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Balloon-borne observations of ozone from the South Pole Station have been reported to reach ozone mixing ratios below the detection limit of about 10 ppbv at the 70 hPa level by late September. After reaching a minimum, ozone mixing ratios increase to above 1 ppmv on the 70 hPa level by late December. While the basic mechanisms causing the ozone hole have been known for more than 20 yr, the detailed chemical processes determining how low the local concentration can fall, and how it recovers from the minimum have not been explored so far. Both of these aspects are investigated here by analysing results from the Chemical Lagrangian Model of the Stratosphere (CLaMS). As ozone falls below about 0.5 ppmv, a balance is maintained by gas phase production of both HCl and HOCl followed by heterogeneous reaction between these two compounds in these simulations. Thereafter, a very rapid, irreversible chlorine deactivation into HCl can occur, either when ozone drops to values low enough for gas phase HCl production to exceed chlorine activation processes or when temperatures increase above the polar stratospheric cloud (PSC) threshold. As a consequence, the timing and mixing ratio of the minimum ozone depends sensitively on model parameters, including the ozone initialisation. The subsequent ozone increase between October and December is linked mainly to photochemical ozone production, caused by oxygen photolysis and by the oxidation of carbon monoxide and methane.

We explore the potential of ozone observations to constrain transport processes in the tropical tropopause layer (TTL), and contrast it with insights that can be obtained from water vapour. Global fields from Halogen Occultation Experiment (HALOE) and in-situ observations are predicted using a backtrajectory approach that captures advection, instantaneous freeze-drying and photolytical ozone production. Two different representations of transport (kinematic and diabatic 3-month backtrajectories based on ERA-Interim data) are used to evaluate the sensitivity to differences in transport. Results show that mean profiles and seasonality of both tracers can be reasonably reconstructed. Water vapour predictions are similar for both transport representations, but predictions for ozone are systematically higher for kinematic transport. Compared to global HALOE observations, the diabatic model prediction underestimates the vertical ozone gradient. Comparison of the kinematic prediction with observations obtained during the tropical SCOUT-O3 campaign shows a large high bias above 390 K potential temperature. We show that ozone predictions and vertical dispersion of the trajectories are highly correlated, rendering ozone an interesting tracer for aspects of transport to which water vapour is not sensitive. We show that dispersion and mean upwelling have similar effects on ozone profiles, with slower upwelling and larger dispersion both leading to higher ozone concentrations. Analyses of tropical upwelling based on mean transport characteristics, and model validation have to take into account this ambiguity between tropical ozone production and in-mixing from the stratosphere. In turn, ozone provides constraints on transport in the TTL and lower stratosphere that cannot be obtained from water vapour.

Airborne in-situ observations of ClO in the tropics were made during the TROCCINOX (Aracatuba, Brazil, February 2005) and SCOUT-O3 (Darwin, Australia, November/December 2005) field campaigns. While during most flights significant amounts of ClO (≈10–20 parts per trillion, ppt) were present only in aged stratospheric air, instances of enhanced ClO mixing ratios of up to 40 ppt – significantly exceeding those expected from gas phase chemistry – were observed in air masses of a more tropospheric character. Most of these observations are associated with low temperatures or with the presence of cirrus clouds (often both), suggesting that cirrus ice particles and/or liquid aerosol at low temperatures may promote significant heterogeneous chlorine activation in the tropical upper troposphere lower stratosphere (UTLS). In two case studies, particularly high levels of ClO observed were reproduced by chemistry simulations only under the assumption that significant denoxification had occurred in the observed air. However, to reproduce the ClO observations in these simulations, O3 mixing ratios higher than observed had to be assumed, and at least for one of these flights, a significant denoxification is in contrast to the observed NO levels, suggesting that the coupling of chlorine and nitrogen compounds in the tropical UTLS may not be completely understood.

