Ab halogen scenario using the chemistry–climate model ECHAM5/MESSy Atmospheric Chemistry (EMAC). In addition to this standard simulation we performed five sensitivity simulations from 2000 to 2050 using the rerun files of the simulation mentioned above. For these sensitivity simulations we used the same model setup as in the standard simulation but changed the boundary conditions for carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone-depleting substances (ODS). In the first sensitivity simulation we fixed the mixing ratios of CO2, CH4, and N2O in the boundary conditions to the amounts for 2000. In each of the four other sensitivity simulations we fixed the boundary ...

Until a decade ago, acetone was assumed to be a dominant HOx source in the dry extra-tropical upper troposphere (ex-UT). New photodissociation quantum yields of acetone and the lack of representative data from the ex-UT challenged that assumption. Regular mass spectrometric observations onboard the Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) passenger aircraft deliver the first representative distribution of acetone in the UT/LMS (UT/lowermost stratosphere). Based on diverse CARIBIC trace gas data and non-observed parameters taken from the model ECHAM5/MESSy for Atmospheric Chemistry, we quantify the HOx source in the UT/LMS from (photo)oxidation of acetone. The findings are contrasted to HOx production from ozone photolysis, overall the dominant tropospheric HOx source. It is shown that HOx production from acetone (photo)oxidation reaches up to 95% of the HOx source from ozone photolysis in autumn in the UT and on average ~61% in summer. That is, acetone is a significant source of HOx in the UT/LMS.

Arctic stratospheric ozone depletion is closely linked to the occurrence of low stratospheric temperatures. There are indications that cold winters in the Arctic stratosphere have been getting colder, raising the question if and to what extent a cooling of the Arctic stratosphere may continue into the future. We use meteorological reanalyses from the European Centre for Medium Range Weather Forecasts (ECMWF) ERA-Interim and NASA's Modern-Era Retrospective-Analysis for Research and Applications (MERRA) for the past 32 yr together with calculations of the chemistry-climate model (CCM) ECHAM/MESSy Atmospheric Chemistry (EMAC) and models from the Chemistry-Climate Model Validation (CCMVal) project to infer radiative and dynamical contributions to long-term Arctic stratospheric temperature changes. For the past three decades the reanalyses show a warming trend in winter and cooling trend in spring and summer, which agree well with trends from the Radiosonde Innovation Composite Homogenization (RICH) adjusted radiosonde data set. Changes in winter and spring are caused by a corresponding change of planetary wave activity with increases in winter and decreases in spring. During winter the increase of planetary wave activity is counteracted by a residual radiatively induced cooling. Stratospheric radiatively induced cooling is detected throughout all seasons, being highly significant in spring and summer. This means that for a given dynamical situation, according to ERA-Interim the annual mean temperature of the Arctic lower stratosphere has been cooling by −0.41 ± 0.11 K decade−1 at 50 hPa over the past 32 yr. Calculations with state-of-the-art models from CCMVal and the EMAC model qualitatively reproduce the radiatively induced cooling for the past decades, but underestimate the amount of radiatively induced cooling deduced from reanalyses. There are indications that this discrepancy could be partly related to a possible underestimation of past Arctic ozone trends in the models. The models project a continued cooling of the Arctic stratosphere over the coming decades (2001–2049) that is for the annual mean about 40% less than the modeled cooling for the past, due to the reduction of ozone depleting substances and the resulting ozone recovery. This projected cooling in turn could offset between 15 and 40% of the Arctic ozone recovery.

Measurement data from the long-term passenger aircraft project IAGOS-CARIBIC are often used to derive climatologies of trace gases in the upper troposphere and lower stratosphere (UTLS). We investigate to what extent such climatologies are representative of the true state of the atmosphere. Climatologies are considered relative to the tropopause in mid-latitudes (35 to 75° N) for trace gases with different atmospheric lifetimes. Using the chemistry–climate model EMAC, we sample the modeled trace gases along CARIBIC flight tracks. Representativeness is then assessed by comparing the CARIBIC sampled model data to the full climatological model state. Three statistical methods are applied for the investigation of representativeness: the Kolmogorov–Smirnov test and two scores based on the variability and relative differences. Two requirements for any score describing representativeness are essential: representativeness is expected to increase (i) with the number of samples and (ii) with decreasing variability of the species considered. Based on these two requirements, we investigate the suitability of the different statistical measures for investigating representativeness. The Kolmogorov–Smirnov test is very strict and does not identify any trace-gas climatology as representative – not even of long-lived trace gases. In contrast, the two scores based on either variability or relative differences show the expected behavior and thus appear applicable for investigating representativeness. For the final analysis of climatological representativeness, we use the relative difference score and calculate a representativeness uncertainty for each trace gas in percent. In order to justify the transfer of conclusions about representativeness of individual trace gases from the model to measurements, we compare the trace gas variability between model and measurements. We find that the model reaches 50–100 % of the measurement variability. The tendency of the model to underestimate the variability is caused by the relatively coarse spatial and temporal model resolution. In conclusion, we provide representativeness uncertainties for several species for tropopause-referenced climatologies. Long-lived species like CO2 have low uncertainties ( ≤ 0.4 %), while shorter-lived species like O3 have larger uncertainties (10–15 %). Finally, we translate the representativeness score into a number of flights that are necessary to achieve a certain degree of representativeness. For example, increasing the number of flights from 334 to 1000 would reduce the uncertainty in CO to a mere 1 %, while the uncertainty for shorter-lived species like NO would drop from 80 to 10 %.

