Quantification and interpretation of long-term uv-visible observations of the stratosphere (QUILT)

The QUILT webpage (http://nadir.nilu.no/quilt) contains complete project documentation, e.g. annual reports, the documents form projects meetings and the project brochure. Parts of it are secured, only accessible for the project QUILT participants, others are open for the general public. For example the RT model validation package can be directly downloaded from this side. During the timeframe of the QUILT project a database (on the server zardoz at NILU) has been created for NRT data, accessible via the project webpage. The site contains a large number of links, e.g. to the NRT data products sides for GOME and SCIAMACHY (University Bremen) and the SAOZ network (CNRS). Consolidated O3, NO2, BrO and OClO column measurements from a large number of ground-based stations are as well accessible via the project webpage.

An operational method to improve accuracy and information content of ground-based measurements of stratospheric NO2 has been developed. The motive is to improve the investigation of trends in NO2, important because the current trend in NO2 appears to contradict the trend in its source, suggesting that the stratospheric circulation has changed. To do so, a new software package for retrieving NO2 vertical profiles from slant columns measured by zenith-sky spectrometers has been created. It uses a Rodgers optimal linear inverse method coupled with a radiative transfer model for calculations of transfer functions between profiles and columns, and a chemical box model for taking into account the NO2 variations during twilight and during the day. Each model has parameters that vary according to season and location. Forerunners of each model have been previously validated. The scheme maps random errors in the measurements and systematic errors in the models and their parameters on to the retrieved profiles. Initialisation for models is derived from well-established climatologies. The software has been tested by comparing retrieved profiles to simultaneous balloon-borne profiles at mid-latitudes in spring. In the frame of the QUILT project the software package has been distributed to participants. More information on the QUILT project can be found at: http://nadir.nilu.no/quilt/index.php.

As during preceeding winters, the total ozone reduction inside the vortex was quantified during the winter of 2001/2002, 2002/2003 and 2003/2004. The analysis is based on the total ozone measurements of the Arctic SAOZ network from which the contribution of transport has been removed using the 3D CTM REPROBUS model. Winter 2001/2002: The temperature at 475 K was below TNAT from mid-December to early January. Otherwise, the temperature was warm. There was only few possibilities for PSC occurrences, during two periods, one in mid-December and one around January 10. That winter was a rather "warm" winter. The final warming occurred in early March. The vortex formed in December 2001 and was persisting until early April. At that date, two bubbles of vortex were well separated. Two periods of O3 reduction are observed, one around mid-December and one in mid-February/early March. During that period the loss was at a rate of 0.25% per day. The cumulative loss at the end of the winter is 10% which corresponds to ~ 46 DU. Winter 2002/2003: The temperature was below TNAT at all levels from end of November to January 15, and the temperature was very cold, around TICE, at the beginning of December. The final warming occurred after March 22. The vortex was persisting until early April. Three periods of O3 reduction are observed, one in December and one in January at a rate of 0.4% per day and one in end of February at a rate of 0.5 % per day. The cumulative loss at the end of the winter is 23 % which corresponds to ~90 DU. Winter 2003/2004: During the winter of 2003/2004, in early December, the vortex was compact and in coincidence with the cold temperature areas. On 24 December, the vortex was elongated, extending down to Southern France that is to sun-illuminated regions. The situation was very similar during the whole month of January. On January 30, the vortex was split in two bulbs, which recombined, by Mid-February. During the second part of February, the vortex was small, the temperature was increasing and cold surface area was vanishing. In March, the small remaining bulb of vortex is compact and cantered on the pole. In April, the temperature was above 203K at all levels. During the month of December, no loss is observed. A reduction at a rate of 0.3% /day is observed during the first twenty days of January, then the loss rate increases up to 0.5% /day between January 20 and February 10. No loss is observed after that date. The cumulative ozone loss during that winter is 18% corresponding to 90 DU. In addition, experimental results since 1993/1994 have also been compared to 3D CTM multi-annual simulations in order to verify the ability of models to reproduce the ozone loss in a variety of meteorological conditions (cold vortex early in the season with little sunlight or at the opposite, cold and persistent vortex until April, etc.) and chlorine loading. In their most updated version, both model SLIMCAT and REPROBUS, are correctly reproducing the observed ozone loss. More information on the QUILT project can be found at: http://nadir.nilu.no/quilt/index.php.

