Observation of mesospheric air inside the arctic stratospheric polar vortex in early 2003

During several balloon flights inside the Arctic polar vortex in early 2003, unusual trace gas distributions were observed, which indicate a strong influence of mesospheric air in the stratosphere. The tuneable diode laser (TDL) instrument SPIRALE (Spectroscopie InFrarouge par Absorption de Lasers Embarqués) measured unusually high CO values (up to 600 ppb) on 27 January at about 30 km altitude. The cryosampler BONBON sampled air masses with very high molecular Hydrogen, extremely low SF6 and enhanced CO values on 6 March at about 25 km altitude. Finally, the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) Fourier Transform Infra-Red (FTIR) spectrometer showed NOy values which are significantly higher than NOy* (the NOy derived from a correlation between N2O and NOy under undisturbed conditions), on 21 and 22 March in a layer centred at 22 km altitude. Thus, the mesospheric air seems to have been present in a layer descending from about 30 km in late January to 25 km altitude in early March and about 22 km altitude on 20 March. We present corroborating evidence from a model study using the KASIMA (KArlsruhe Simulation model of the Middle Atmosphere) model that also shows a layer of mesospheric air, which descended into the stratosphere in November and early December 2002, before the minor warming which occurred in late December 2002 lead to a descent of upper stratospheric air, cutting of a layer in which mesospheric air is present. This layer then descended inside the vortex over the course of the winter. The same feature is found in trajectory calculations, based on a large number of trajectories started in the vicinity of the observations on 6 March. Based on the difference between the mean age derived from SF6 (which has an irreversible mesospheric loss) and from CO2 (whose mesospheric loss is much smaller and reversible) we estimate that the fraction of mesospheric air in the layer observed on 6 March, must have been somewhere between 35% and 100%

Subtropical trace gas profiles determined by ground-based FTIR spectroscopy at Izaña (28° N, 16° W): Five-year record, error analysis, and comparison with 3-D CTMs

International audienceWithin the framework of the NDSC (Network for the Detection of Stratospheric Change) ground-based FTIR solar absorption spectra have been routinely recorded at Izaña Observatory (28° N, 16° W) on Tenerife Island since March 1999. By analyzing the shape of the absorption lines, and their different temperature sensitivities, the vertical distribution of the absorbers can be retrieved. Unique time series of subtropical profiles of O3, HCl, HF, N2O, and CH4 are presented. The effects of both dynamical and chemical annually varying cycles can be seen in the retrieved profiles. These include enhanced upwelling and photochemistry in summer and a more disturbed atmosphere in winter, which are typical of the subtropical stratosphere. A detailed error analysis has been performed for each profile. The output from two different three-dimensional (3-D) chemical transport models (CTMs), which are forced by ECMWF analyses, are compared to the measured profiles. Both models agree well with the measurements in tracking abrupt variations in the atmospheric structure, e.g. due to tropical streamers, in particular for the lower stratosphere. Simulated and measured profiles also reflect similar dynamical and chemical annual cycles. However, the differences between their mixing ratios clearly exceed the error bars estimated for the measured profiles. Possible reasons for this are discussed

Global stratospheric hydrogen peroxide distribution from MIPAS-Envisat full resolution spectra compared to KASIMA model results

MIPAS-ENVISAT full resolution spectra were analyzed to obtain a global distribution of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) in the stratosphere. H<sub>2</sub>O<sub>2</sub> acts as reservoir gas for the HO<sub>x</sub> family (= H+OH+HO<sub>2</sub>) and thus, observations of H<sub>2</sub>O<sub>2</sub> provide a better understanding of the HO<sub>x</sub> 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 H<sub>2</sub>O<sub>2</sub> 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 H<sub>2</sub>O<sub>2</sub> 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 HO<sub>2</sub> + HO<sub>2</sub>→H<sub>2</sub>O<sub>2</sub> + O<sub>2</sub> 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 H<sub>2</sub>O<sub>2</sub> 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

Heppa III Intercomparison Experiment on Electron Precipitation Impacts: 2. Model‐Measurement Intercomparison of Nitric Oxide (NO) During a Geomagnetic Storm in April 2010