The winter 2009/2010 was characterized by a strong Arctic vortex with extremely cold mid-winter temperatures in the lower stratosphere associated with an intense activation of reactive chlorine compounds (ClOx) from reservoir species. Stratospheric limb emission spectra were recorded during a flight of the balloon version of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) from Kiruna (Sweden) on 24 January 2010 inside the Arctic vortex. Several fast limb sequences of spectra (in time steps of about 10 min) were measured from nighttime photochemical equilibrium to local noon allowing the retrieval of chlorine- and nitrogen-containing species which change rapidly their concentration around the terminator between night and day. Mixing ratios of species like ClO, NO2, and N2O5 show significant changes around sunrise, which are temporally delayed due to polar stratospheric clouds reducing the direct radiative flux from the sun. ClO variations were derived for the first time from MIPAS-B spectra. Daytime ClO values of up to 1.6 ppbv are visible in a broad chlorine activated layer below 26 km correlated with low values (below 0.1 ppbv) of the chlorine reservoir species ClONO2. Observations are compared and discussed with calculations performed with the 3-dimensional Chemistry Climate Model EMAC (ECHAM5/MESSy Atmospheric Chemistry). Mixing ratios of the species ClO, NO2, and N2O5 are well reproduced by the model during night and noon. However, the onset of ClO production and NO2 loss around the terminator in the model is not consistent with the measurements. The MIPAS-B observations along with Tropospheric Ultraviolet-Visible (TUV) radiation model calculations suggest that polar stratospheric clouds lead to a delayed start followed by a faster increase of the photodissoziation of ClOOCl and NO2 near the morning terminator since stratospheric clouds alter the direct and the diffuse flux of solar radiation. These effects are not considered in the EMAC model simulations which assume a cloudless atmosphere.

MIPAS-ENVISAT full resolution spectra were analyzed to obtain a global distribution of hydrogen peroxide (H2O2) in the stratosphere. H2O2 acts as reservoir gas for the HOx family (= H+OH+HO2) and thus, observations of H2O2 provide a better understanding of the HOx chemistry in the atmosphere. A retrieval approach based on constrained least squares fitting was developed and applied to small dedicated spectral analysis windows with maximum H2O2 information and minimum contribution of interfering gases. Due to a low signal to noise ratio in the measured spectra single profiles cannot be used for scientific interpretation and about 100 profiles have to be averaged temporally or spatially. Our retrievals of H2O2 from MIPAS measurements provide meaningful results between approximately 20 and 60 km. A possible impact by the high uncertainty of the reaction rate constant for HO2 + HO2→H2O2 + O2 in our 3D-CTM KASIMA is discussed. We find best agreement between model and observations for applying rate constants according to Christensen et al. (2002) however, a mismatch in vertical profile shape remains. The observations were compared to the model results of KASIMA focusing on low to mid latitudes. Good agreement in spatial distribution and in temporal evolution was found. Highest vmr of H2O2 in the stratosphere were observed and modeled in low latitudes shortly after equinox at about 30 km. The modelled diurnal cycle with lowest vmr shortly after sunrise and highest vmr in the afternoon is confirmed by the MIPAS observations.