Multi-annual simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) were conducted to study the seasonality of O3 within the stratospheric part of the tropical tropopause layer (TTL), i.e. above θ=360 K potential temperature level. In agreement with satellite (HALOE) and in-situ observations (SHADOZ), CLaMS simulations show a pronounced annual cycle in O3, at and above θ=380 K, with the highest mixing ratios in the late boreal summer. Within the model, this cycle is driven by the seasonality of both upwelling and in-mixing. The latter process occurs through enhanced horizontal transport from the extratropics into the TTL that is mainly driven by the meridional, isentropic winds. The strongest in-mixing occurs during the late boreal summer from the Northern Hemisphere in the potential temperature range between 370 and 420 K. Complementary, the strongest upwelling occurs in winter reducing O3 to the lowest values in early spring. Both CLaMS simulations and Aura MLS O3 observations consistently show that enhanced in-mixing in summer is mainly driven by the Asian monsoon anticyclone.

High-resolution water measurements from three tropical airborne missions in Northern Australia, Southern Brazil and West Africa in different seasons are analysed to study the transport and transformation of water in the tropical tropopause layer (TTL) and its impact on the stratosphere. The mean profiles are quite different according to the season and location of the campaigns, with lowest mixing ratios below 2 ppmv at the cold point tropopause during the Australian mission in November/December and high TTL mixing ratios during the African measurements in August. We present backward trajectory calculations considering freeze-drying of the air to the minimum saturation mixing ratio and initialised with climatological satellite data. This trajectory-based reconstruction of water agrees well with the observed H2O average profiles and therefore demonstrates that the water vapour set point in the TTL is primarily determined by the Lagrangian saturation history. Deep convection was found to moisten the TTL, in several events even above the cold point up to 420 K potential temperatures. However, our study does not provide evidence for a larger impact of these highly-localised events on global scales.

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 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. This analysis demonstrated that mainly the displacement of the ozone depleted polar vortex contributed to the ozone column decrease. Advection of ozone-poor low-latitude air masses was important in the UTLS region. The vertical displacement of isentropes connected with divergence of air out of the column was found to be of minor importance compared to the horizontal transport processes. Severe low total ozone episodes seem to occur when the mentioned mechanisms are superimposed. 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.

Satellite observations show that the enormous solar proton events (SPEs) in October–November 2003 had significant effects on the composition of the stratosphere and mesosphere in the polar regions. After the October–November 2003 SPEs and in early 2004, significant enhancements of NOx(=NO+NO2) in the upper stratosphere and lower mesosphere in the Northern Hemisphere were observed by several satellite instruments. Here we present global full chemistry calculations performed with the CLaMS model to study the impact of mesospheric NOx intrusions on Arctic polar ozone loss processes in the stratosphere. Several model simulations are preformed with different upper boundary conditions for NOx at 2000 K potential temperature (≈50 km altitude). In our study we focus on the impact of the non-local production of NOx, which means the downward transport of enhanced NOx from the mesosphere to the stratosphere. The local production of NOx in the stratosphere is neglected. Our findings show that intrusions of mesospheric air into the stratosphere, transporting high burdens of NOx, affect the composition of the Arctic polar region down to about 400 K (≈17–18 km). We compare our simulated NOx and O3 mixing ratios with satellite observations by ACE-FTS and MIPAS processed at IMK/IAA and derive an upper limit for the ozone loss caused by enhanced mesospheric NOx. Our findings show that in the Arctic polar vortex (equivalent lat.>70° N) the accumulated column ozone loss between 350–2000 K potential temperature (≈14–50 km altitude) caused by the SPEs in October–November 2003 in the stratosphere is up to 3.3 DU with an upper limit of 5.5 DU until end of November. Further, we found that about 10 DU, but in any case lower than 18 DU, accumulated ozone loss additionally occurred until end of March 2004 caused by the transport of mesospheric NOx-rich air in early 2004. The solar-proton-produced NOx above 55 km due to the SPEs of October–November 2003 had a negligibly small impact on ozone loss processes through the end of November in the lower stratosphere (350–700 K≈14–27 km). The mesospheric NOx intrusions in early 2004 yielded a lower stratospheric ozone loss of about 3.5 DU, and clearly lower than 6.5 DU through the end of March. Overall, the non-local production of NOx is an additional variability in the existing variations of the ozone loss observed in the Arctic.