Precipitating auroral and radiation belt electrons are considered to play an important part in the natural forcing of the middle atmosphere with a possible impact on the climate system. Recent studies suggest that this forcing is underestimated in current chemistry-climate models. The HEPPA III intercomparison experiment is a collective effort to address this point. In this study, we apply electron ionization rates from three data-sets in four chemistry-climate models during a geomagnetically active period in April 2010. Results are evaluated by comparison with observations of nitric oxide (NO) in the mesosphere and lower thermosphere. Differences between the ionization rate data-sets have been assessed in a companion study. In the lower thermosphere, NO densities differ by up to one order of magnitude between models using the same ionization rate data-sets due to differences in the treatment of NO formation, model climatology, and model top height. However, a good agreement in the spatial and temporal variability of NO with observations lends confidence that the electron ionization is represented well above 80 km. In the mesosphere, the averages of model results from all chemistry-climate models differ consistently with the differences in the ionization-rate data-sets, but are within the spread of the observations, so no clear assessment on their comparative validity can be provided. However, observed enhanced amounts of NO in the mid-mesosphere below 70 km suggest a relevant contribution of the high-energy tail of the electron distribution to the hemispheric NO budget during and after the geomagnetic storm on April 6

HEPPA III Intercomparison Experiment on Electron Precipitation Impacts: 1. Estimated Ionization Rates During a Geomagnetic Active Period in April 2010

Precipitating auroral and radiation belt electrons are considered an important part of the natural forcing of the climate system. Recent studies suggest that this forcing is underestimated in current chemistry-climate models. The High Energy Particle Precipitation in the Atmosphere III intercomparison experiment is a collective effort to address this point. Here, eight different estimates of medium energy electron (MEE) (urn:x-wiley:21699380:media:jgra56926:jgra56926-math-0001) ionization rates are assessed during a geomagnetic active period in April 2010. The objective is to understand the potential uncertainty related to the MEE energy input. The ionization rates are all based on the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA/POES and EUMETSAT/MetOp spacecraft series. However, different data handling, ionization rate calculations, and background atmospheres result in a wide range of mesospheric electron ionization rates. Although the eight data sets agree well in terms of the temporal variability, they differ by about an order of magnitude in ionization rate strength both during geomagnetic quiet and disturbed periods. The largest spread is found in the aftermath of enhanced geomagnetic activity. Furthermore, governed by different energy limits, the atmospheric penetration depth varies, and some differences related to latitudinal coverage are also evident. The mesospheric NO densities simulated with the Whole Atmospheric Community Climate Model driven by highest and lowest ionization rates differ by more than a factor of eight. In a follow-up study, the atmospheric responses are simulated in four chemistry-climate models (CCM) and compared to satellite observations, considering both the CCM structure and the ionization forcing

An emission module for ICON-ART 2.0: implementation and simulations of acetone

We present a recently developed emission module for the ICON (ICOsahedral Non-hydrostatic)-ART (Aerosols and Reactive Trace gases) modelling framework. The emission module processes external flux data sets and increments the tracer volume mixing ratios in the boundary layer accordingly. The performance of the emission 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 ground-based as well as 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 uncertainties remain concerning the magnitude of the acetone mixing ratio in the UTLS region. 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. In a sensitivity study we show how different parametrisations of the leaf area index (LAI) change the emission fluxes calculated by MEGAN2.1 and demonstrate the importance of an adequate treatment of the LAI within MEGAN2.1. We conclude that the emission module performs well with offline and online emission fluxes and allows the simulation of the annual cycles of emissions-dominated substances

Validation of MIPAS ClONO2 measurements

Altitude profiles of ClONO2 retrieved with the IMK (Institut fur 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, Izana, 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 x 10(14) cm(-2) (1.0 +/- 1.1%) and -0.09 +/- 0.19 x 10(14) cm(-2) (-0.8 +/- 1.7%), depending on the coincidence criterion applied. chi(2) 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 chi(2) deviations. From the resulting chi(2) profiles there is no evidence for a systematic over/underestimation of the MIPAS random error analysis.Peer reviewe

Observed and simulated time evolution of HCl, ClONO2, and HF total column abundances

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.Peer reviewe