Time series of total column abundances of hydrogen chloride (HCl), chlorine nitrate (ClONO2), and hydrogen fluoride (HF) were determined from ground-based Fourier transform infrared (FTIR) spectra recorded at 17 sites belonging to the Network for the Detection of Atmospheric Composition Change (NDACC) and located between 80.05° N and 77.82° S. By providing such a near-global overview on ground-based measurements of the two major stratospheric chlorine reservoir species, HCl and ClONO2, the present study is able to confirm the decrease of the atmospheric inorganic chlorine abundance during the last few years. This decrease is expected following the 1987 Montreal Protocol and its amendments and adjustments, where restrictions and a subsequent phase-out of the prominent anthropogenic chlorine source gases (solvents, chlorofluorocarbons) were agreed upon to enable a stabilisation and recovery of the stratospheric ozone layer. The atmospheric fluorine content is expected to be influenced by the Montreal Protocol, too, because most of the banned anthropogenic gases also represent important fluorine sources. But many of the substitutes to the banned gases also contain fluorine so that the HF total column abundance is expected to have continued to increase during the last few years. The measurements are compared with calculations from five different models: the two-dimensional Bremen model, the two chemistry-transport models KASIMA and SLIMCAT, and the two chemistry-climate models EMAC and SOCOL. Thereby, the ability of the models to reproduce the absolute total column amounts, the seasonal cycles, and the temporal evolution found in the FTIR measurements is investigated and inter-compared. This is especially interesting because the models have different architectures. The overall agreement between the measurements and models for the total column abundances and the seasonal cycles is good. Linear trends of HCl, ClONO2, and HF are calculated from both measurement and model time series data, with a focus on the time range 2000–2009. This period is chosen because from most of the measurement sites taking part in this study, data are available during these years. The precision of the trends is estimated with the bootstrap resampling method. The sensitivity of the trend results with respect to the fitting function, the time of year chosen and time series length is investigated, as well as a bias due to the irregular sampling of the measurements. The measurements and model results investigated here agree qualitatively on a decrease of the chlorine species by around 1% yr−1. The models simulate an increase of HF of around 1% yr−1. This also agrees well with most of the measurements, but some of the FTIR series in the Northern Hemisphere show a stabilisation or even a decrease in the last few years. In general, for all three gases, the measured trends vary more strongly with latitude and hemisphere than the modelled trends. Relative to the FTIR measurements, the models tend to underestimate the decreasing chlorine trends and to overestimate the fluorine increase in the Northern Hemisphere. At most sites, the models simulate a stronger decrease of ClONO2 than of HCl. In the FTIR measurements, this difference between the trends of HCl and ClONO2 depends strongly on latitude, especially in the Northern Hemisphere.

We present an original multi-spectrum fitting procedure to retrieve volume mixing ratio (VMR) profiles of carbonyl fluoride (COF2) from ground-based high resolution Fourier transform infrared (FTIR) solar spectra. The multi-spectrum approach consists of simultaneously combining, during the retrievals, all spectra recorded consecutively during the same day and with the same resolution. Solar observations analyzed in this study with the SFIT-2 v3.91 fitting algorithm correspond to more than 2900 spectra recorded between January 2000 and December 2007 at high zenith angles, with a Fourier Transform Spectrometer operated at the high-altitude International Scientific Station of the Jungfraujoch (ISSJ, 46.5° N latitude, 8.0° E longitude, 3580 m altitude), Switzerland. The goal of the retrieval strategy described here is to provide information about the vertical distribution of carbonyl fluoride. The microwindows used are located in the ν4 or in the ν4 COF2 infrared (IR) absorption bands. Averaging kernel and eigenvector analysis indicates that our FTIR retrieval is sensitive to COF2 inversion between 17 and 30 km, with the major contribution to the retrieved information always coming from the measurement. Moreover, there was no significant bias between COF2 partial columns, total columns or VMR profiles retrieved from the two bands. For each wavenumber region, a complete error budget including all identified sources has been carefully established. In addition, comparisons of FTIR COF2 17–30 km partial columns with KASIMA and SLIMCAT 3-D CTMs are also presented. If we do not notice any significant bias between FTIR and SLIMCAT time series, KASIMA COF2 17–30 km partial columns are lower of around 25%, probably due to incorrect lower boundary conditions. For each times series, linear trend estimation for the 2000–2007 time period as well as a seasonal variation study are also performed and critically discussed. For FTIR and KASIMA time series, very low COF2 growth rates (0.4±0.2%/year and 0.3±0.2%/year, respectively) have been derived. However, the SLIMCAT data set gives a slight negative trend (−0.5±0.2%/year), probably ascribable to discontinuities in the meteorological data used by this model. We further demonstrate that all time series are able to reproduce the COF2 seasonal cycle, which main seasonal characteristics deduced from each data set agree quite well.