The Match method for the quantification of polar chemical ozone loss is investigated mainly with respect to the impact of the transport of air masses across the vortex edge. For the winter 2002/03, we show that significant transport across the vortex edge occurred and was simulated by the Chemical Lagrangian Model of the Stratosphere. In-situ observations of inert tracers and ozone from HAGAR on the Geophysica aircraft and balloon-borne sondes, and remote observations from MIPAS on the ENVISAT satellite were reproduced well by CLaMS. The model even reproduced a small vortex remnant that remained a distinct feature until June 2003 and was also observed in-situ by a balloon-borne whole air sampler. We use this CLaMS simulation to quantify the impact of transport across the vortex edge on ozone loss estimates from the Match method. We show that a time integration of the determined vortex average ozone loss rates, as performed in Match, results in a larger ozone loss than the polar vortex average ozone loss in CLaMS. The determination of the Match ozone loss rates is also influenced by the transport of air across the vortex edge. We use the model to investigate how the sampling of the ozone sondes on which Match is based represents the vortex average ozone loss rate. Both the time integration of ozone loss and the determination of ozone loss rates for Match are evaluated using the winter 2002/2003 CLaMS simulation. These impacts can explain the majority of the differences between CLaMS and Match column ozone loss. While the investigated effects somewhat reduce the apparent discrepancy in January ozone loss rates reported earlier, a distinct discrepancy between simulations and Match remains. However, its contribution to the accumulated ozone loss over the winter is not large.

We investigate the extent to which quantities that are based on total column ozone are applicable as measures of ozone loss in the polar vortices. Such quantities have been used frequently in ozone assessments by the World Meteorological Organization (WMO) and also to assess the performance of chemistry-climate models. The most commonly considered quantities are March and October mean column ozone poleward of geometric latitude 63° and the spring minimum of daily total ozone minima poleward of a given latitude. Particularly in the Arctic, the former measure is affected by vortex variability and vortex break-up in spring. The minimum of daily total ozone minima poleward of a particular latitude is debatable, insofar as it relies on one single measurement or model grid point. We find that, for Arctic conditions, this minimum value often occurs in air outside the polar vortex, both in the observations and in a chemistry-climate model. Neither of the two measures shows a good correlation with chemical ozone loss in the vortex deduced from observations. We recommend that the minimum of daily minima should no longer be used when comparing polar ozone loss in observations and models. As an alternative to the March and October mean column polar ozone we suggest considering the minimum of daily average total ozone poleward of 63° equivalent latitude in spring (except for winters with an early vortex break-up). Such a definition both obviates relying on one single data point and reduces the impact of year-to-year variability in the Arctic vortex break-up on ozone loss measures. Further, this measure shows a reasonable correlation (r=–0.75) with observed chemical ozone loss. Nonetheless, simple measures of polar ozone loss must be used with caution; if possible, it is preferable to use more sophisticated measures that include additional information to disentangle the impact of transport and chemistry on ozone.

Among the major factors controlling ozone loss in the polar vortices in winter/spring is the kinetics of the ClO dimer catalytic cycle. Here, we propose a strategy to test and improve our understanding of these kinetics by comparing and combining information on the thermal equilibrium between ClO and Cl2O2, the rate of Cl2O2 formation, and the Cl2O2 photolysis rate from laboratory experiments, theoretical studies and field observations. Concordant with a number of earlier studies, we find considerable inconsistencies of some recent laboratory results with rate theory calculations and stratospheric observations of ClO and Cl2O2. The set of parameters for which we find the best overall consistency – namely the ClO/Cl2O2 equilibrium constant suggested by Plenge et al. (2005), the Cl2O2 recombination rate constant reported by Nickolaisen et al. (1994) and Cl2O2 photolysis rates based on absorption cross sections in the range between the JPL 2006 assessment and the laboratory study by Burkholder et al. (1990) – is not congruent with the latest recommendations given by the JPL and IUPAC panels and does not represent the laboratory studies currently regarded as the most reliable experimental values. We show that the incorporation of new Pope et al. (2007) Cl2O2 absorption cross sections into several models, combined with best estimates for other key parameters (based on either JPL and IUPAC evaluations or on our study), results in severe model underestimates of observed ClO and observed ozone loss rates. This finding suggests either the existence of an unknown process that drives the partitioning of ClO and Cl2O2, or else some unidentified problem with either the laboratory study or numerous measurements of atmospheric ClO. Our mechanistic understanding of the ClO/Cl2O2 system is grossly lacking, with severe implications for our ability to simulate both present and future polar ozone depletion.