GOME is one of several instruments aboard the European research satellite ERS-2 (Burrows et al., 1999). It consists of a set of four spectrometers that simultaneously measure sunlight reflected from the earths atmosphere and surface in four spectral windows covering the wavelength range between 240nm and 790nm with moderate spectral resolution (0.2 - 0.4 nm). The satellite operates in a nearly polar, sun-synchronous orbit at an altitude of 780 km with a local equator crossing time at approximately 10:30 local time. While the satellite orbits in an almost north-south direction the GOME instrument sweeps in the perpendicular east west direction. During one sweep, three individual spectral scans are performed. The corresponding three ground pixels covering an area of 320 km (eastwest) by 40km (north-south) lie side by side (a western, a center and an eastern pixel). The Earths surface is totally covered within three days, and polewards from about 70° latitude within one day. From the raw spectra measured by GOME, the slant column density (SCD) is determined using the Differential Optical Absorption Spectroscopy (DOAS) technique [Platt, 1994]. From the inferred differential absorption and knowledge of the absorption cross sections, the SCD of the QUILT-relevant trace species (O3, NO2, BrO and OClO) can be determined. Work on historical satellite data at University of Bremen, University of Heidelberg and BIRA-IASB focused on the homogenisation of the GOME NO2, OClO and BrO time series. Satellite data dissemination: The consolidated GOME BrO data produced at BIRA-IASB - at global scale and extracted for the QUILT stations (200 km radius overpasses) - can be downloaded in ASCII format from http://www.oma.be/BIRA-IASB/Molecules/BrO/index.html. Consolidated NO2, BrO and OClO data retrieved using scientific algorithms from the University of Bremen and the University of Heidelberg can be obtained on the following web-sites: http://www.iup.physik.uni-bremen.de/gome/; and http://satellite.iup.uni-heidelberg.de/. In addition, GOME NO2 data extracted at the University of Bremen for all QUILT stations can be downloaded in ASCII format from http//www.doas-bremen.de/gome_no2_data_quilt.htm. In order to assess the reliability of the consolidated data sets, comparisons of the GOME NO2 and BrO columns with ground-based measurements have been performed by the Bremen University and BIRA-IASB for several stations of the QUILT network. More information on the QUILT project can be found at: http://nadir.nilu.no/quilt/index.php.

The present radiative transfer (RT) model validation package is the deliverable of the QUILT project. Its purpose is to enable the testing of RT codes aiming to the calculation of AMFs(air mass factors)/slant columns as required for ground-based DOAS (Differential Optical Absorption Spectroscopy) and MAX-DOAS applications. The package focus on the simulation of: - Zenith-sky observations of BrO, NO2, and OClO, taking into account the photochemical variation of these species and - Off-axis observations of BrO, NO2, and HCHO, the photochemical variation of the species being not taken into account in this case. The initialisation data and results come from an intercomparison exercise between seven RT models carried out within the framework of QUILT. The different models and comparison tests are described in a word document included in the package. In brief, the following tests have been performed: - Calculation of BrO, OClO, and NO2 SCDs for sunset conditions in zenith-sky geometry, single (SS) and multiple scattering (MS) modes ["preliminary test 1"]. - Calculation of BrO and HCHO SCDs in off-axis geometry, MS mode and for 90° of azimuthal angle of line of sight. The elevation degrees are 5°, 20°, and 40° for BrO, and 5°, 10°, and 20° for HCHO ["preliminary test 2"]. - Test on the impact of aerosol scattering in off-axis geometry: Calculation of NO2 SCDs with and without aerosol scattering in MS mode, for 30° of azimuthal angle of line of sight, and for 5°, 20°, and 40° of elevation. - Test on the impact of ground albedo in off-axis geometry: Calculation of HCHO SCDs for ground albedo values of 0 and 0.9 in MS mode, for 90° of azimuthal angle of line of sight, and for 5°, 20°, and 40° of elevation. The plots of the single profiles used for the tests in off-axis geometry appear in the file "offaxis_profiles_plots.jpeg". Absorption by O3 is included in all calculations. Absorption by NO2 is also included in the calculation of BrO, OClO, and HCHO SCDs. More information on the QUILT project can be found at: http://nadir.nilu.no/quilt/index.php.

Multi- Axis- (MAX-) DOAS, a measurement technique that has become more and more popular in recent years, has the potential to discriminate between tropospheric and stratospheric absorbers. By observing scattered light not only from the zenith but also from sky close to the horizon, a long light path through the lowermost atmospheric layers and a strongly increased sensitivity to trace gases located close to the surface can be achieved. During the QUILT project period, MAX-DOAS measurements of atmospheric trace gases have been performed at various locations, covering the Arctic, the northern mid- latitudes, the tropics and Antarctica, as well as 4 ship cruises from Bremerhaven to Antarctica onboard the Polarstern research vessel. In addition, the owners of this result particapated in the NDSC intercomparison in Andoya in February 2003 and in two campaigns within the FORMAT project with a MAX-DOAS setup. The derived data sets illustrate, that MAX-DOAS is able to: - Measure halogen oxides in the boundary layer. - Measure local pollution, e.g. NO2 in the Arctic. - Give information on the aerosol extinction profiles and the optical properties of aerosol. In summary, Multi-Axis DOAS measurements are a promising tool for continuous measurements of several tropospheric species such as NO2, BrO, SO2, HCHO, and O3. Quantitative analysis of the measurements depends on a number of parameters in the radiative transfer, and models such as those described in the project are now in a position to include all these factors with the necessary accuracy. In the near future other applications for off-axis or MAX-DOAS observations are likely: - Monitoring of air pollution; - Monitoring of volcanoes; - Tomography of urban areas. It is also expected, that advances in software development raises new applications. While radiative transfer models are available to interpret MAX-DOAS data, the algorithms to derive vertical profiles of trace gases are still under development. More information on the QUILT project can be found at: http://nadir.nilu.no/quilt/index.php.