Total column and stratospheric ozone levels at mid-latitudes often reveal strong fluctuations on time scales of days caused by dynamic processes. In some cases the total ozone column is distinctly reduced below climatological values. Here, a very low total ozone episode around 19 January 2006 over Western Europe is investigated when the observed total ozone column over Uccle (BE), measured by a Brewer spectrophotometer, reached a daily minimum of 200 DU, the lowest recorded value at this station. In order to investigate the mechanisms leading to the ozone minimum, the present study used data from (i) six ozone sounding stations in Western and Middle Europe, (ii) ECMWF meteorological fields, (iii) a simulation of the CLaMS model for January 2006, (iv) a multi-year run of the chemistry transport model KASIMA, and (v) a six-year run of the climate chemistry model ECHAM5/MESSy1. The ozone decrease at different heights was quantified and it was determined to what extent different transport mechanisms, and instantaneous, in-situ chemical ozone depletion contributed to the event. All three models reproduced well the evolution and formation of the event. The ozone column decrease between Θ=300 and 750 K was strongest at Uccle (BE) and De Bilt (NL) with 108 and 103 DU, respectively, and somewhat lower at Hohenpeissenberg (DE), Payerne (CH), Prague (CZ) and Lerwick (UK) with 85, 84, 83 and 74 DU, respectively. Our analysis demonstrated that mainly the displacement of the ozone depleted polar vortex contributed to the ozone column decrease (between 55 and 82%), compared to the advection of ozone-poor low-latitude air in the UTLS region, connected with divergence of air out of the column caused by uplift of isentropes in the lower stratosphere. This dominance was significant only at Lerwick, De Bilt and Uccle. Severe low total ozone episodes seem to occur preferentially when the two mentioned transport mechanisms occur at the same time. Instantaneous, in-situ chemical ozone depletion accounted for only 2±1% of the overall total ozone decrease at the sounding stations.

1-year data sets of monthly averaged nitrous oxide (N2O) and ozone (O3) derived from satellite measurements were used as a tool for the evaluation of atmospheric photochemical models. Two 1-year data sets, one solar occultation data set derived from the Improved Limb Atmospheric Spectrometer (ILAS and ILAS-II) and one limb sounding data set derived from the Odin Sub-Millimetre Radiometer (Odin/SMR) were employed. Here, these data sets are used for the evaluation of two Chemical Transport Models (CTMs), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) and the Chemical Lagrangian Model of the Stratosphere (CLaMS) as well as for one Chemistry-Climate Model (CCM), the atmospheric chemistry general circulation model ECHAM5/MESSy1 (E5M1) in the lower stratosphere with focus on the Northern Hemisphere. Since the Odin/SMR measurements cover the entire hemisphere, the evaluation is performed for the entire hemisphere as well as for the low latitudes, midlatitudes and high latitudes using the Odin/SMR 1-year data set as reference. To assess the impact of using different data sets for such an evaluation study we repeat the evaluation for the polar lower stratosphere using the ILAS/ILAS-II data set. Only small differences were found using ILAS/ILAS-II instead of Odin/SMR as a reference, thus, showing that the results are not influenced by the particular satellite data set used for the evaluation. The evaluation of CLaMS, KASIMA and E5M1 shows that all models are in agreement with Odin/SMR and ILAS/ILAS-II. Differences are generally in the range of ±20%. Larger differences (up to −40%) are found in all models at 500±25 K for N2O mixing ratios greater than 200 ppbv, thus in air masses of tropical character. Generally, the largest differences were found for the tropics and the lowest for the polar regions. However, an underestimation of polar winter ozone loss was found both in KASIMA and E5M1 both in the Northern and Southern Hemisphere.

This paper presents the spatio-temporal distribution of NOy species at altitudes between 14 and 31 km as measured with the MIPAS-B instrument on the morning of 21 March 2003 in northern Scandinavia. At lower altitudes (below about 22 km), temperature variations, the distribution of ClONO2, and the tracer N2O reveal the dynamics through the edge of the late arctic polar vortex. At higher altitudes, continuous measurement before, during, and after sunrise provides information about photochemistry illustrating the evolution of the photochemically active gases NO2 and N2O5 around sunrise. The measured temporal evolution of NO2 and N2O5 is compared to box modelling that is run along backward calculated trajectories. While the comparison of measured and modelled N2O5 reveals significant differences, there is a good agreement between the model and observations for NO2 in terms of volume mixing ratios but the simulated decrease shortly after sunrise is underestimated compared to the measurements. The differences are attributed to the photolysis rates used in the box model calculations.