A one-dimensional chemistry model is applied to study the stable hydrogen (D) and stable oxygen isotope (17O, 18O) composition of water vapour in stratosphere and mesosphere. In the troposphere, this isotope composition is determined by 'physical'' fractionation effects, that are phase changes (e.g. during cloud formation), diffusion processes (e.g. during evaporation from the ocean), and mixing of air masses. Due to these processes water vapour entering the stratosphere first shows isotope depletions in D/H relative to ocean water, which are ~5 times of those in 18O/16O, and secondly is mass-dependently fractionated (MDF), i.e. changes in the isotope ratio 17O/16O are ~0.52 times of those of 18O/16O. In contrast, in the stratosphere and mesosphere 'chemical'' fractionation mechanisms, that are the production of HO due to the oxidation of methane, re-cycling of H2O via the HOx family, and isotope exchange reactions considerably enhance the isotope ratios in the water vapour imported from the troposphere. The model reasonably predicts overall enhancements of the stable isotope ratios in H2O by up to ~25% for D/H, ~8.5% for 17O/16O, and ~14% for 18O/16O in the mesosphere relative to the tropopause values. The 17O/16O and 18O/16O ratios in H2O are shown to be a measure of the relative fractions of HOx that receive the O atom either from the reservoirs O2 or O3. Throughout the middle atmosphere, MDF O2 is the major donator of oxygen atoms incorporated in OH and HO2 and thus in H2O. In the stratosphere the known mass-independent fractionation (MIF) signal in O3 is in a first step transferred to the NOx family and only in a second step to HOx and H2O. In contrast to CO2, O(1D) only plays a minor role in this MIF transfer. The major uncertainty in our calculation arises from poorly quantified isotope exchange reaction rate coefficients and kinetic isotope fractionation factors.

The Halogen Occultation Experiment (HALOE) on board the Upper Atmosphere Research Satellite (UARS) has observed mixing ratios of important trace species in the stratosphere for more than a decade since 1991. Here we present a climatology for the stratosphere compiled from HALOE O3, H2O, CH4, NOx, HCl, and HF data for the period from 1991 to 2002. In this approach, the data are averaged over equivalent latitude instead of latitude in order to correctly reproduce the gradients at the transport barriers like the polar vortex edge. The climatology is compiled for 5 degree equivalent latitude bins. Seasonal dependence is taken into account by choosing intervals of one month. The climatology is available as an electronic supplement.

In-situ measurements of ClO and its dimer carried out during the SOLVE II/VINTERSOL-EUPLEX and ENVISAT Validation campaigns in the Arctic winter 2003 suggest that the thermal equilibrium between the dimer formation and dissociation is shifted significantly towards the monomer compared to the current JPL 2002 recommendation. Detailed analysis of observations made in thermal equilibrium allowed to re-evaluate the magnitude and temperature dependence of the equilibrium constant. A fit of the JPL format for equilibrium constants yields KEQ=3.61x10-27exp(8167/T), but to reconcile the observations made at low temperatures with the existing laboratory studies at room temperature, a modified equation, KEQ=5.47x10-25(T/300)-2.29exp(6969/T), is required. This format can be rationalised by a strong temperature dependence of the reaction enthalpy possibly induced by Cl2O2 isomerism effects. At stratospheric temperatures, both equations are practically equivalent. Using the equilibrium constant reported here rather than the JPL 2002 recommendation in atmospheric models does not have a large impact on simulated ozone loss. Solely at large zenith angles after sunrise, a small decrease of the ozone loss rate due to the ClO dimer cycle and an increase due to the ClO-BrO cycle (attributed to the enhanced equilibrium ClO concentrations) is observed, the net effect being a slightly stronger ozone loss rate.