- The new QUILT validated time series of ground-based data have been compared with SLIMCAT 3D CTM results. The model output acts as a transfer standard and shows large differences in column NO2 measured at nearby stations - probably indicating tropospheric pollution at one site. At high latitudes the model overestimates column NO2 due to strong transport in the ECMWF analyses. The BrO comparisons are consistent with a current stratospheric loading of around 21pptv, although the model does not reproduce the sub tropical observations well. There is an inconsistency in the OClO comparisons: The model captures the magintude of the Arctic observations but overestimates the Antarctic ones. - 3D model runs have been performed to look at the observed long-term trend in groundbased NO2 at sites such as Lauder, NZ. Over the period 1990-2003 the basic model does produce a trend similar in magnitude to the observations of around 8%/decade over this time. The observed trend is clearly larger than the observed trend of 3%/decade in the NOy source gas, N2O. There is a large contribution of decreasing aerosol loading during this period. However, the model agreement with the observed trend is also due to a 'dynamical' trend in the model NOy column (i.e. a trend which is present even in a run with constant NOy) and related to a change in the altitude of the NOy profile (e.g. change in tropopause height). More information on the QUILT project can be found at: http://nadir.nilu.no/quilt/index.php.

Over the past decade, in the framework of the international Network for the Detection of Stratospheric Change (NDSC), European capability to monitor stratospheric ozone and the related active radicals NO2, BrO and OClO using ground-based UV-visible spectrometry has been established. A key-aspect of the NDSC is the need to establish methods to improve and consolidate the existing long-term data sets, so these data can be safely used for trend assessment and satellite validation. This result consists in the establishment and collection of a homogenised data set of column measurements of ozone, NO2, BrO and OClO at approximately 30 stations covering all latitudinal bands of both hemispheres. This data set is made available to the scientific community through the QUILT web site. It also provides a valuable European input to the NDSC. More information on the QUILT project can be found at: http://nadir.nilu.no/quilt/index.php.

In this project ozone profiles retrieved from GOME measurements were validated with more then 750 ozone sonde measurements at different geo-locations. The validation showed that daily and seasonal variability of the sonde profiles were accurately reproduced by the GOME measurements. The measurements are in agreement within uncertainty limits (better than 5 %) of the O3 profile measurements and the different footprints for the sonde and GOME measurements (with the exception of the tropics where exist small biases in the GOME O3 profiles: tropospheric O3 is slightly underestimated and stratospheric O3 is overestimated, both by about 5-10 %). A comparison of TM5 model results with the GOME measurements was made for the period 1996-1998. The TM5 model is a chemistry-transport model, i.e. wind fields - which are obtained from a weather forecast model - are used to calculate transport of chemical species. The comparison showed that the model reproduces the observed stratospheric O3 variability, both on short and long timescales. Three biases were identified: the lack of arctic stratospheric O3 depletion in the model results in too much high latitude O3 while the model also overestimates tropospheric O3 due to an incorrect representation of tropospheric chemistry by the model. The third and most prominent bias was the overestimation of mid-latitude lower and middle stratospheric O3 and subsequent underestimation of tropical stratospheric O3. This bias is caused by the too fast stratospheric transport from the tropics to the poles, showing the potential of using stratospheric O3 measurements to study this topic (which is a known feature of chemistry-transport models). More information on the QUILT project can be found at: http://nadir.nilu.no/quilt/index.php.

A large number of O3, NO2, BrO, OClO, IO, and OIO vertical profiles mainly arising from an existing series of balloon-borne UV/visible measurements have been consolidated. The data set stems from a large series of stratospheric balloon flights conducted by either the SAOZ (Systeme dAnalyse par Observation Zénithale from partner 4, CNRS-SA) or LPMA/DOAS payloads (Laboratoire de Physique Moléculaire et Applications/ Differential Optical Absorption Spectrometry from partner 5, IUP-Heidelberg). Earlier data from about 100 SAOZ balloon flights and 13 LPMA/DOAS balloon flights were reprocessed including the most recent available information on - The balloon trajectories, - The spectral retrieval, - Absorption cross sections, and - The radiative transfer. After the data had been reprocessed, they were carefully inter-compared with simultaneous or quasi-simultaneous measurements obtained from independent sensors. More information on the QUILT project can be found at: http://nadir.nilu.no/quilt/index.php.

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