The Atmospheric Chemistry Experiment (ACE) satellite was launched on 12 August 2003. Its two instruments measure vertical profiles of over 30 atmospheric trace gases by analyzing solar occultation spectra in the ultraviolet/visible and infrared wavelength regions. The reservoir gases HNO3, ClONO2, and N2O5 are three of the key species provided by the primary instrument, the ACE Fourier Transform Spectrometer (ACE-FTS). This paper describes the ACE-FTS version 2.2 data products, including the N2O5 update, for the three species and presents validation comparisons with available observations. We have compared volume mixing ratio (VMR) profiles of HNO3, ClONO2, and N2O5 with measurements by other satellite instruments (SMR, MLS, MIPAS), aircraft measurements (ASUR), and single balloon-flights (SPIRALE, FIRS-2). Partial columns of HNO3 and ClONO2 were also compared with measurements by ground-based Fourier Transform Infrared (FTIR) spectrometers. Overall the quality of the ACE-FTS v2.2 HNO3 VMR profiles is good from 18 to 35 km. For the statistical satellite comparisons, the mean absolute differences are generally within ±1 ppbv ±20%) from 18 to 35 km. For MIPAS and MLS comparisons only, mean relative differences lie within±10% between 10 and 36 km. ACE-FTS HNO3 partial columns (~15–30 km) show a slight negative bias of −1.3% relative to the ground-based FTIRs at latitudes ranging from 77.8° S–76.5° N. Good agreement between ACE-FTS ClONO2 and MIPAS, using the Institut für Meteorologie und Klimaforschung and Instituto de Astrofísica de Andalucía (IMK-IAA) data processor is seen. Mean absolute differences are typically within ±0.01 ppbv between 16 and 27 km and less than +0.09 ppbv between 27 and 34 km. The ClONO2 partial column comparisons show varying degrees of agreement, depending on the location and the quality of the FTIR measurements. Good agreement was found for the comparisons with the midlatitude Jungfraujoch partial columns for which the mean relative difference is 4.7%. ACE-FTS N2O5 has a low bias relative to MIPAS IMK-IAA, reaching −0.25 ppbv at the altitude of the N2O5 maximum (around 30 km). Mean absolute differences at lower altitudes (16–27 km) are typically −0.05 ppbv for MIPAS nighttime and ±0.02 ppbv for MIPAS daytime measurements.

We present a new emissions module for the ICON (ICOsahedral Non-hydrostatic)-ART (Aerosols and Reactive Trace gases) modelling framework. The emissions module processes external flux data sets and increments the tracer volume mixing ratios in the boundary layer accordingly. In addition, the module for online calculations of biogenic emissions (MEGAN2.1) is implemented in ICON-ART and can replace the offline biogenic emission data sets. The performance of the emissions module is illustrated with simulations of acetone, using a simplified chemical depletion mechanism based on a reaction with OH and photolysis only. In our model setup, we calculate a tropospheric acetone lifetime of 33 days, which is in good agreement with the literature. We compare our results with airborne IAGOS-CARIBIC measurements in the upper troposphere and lowermost stratosphere (UTLS) in terms of phase and amplitude of the annual cycle. In all our ICON-ART simulations the general seasonal variability is well represented but questions remain concerning the magnitude of the acetone emissions and its atmospheric lifetime. We conclude that the new emissions module performs well and allows the simulation of the annual cycles of emissions dominatedconcentrations even with a simple chemistry only.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument was launched aboard the environmental satellite ENVISAT into its sun-synchronous orbit on 1 March 2002. The short-lived species NO2 is one of the key target products of MIPAS that are operationally retrieved from limb emission spectra measured in the stratosphere and mesosphere. Within the MIPAS validation activities, a large number of independent observations from balloons, satellites and ground-based stations have been compared to European Space Agency (ESA) version 4.61 operational NO2 data comprising the time period from July 2002 until March 2004 where MIPAS measured with full spectral resolution. Comparisons between MIPAS and balloon-borne observations carried out in 2002 and 2003 in the Arctic, at mid-latitudes, and in the tropics show a very good agreement below 40 km altitude with a mean deviation of roughly 3%, virtually without any significant bias. The comparison to ACE satellite observations exhibits only a small negative bias of MIPAS which appears not to be significant. The independent satellite instruments HALOE, SAGE II, and POAM III confirm in common for the spring-summer time period a negative bias of MIPAS in the Arctic and a positive bias in the Antarctic middle and upper stratosphere exceeding frequently the combined systematic error limits. In contrast to the ESA operational processor, the IMK/IAA retrieval code allows accurate inference of NO2 volume mixing ratios under consideration of all important non-LTE processes. Large differences between both retrieval results appear especially at higher altitudes, above about 50 to 55 km. These differences might be explained at least partly by non-LTE under polar winter conditions but not at mid-latitudes. Below this altitude region mean differences between both processors remain within 5% (during night) and up to 10% (during day) under undisturbed (September 2002) conditions and up to 40% under perturbed polar night conditions (February and March 2004). The intercomparison of ground-based NDACC observations shows no significant bias between the FTIR measurements in Kiruna (68° N) and MIPAS in summer 2003 but larger deviations in autumn and winter. The mean deviation over the whole comparison period remains within 10%. A mean negative bias of 15% for MIPAS daytime and 8% for nighttime observations has been determined for UV-vis comparisons over Harestua (60° N). Results of a pole-to-pole comparison of ground-based DOAS/UV-visible sunrise and MIPAS mid-morning column data has shown that the mean agreement in 2003 falls within the accuracy limit of the comparison method. Altogether, it can be indicated that MIPAS NO2 profiles yield valuable information on the vertical distribution of NO2 in the lower and middle stratosphere (below about 45 km) during day and night with an overall accuracy of about 10–20% and a precision of typically 5–15% such that the data are useful for scientific studies. In cases where extremely high NO2 occurs in the mesosphere (polar winter) retrieval results in the lower and middle stratosphere are less accurate than under undisturbed atmospheric conditions.