Chlorine monoxide (ClO) plays a key role in stratospheric ozone loss processes at midlatitudes. We present two balloon-borne in situ measurements of ClO conducted in northern hemisphere midlatitudes during the period of the maximum of total inorganic chlorine loading in the atmosphere. Both ClO measurements were conducted on board the TRIPLE balloon payload, launched in November 1996 in León, Spain, and in May 1999 in Aire sur l'Adour, France. For both flights a ClO daylight and night-time vertical profile was derived over an altitude range of approximately 15-35 km. ClO mixing ratios are compared to model simulations performed with the photochemical box model version of the Chemical Lagrangian Model of the Stratosphere (CLaMS). Simulations along 24-hour backward trajectories were performed to study the diurnal variation of ClO in the midlatitude lower stratosphere. Model simulations for the flight launched in Aire sur l'Adour 1999 show an excellent agreement with the ClO measurements. For the flight launched in León 1996, an overall good agreement is found, whereas the flight is characterized by a more complex dynamical situation due to a possible mixture of vortex and non-vortex air. We note that for both flights at solar zenith angles greater than 86°-87° simulated ClO mixing ratios are higher than observed ClO mixing ratios. However, the present findings indicate that no substantial uncertainties exist in midlatitude chlorine chemistry of the stratosphere.

We present simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) for the Arctic winter 2002/2003. We integrated a Lagrangian denitrification scheme into the three-dimensional version of CLaMS that calculates the growth and sedimentation of nitric acid trihydrate (NAT) particles along individual particle trajectories. From those, we derive the HNO3 downward flux resulting from different particle nucleation assumptions. The simulation results show a clear vertical redistribution of total inorganic nitrogen ( ), with a maximum vortex average permanent removal of over 5ppb in late December between 500 and 550K and a corresponding increase of of over 2ppb below about 450K. The simulated vertical redistribution of is compared with balloon observations by MkIV and in-situ observations from the high altitude aircraft Geophysica. Assuming a globally uniform NAT particle nucleation rate of 7.8x10-6cm-3h-1 in the model, the observed denitrification is well reproduced. In the investigated winter 2002/2003, the denitrification has only moderate impact (≤14%) on the simulated vortex average ozone loss of about 1.1ppm near the 460K level. At higher altitudes, above 600K potential temperature, the simulations show significant ozone depletion through -catalytic cycles due to the unusual early exposure of vortex air to sunlight.

Tracer-tracer relations have been used for a long time to separate physico-chemical change from change caused by transport processes. In particular, for more than a decade, ozone-tracer relations have been used to quantify chemical ozone loss in the polar vortex. The application of ozone-tracer relations for quantifying ozone loss relies on two hypotheses: that a compact ozone-tracer relation is established in the 'early' polar vortex and that any change of the ozone-tracer relation in the vortex over the course of winter is caused predominantly by chemical ozone loss. Here, we revisit this issue by analysing various sets of measurements and the results from several models. We find that mixing across the polar vortex edge impacts ozone-tracer relations in a way that may solely lead to an 'underestimation' of chemical ozone loss and not to an overestimation. Further, differential descent in the vortex and internal mixing has only a negligible impact on ozone loss estimates. Moreover, the representation of mixing in three-dimensional atmospheric models can have a substantial impact on the development of tracer relations in the model. Rather compact ozone-tracer relations develop – in agreement with observations – in the vortex of a Lagrangian model (CLaMS) where mixing is anisotropic and driven by the deformation of the flow. We conclude that, if a reliable 'early vortex' reference can be obtained and if vortex measurements are separated from mid-latitude measurements, ozone-tracer relations constitute a reliable tool for the quantitative determination of chemical ozone loss in the polar vortex.

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.