Altitude profiles of ClONO2 retrieved with the IMK (Institut für Meteorologie und Klimaforschung) science-oriented data processor from MIPAS/Envisat (Michelson Interferometer for Passive Atmospheric Sounding on Envisat) mid-infrared limb emission measurements between July 2002 and March 2004 have been validated by comparison with balloon-borne (Mark IV, FIRS2, MIPAS-B), airborne (MIPAS-STR), ground-based (Spitsbergen, Thule, Kiruna, Harestua, Jungfraujoch, Izaña, Wollongong, Lauder), and spaceborne (ACE-FTS) observations. With few exceptions we found very good agreement between these instruments and MIPAS with no evidence for any bias in most cases and altitude regions. For balloon-borne measurements typical absolute mean differences are below 0.05 ppbv over the whole altitude range from 10 to 39 km. In case of ACE-FTS observations mean differences are below 0.03 ppbv for observations below 26 km. Above this altitude the comparison with ACE-FTS is affected by the photochemically induced diurnal variation of ClONO2. Correction for this by use of a chemical transport model led to an overcompensation of the photochemical effect by up to 0.1 ppbv at altitudes of 30–35 km in case of MIPAS-ACE-FTS comparisons while for the balloon-borne observations no such inconsistency has been detected. The comparison of MIPAS derived total column amounts with ground-based observations revealed no significant bias in the MIPAS data. Mean differences between MIPAS and FTIR column abundances are 0.11±0.12×1014 cm−2 (1.0±1.1%) and −0.09±0.19×1014 cm−2 (−0.8±1.7%), depending on the coincidence criterion applied. χ2 tests have been performed to assess the combined precision estimates of MIPAS and the related instruments. When no exact coincidences were available as in case of MIPAS – FTIR or MIPAS – ACE-FTS comparisons it has been necessary to take into consideration a coincidence error term to account for χ2 deviations. From the resulting χ2 profiles there is no evidence for a systematic over/underestimation of the MIPAS random error analysis.

During the CRISTA-1 mission three pronounced fingerlike structures reaching from the lower latitudes to the mid-latitudes, so-called streamers, were observed in the measurements of several trace gases in early November 1994. A simulation of these streamers in previous studies employing the KASIMA (Karlsruhe Simulation Model of the Middle Atmosphere) and ROSE (Research on Ozone in the Stratosphere and its Evolution) model, both being Eulerian models, show that their formation is due to adiabatic transport processes. Here, the impact of mixing on the development of these streamers is investigated. These streamers were simulated with the CLaMS model (Chemical Lagrangian Model of the Stratosphere), a Lagrangian model, using N2O as long-lived tracer. Using several different initialisations the results were compared to the KASIMA simulations and CRISTA (Cryogenic Infrared Spectrometer and Telescope for the Atmosphere) observations. Further, since the KASIMA model was employed to derive a 9-year climatology, the quality of the reproduction of streamers from such a study was tested by the comparison of the KASIMA results with CLaMS and CRISTA. The streamers are reproduced well for the Northern Hemisphere in the simulations of CLaMS and KASIMA for the 6 November 1994. However, in the CLaMS simulation a stronger filamentation is found while larger discrepancies between KASIMA and CRISTA were found especially for the Southern Hemisphere. Further, compared to the CRISTA observations the mixing ratios of N2O are in general underestimated in the KASIMA simulations. An improvement of the simulations with KASIMA was obtained for a simulation time according to the length of the CLaMS simulation. To quantify the differences between the simulations with CLaMS and KASIMA, and the CRISTA observations, the probability density function technique (PDF) is used to interpret the tracer distributions. While in the PDF of the KASIMA simulation the small scale structures observed by CRISTA are smoothed out due to the numerical diffusion in the model, the PDFs derived from CRISTA observations can be reproduced by CLaMS by optimising the mixing parameterisation. Further, this procedure gives information on small-scale variabilities not resolved by the CRISTA observations.

Several stratospheric chemistry modules from box, 2-D or 3-D models, have been intercompared. The intercomparison was focused on the ozone loss and associated reactive species under the conditions found in the cold, wintertime Arctic and Antarctic vortices. Comparisons of both gas phase and heterogeneous chemistry modules show excellent agreement between the models under constrained conditions for photolysis and the microphysics of polar stratospheric clouds. While the mean integral ozone loss ranges from 4–80% for different 30–50 days long air parcel trajectories, the mean scatter of model results around these values is only about ±1.5%. In a case study, where the models employed their standard photolysis and microphysical schemes, the variation around the mean percentage ozone loss increases to about ±7%. This increased scatter of model results is mainly due to the different treatment of the PSC microphysics and heterogeneous chemistry in the models, whereby the most unrealistic assumptions about PSC processes consequently lead to the least representative ozone chemistry. Furthermore, for this case study the model results for the ozone mixing ratios at different altitudes were compared with a measured ozone profile to investigate the extent to which models reproduce the stratospheric ozone losses. It was found that mainly in the height range of strong ozone depletion all models underestimate the ozone loss by about a factor of two. This finding corroborates earlier studies and implies a general deficiency in our understanding of the stratospheric ozone loss chemistry rather than a specific problem related to a particular model simulation.

The Arctic winter 2010/2011 was characterized by a persistent vortex with extremely low temperatures in the lower stratosphere above northern Scandinavia leading to a strong activation of chlorine compounds (ClOx) like Cl, Cl2, ClO, ClOOCl, OClO, and HOCl, which rapidly destroyed ozone when sunlight returned after winter solstice. The MIPAS-B (Michelson Interferometer for Passive Atmospheric Sounding) and TELIS (TErahertz and submillimeter LImb Sounder) balloon measurements obtained in northern Sweden on 31 March 2011 inside the polar vortex have provided vertical profiles of inorganic and organic chlorine species as well as diurnal variations of ClO around sunrise over the whole altitude range in which chlorine has been undergoing activation and deactivation. This flight was performed at the end of the winter during the last phase of ClOx deactivation. The complete inorganic and organic chlorine partitioning and budget for 31 March 2011, assumed to be representative for the late-winter Arctic stratosphere, has been derived by combining MIPAS-B and TELIS simultaneously observed molecules. A total chlorine amount of 3.41 ± 0.30 parts per billion by volume is inferred from the measurements (above 24 km). This value is in line with previous stratospheric observations carried out outside the tropics confirming the slightly decreasing chlorine amount in the stratosphere. Observations are compared and discussed with the output of a multi-year simulation performed with the chemistry climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry). The simulated stratospheric total chlorine amount is in accordance with the MIPAS-B/TELIS observations, taking into account the fact that some chlorine source gases and very short-lived species are not included in the model.

Heterogeneous reactions in the Antarctic stratosphere are the cause of chlorine activation and ozone depletion, but the relative roles of different types of polar stratospheric clouds (PSCs) in chlorine activation is an open question. We use multi-year simulations of the chemistry-climate model ECHAM5/MESSy for Atmospheric Chemistry (EMAC) to investigate the impact that the various types of PSCs have on Antarctic chlorine activation and ozone loss. One standard and three sensitivity EMAC simulations have been performed. In all simulations a Newtonian relaxation technique using the ERA-Interim reanalysis was applied to simulate realistic synoptic conditions. In the three sensitivity simulations, we only changed the heterogeneous chemistry on PSC particles by switching the chemistry on liquid, nitric acid trihydrate (NAT) and ice particles on and off. The results of these simulations show that the significance of heterogeneous reactions on NAT and ice particles for chlorine activation and ozone depletion in Antarctic winter and spring is small in comparison to the significance of heterogeneous reactions on liquid particles. Liquid particles alone are sufficient to activate almost all of the available chlorine, with the exception of the upper PSC regions between 10 and 30 hPa, where temporarily ice particles show a relevant contribution. Shortly after the first PSC occurrence, NAT particles contribute a small fraction to chlorine activation. Heterogeneous chemistry on liquid particles is responsible for more than 90% of the ozone depletion in Antarctic spring in the model simulations. In high southern latitudes, heterogeneous chemistry on ice particles causes only up to 5 DU of additional ozone depletion in the column and heterogeneous chemistry on NAT particles less than 0.5 DU. The simulated HNO3, ClO and O3 results agree closely with observations from the Microwave Limb Sounder (MLS) onboard NASA's Aura satellite.