Chemical ozone loss in the Arctic stratosphere was investigated for the twelve years between 1991 and 2003 employing the ozone-tracer correlation method. For this method, the change in the relation between ozone and a long-lived tracer is considered for all twelve years over the lifetime of the polar vortex to calculate chemical ozone loss. Both the accumulated local ozone loss in the lower stratosphere and the column ozone loss were derived consistently, mainly on the basis of HALOE satellite observations. HALOE measurements do not cover the polar region homogeneously over the course of the winter. Thus, to derive an early winter reference function for each of the twelve years, all available measurements were additionally used; for two winters climatological considerations were necessary. Moreover, a detailed quantification of uncertainties was performed. This study further demonstrates the interaction between meteorology and ozone loss. The connection between temperature conditions and chlorine activation, and in turn, the connection between chlorine activation and ozone loss, becomes obvious in the HALOE HCl measurements. Additionally, the degree of homogeneity of ozone loss within the vortex was shown to depend on the meteorological conditions. Results derived here are in general agreement with the results obtained by other methods for deducing polar ozone loss. Differences occur mainly owing to different time periods considered in deriving accumulated ozone loss. However, very strong ozone losses as deduced from SAOZ for January in winters 1993-1994 and 1995-1996 cannot be identified using available HALOE observations in the early winter. In general, strong accumulated ozone loss was found to occur in conjunction with a strong cold vortex containing a large volume of possible PSC existence (VPSC), whereas moderate ozone loss was found if the vortex was less strong and moderately warm. Hardly any ozone loss was calculated for very warm winters with small amounts of VPSC during the entire winter. This study supports the linear relationship between VPSC and the accumulated ozone loss reported by Rex et al. (2004) if VPSC was averaged over the entire winter period. Here, further meteorological factors controlling ozone loss were additionally identified if VPSC was averaged over the same time interval as that for which the accumulated ozone loss was deduced. A significant difference in ozone loss (of ≈36DU) was found due to the different duration of solar illumination of the polar vortex of at maximum 4 hours per day in the observed years. Further, the increased burden of aerosols in the atmosphere after the Pinatubo volcanic eruption in 1991 significantly increased the extent of chemical ozone loss.

Mixing ratios of water (H2O) in the stratosphere appear to increase due to increased input of H2O and methane from the troposphere and due to intensified oxidation of CH4 in the stratosphere, but many of the underlying mechanisms are not yet understood. Here we identify and quantify three chemical mechanisms which must have led to more efficient oxidation of CH4 in the stratosphere over the past several decades: 1) The increase in stratospheric chlorine levels due to anthropogenic CFC emissions, 2) the thinning of the stratospheric ozone column and 3) enhanced OH levels in the stratosphere due to increasing H2O levels themselves. In combination with the increase in tropospheric CH4 mixing ratios and with solar cycle related variations of upper stratospheric ozone, these effects can explain about 50% of the additional conversion of CH4 to H2O as observed throughout the stratosphere. The relative contributions from the individual processes have varied over the past decades.

High-resolution simulations of the chemical composition of the Arctic stratosphere during late spring 1997 and 2000 were performed with the Chemical Lagrangian Model of the Stratosphere (CLaMS). The simulations were performed for the entire northern hemisphere on two isentropic levels 450 K (~18 km) and 585 K (~24 km). The spatial distribution and the lifetime of the vortex remnants formed after the vortex breakup in May 1997 display different behavior above and below 20 km. Above 20 km, vortex remnants propagate southward (up to 40°N) and are "frozen in'' in the summer circulation without significant mixing. Below 20 km the southward propagation of the remnants is bounded by the subtropical jet. Their lifetime is shorter by a factor of 2 than that above 20 km, owing to significant stirring below this altitude. The behavior of vortex remnants formed in March 2000 is similar but, due to an earlier vortex breakup, dominated during the first 6 weeks after the vortex breakup by westerly winds, even above 20 km. Vortex remnants formed in May 1997 are characterized by large mixing ratios of HCl indicating negligible, halogen-induced ozone loss. In contrast, mid-latitude ozone loss in late boreal spring 2000 is dominated, until mid-April, by halogen-induced ozone destruction within the vortex remnants, and subsequent transport of the ozone-depleted polar air masses (dilution) into the mid-latitudes. By varying the intensity of mixing in CLaMS, the impact of mixing on the formation of ClONO2 and ozone depletion is investigated. We find that the photochemical decomposition of HNO3 and not mixing with NOx-rich mid-latitude air is the main source of NOx within the vortex remnants in March and April 2000. Ozone depletion in the remnants is driven by ClOx photolytically formed from ClONO2. At the end of May 1997, the halogen-induced ozone deficit at 450 K poleward of 30°N amounts to ~12% with ~10% in the polar vortex and ~2% in well-isolated vortex remnants after the vortex breakup.