We present the first stage of a new online-coupled global to regional-scale modeling framework for the simulation of the spatiotemporal evolution of aerosols and trace gases. The underlying meteorological model is the new nonhydrostatic model system ICON (ICOsahedral Nonhydrostatic) which allows a local grid refinement with two-way interactions between the grids. We develop the extension ART (Aerosol and Reactive Trace gases) with the goal of simulating interactions between trace substances and the state of the atmosphere. Within this paper, we present the basic equations and give an overview of the physical parameterizations as well as numerical methods we use. First applications of the new model system for trace gases, monodisperse particles and polydisperse particles are shown. The simulated distribution of two very short-lived substances (VSLS), bromoform (CHBr3) and dibromomethane (CH2Br2) reflecting the fast upward transport shows a good agreement with observations and previous model studies. Also, the shape of the simulated tropical profiles is well reproduced. As an example for the treatment of monodisperse particles we present the simulated ash plume of the Eyjafjallajökull eruption in April 2010. Here, a novel approach for the source function is applied. The pattern of the simulated distribution of volcanic ash particles shows a good agreement with previous studies. As an example for the treatment of a polydisperse aerosol, where number densities and mass concentrations are accounted for, we simulated the annual emissions of sea salt. We obtain a total emission flux of 26.0 Pg yr−1 and an emission flux of particles with diameter less than 10 μm of 7.36 Pg yr−1.

This study presents an extended vertical grid for the regional atmospheric model COSMO, used for numerical weather prediction, reaching up to 33 km. The extended setup has been used to stably simulate eleven months in a domain covering central and northern Europe. Temperature and relative humidity have been validated using radio sonde data in polar and temperate latitudes, focussing on the stratosphere. Temperature values are reproduced very well by the model. Relative humidity could only be met in the mean over the whole time period after excluding data from Russian stations, which showed significantly higher values. A study of orographically induced lee waves over Iceland, well visible in the model but not in the regridded boundary data (ERA-Interim and NCEP reanalysis), showcases the advantage and applicability of the model in the extended vertical grid.

Multiannual integrations with the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) have been performed using meteorological analyses of vorticity and divergence up to 10 hPa to analyze the influence of a simplified SF 6 mesospheric chemistry on estimation of mean age of air and to compare profiles of SF 6 mixing ratios observed in the stratosphere with model simulations. The chemical degradation scheme includes electron attachment of SF 6 and subsequent reactions of SF 6 - , such as photodetachment and charge transfer with ozone. Several combinations of reaction rate constants and electron profiles have been tested. Good agreement with observations is found for inert SF 6 transport. However, when mesospheric loss is inclnded in the model, significant deviations are found for polar winter observations above 25 km. Chemical loss by electron attachment without reactions yielding SF 6 again is not compatible with observations. The atmospheric lifetime of SF 6 spans 400 to 10,000 years, depending on the assumed loss mechanism and the value for the electron density in the stratosphere.

Data from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) showed three narrow streamers of air with tropical mixing ratios of HNO3 and N2O pointing from the tropics toward middle latitudes in the middle stratosphere on November 6, 1994. By means of the mechanistic prognostic model, the diagnostic chemical transport model (CTM) and the combined nudged model, which are all versions of the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the hypothesis is checked of whether these streamers are due to adiabatic transport processes on a timescale of days. Whereas the prognostic model reproduces the northern hemisphere streamers only qualitatively in their position, the CTM and the nudged model show a good agreement between their simulated tracer structures and the observed streamers. Because of the clear streamer signal in the nudged model compared to the CTM, its data are used for the investigation of isentropic tracer deformations. They show that the northern hemisphere streamers are mainly built by adiabatic transport on a timescale of days. Rossby wave breaking plays a role in the dissolution of the streamers. In the southern hemisphere, the production of Ertel's potential vorticity (EPV) and the net heating rate is large, and the observed streamers are therefore not reproduced in the EPV. Moreover, the isentropic deformations of the EPV due to the horizontal flow are that strong during a minor warming in the end of October that the reproduction of the southern hemisphere streamer by means of artificial tracers fails.

Results of the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) are compared with vertical ClO profiles measured by the groundbased Millimeter Wave Radiometer MIRA2 inside the vortex during March 1997 at Ny-Alesund. The influence of the OH + ClO and HO 2 + ClO reaction branching ratio and of the absorption cross section of Cl 2 O 2 on the calculated mixing ratios of ClO and ozone has been investigated. In the upper stratosphere the ClO mixing ratio is reduced by 90% by using a minor channel of the OH + ClO reaction with a branching ratio of 0.07. A temperature dependent minor channel of the HO 2 + ClO reaction reduces the upper stratospheric ClO mixing ratio by 22%. Different absorption spectra of Cl 2 O 2 alter the ClO mixing ratios up to 12% at noon at 20 km. This causes differences of 15% in the ozone loss during winter.