Current stratospheric chemical model simulations underestimate substantially the large ozone loss rates that are derived for the Arctic from ozone sondes for January of some years. Until now, no explanation for this discrepancy has been found. Here, we examine the influence of intrusions of mid-latitude air into the polar vortex on these ozone loss estimates. This study focuses on the winter 1991/92, because during this winter the discrepancy between simulated and experimentally derived ozone loss rates is reported to be the largest. Also during the considered period the vortex was disturbed by a strong warming event with large-scale intrusions of mid-latitude air into the polar vortex, which is quite unusual for this time of the year. The study is based on simulations performed with the Chemical Lagrangian Model of the Stratosphere (CLaMS). Two methods for determination the ozone loss are investigated, the so-called vortex average approach and the Match method. The simulations for January 1992 show that the intrusions induce a reduction of vortex average ozone mixing ratio corresponding to a systematic offset of the ozone loss rate of about 12 ppb per day. This should be corrected for in the vortex average method. The simulations further suggest, that these intrusions do not cause a significant bias for the Match method due to effective quality control measures in the Match technique.

Severe chemical ozone loss has been detected in the Arctic in the winter and spring of 1995-96 by a variety of methods. Extreme reductions in column ozone due to halogen catalysed chemistry were derived from measurements of the Halogen Occultation Experiment (HALOE) on board the Upper Atmosphere Research Satellite in the Arctic vortex. Here, we discuss further aspects of the HALOE observations in the Arctic over this period. Potential problems, both in the data themselves and in the methodology of the data analysis are considered and the reason for the differences between the Arctic ozone losses deduced from HALOE data version 17 and 18 is analysed. Moreover, it is shown that HALOE measurements in the Arctic in winter and spring 1995-96 compare well with observations by other ground-based and satellite instruments.Key words. Chemical ozone loss (Arctic vortex 1995 · 96; halogen chemistry)

We investigated chemical and microphysical processes in the late winter in the Antarctic lower stratosphere, after the first chlorine activation and initial ozone depletion. We focused on a time interval when both further chlorine activation and ozone loss, but also chlorine deactivation, occur. We performed a comprehensive Lagrangian analysis to simulate the evolution of an air mass along a 10-day trajectory, coupling a detailed microphysical box model to a chemistry model. Model results have been compared with in situ and remote sensing measurements of particles and ozone at the start and end points of the trajectory, and satellite measurements of key chemical species and clouds along it. Different model runs have been performed to understand the relative role of solid and liquid polar stratospheric cloud (PSC) particles for the heterogeneous chemistry, and for the denitrification caused by particle sedimentation. According to model results, under the conditions investigated, ozone depletion is not affected significantly by the presence of nitric acid trihydrate (NAT) particles, as the observed depletion rate can equally well be reproduced by heterogeneous chemistry on cold liquid aerosol, with a surface area density close to background values. Under the conditions investigated, the impact of denitrification is important for the abundances of chlorine reservoirs after PSC evaporation, thus stressing the need to use appropriate microphysical models in the simulation of chlorine deactivation. We found that the effect of particle sedimentation and denitrification on the amount of ozone depletion is rather small in the case investigated. In the first part of the analyzed period, when a PSC was present in the air mass, sedimentation led to a smaller available particle surface area and less chlorine activation, and thus less ozone depletion. After the PSC evaporation, in the last 3 days of the simulation, denitrification increases ozone loss by hampering chlorine deactivation.

The three-dimensional quantification of small-scale processes in the upper troposphere and lower stratosphere is one of the challenges of current atmospheric research and requires the development of new measurement strategies. This work presents the first results from the newly developed Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) obtained during the ESSenCe (ESa Sounder Campaign) and TACTS/ESMVal (TACTS: Transport and composition in the upper troposphere/lowermost stratosphere, ESMVal: Earth System Model Validation) aircraft campaigns. The focus of this work is on the so-called dynamics-mode data characterized by a medium-spectral and a very-high-spatial resolution. The retrieval strategy for the derivation of two- and three-dimensional constituent fields in the upper troposphere and lower stratosphere is presented. Uncertainties of the main retrieval targets (temperature, O3, HNO3, and CFC-12) and their spatial resolution are discussed. During ESSenCe, high-resolution two-dimensional cross-sections have been obtained. Comparisons to collocated remote-sensing and in situ data indicate a good agreement between the data sets. During TACTS/ESMVal, a tomographic flight pattern to sense an intrusion of stratospheric air deep into the troposphere was performed. It was possible to reconstruct this filament at an unprecedented spatial resolution of better than 500 m vertically and 20 × 20 km horizontally.