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Current procedures used in the clearance/analysis of gain scheduled flight control laws for Launch Vehicles (LV) tend to be very time consuming and complicated. Furthermore, the clearance procedure is in many cases limited to only certain vertices (determined on prior experience) of the uncertain parameter space, in the hope that the worst case scenario is not somewhere in the middle. The main objective of this study is to apply the modern control concepts of Linear Fractional Transformations (LFT), µ-analysis and Linear Parameter Varying (LPV) techniques, to the clearance/analysis of gain scheduled flight control laws for a single axis attitude Thrust Vector Control (TVC) Launch Vehicle.

Studies have shown that the total ozone amount and the ozone mixing ratio on the Earth's equator correlate with the 27-day rotational modulation of the solar activity. We report some first results of our research, concerning possible links between the short-term solar activity, i.e., variations on the rotational time-scale, and the stratospheric NO2. The solar activity variations and the atmospheric responses were studied by means of wavelet analysis, cross-correlation and cross-wavelet analysis. It is found out that the 27-day solar cycle in the UV irradiation is modulated by a longer period of several months; this longer period is probably related with active region growth and decay (ARGD). That kind of modulation produces shortening or extension of the 27-day basic period, ranging from 22 to 34 days. The modulation is a consequence of ARGD and hence, results from a variable pattern of spots and active regions on the solar surface. We find a similar time-frequency behaviour in the wavelet maps of NO2 slant column amounts, derived from DOAS measurements at the Stara Zagora station as well as in the total ozone amount, obtained by the GOME instrument during solar cycle 23. For those time intervals when the cross-correlation between the variation of the short-term solar activity and that of the trace gas species is positive, the cross-correlation coefficients are up to 0.6 for the NO2 and up to 0.5 for the ozone variation.

In the field of air quality control, optical remote sensing systems can measure the spatial distribution of gas pollutants, offering numerous advantages over conventional networks of in situ analysers. We propose some innovative solutions in the field of DOAS (Differential Optical Absorption Spectroscopy) remote systems, utilizing diffuse solar light as the radiation source. We examine the numerous potentialities of minor gas slant column calculations, applying the "off-axis" methodology for collecting the diffuse solar radiation. One of these particular approaches, using measurements along horizontal paths, has already been tested with the spectrometer installed on board the Geophysica aircraft during stratospheric flights up to altitudes of 20 km. The theoretical basis of these new measurement techniques using DOAS remote sensing systems are delineated to assess whether low altitude flights can provide 2D and 3D pollution tomography over metropolitan areas. The 2D or 3D trace gas total column mapping could be used to investigate: i) transport and dispersion phenomena of air pollution, ii) photochemical process rates, iii) gas plume tomography, iv) minor gas vertical profiles into the Planetary Boundary Layer and v) minor gas flux divergence over a large area.

A correct interpretation of diffuse solar radiation measurements made by Differential Optical Absorption Spectroscopy (DOAS) remote sensors require the use of radiative transfer models of the atmosphere. The simplest models consider radiation scattering in the atmosphere as a single scattering process. More realistic atmospheric models are those which consider multiple scattering and their application is useful and essential for the analysis of zenith and off-axis measurements regarding the lowest layers of the atmosphere, such as the boundary layer. These are characterized by the highest values of air density and quantities of particles and aerosols acting as scattering nuclei.

The results of a comparison exercise of radiative transfer models (RTM) of various international research groups for Multiple AXis Differential Optical Absorption Spectroscopy(MAX-DOAS) viewing geometry are presented. In contrast to previous comparison exercises, box-air-mass 5 -factors (box-AMFs) for various atmospheric height layers were modelled, which describe the sensitivity of the measurements as a function of altitude. In addition, radiances were calculated allowing the identification of potential errors, which might be overlooked if only AMFs are compared. Accurate modelling of radiances is also a prerequisite for the correct interpretation of satellite observations, for which the received radiance can strongly vary across the large ground pixels, and might be also important for the retrieval of aerosol properties as a future application of MAXDOAS. The comparison exercises included different wavelengths and atmospheric scenarios (with and without aerosols). The results were systematically investigated with respect to their dependence on the telescope’s elevation angle and the azimuth angle. For both dependencies, a strong and systematic influence of aerosol scattering was found indicating that from MAX-DOAS observations also information on atmospheric aerosols can be retrieved. During the various iterations of the exercises, the results from all models showed a substantial convergence, and the final data sets agreed for most cases within about 5%. Larger deviations were found for cases with low atmospheric optical depth, for which the photon path lengths along the line of sight of the instrument can become very large. The differences occurred between models including full spherical geometry and those using only plane parallel approximation indicating that the correct treatment of the Earth’s sphericity becomes indispensable. The modelled box-AMFs constitute an universal data base for the calculation of arbitrary (total) AMFs by simple convolution with a given trace gas concentration profile. Together with the modelled radiances and the specified settings for the various exercises, they can serve as test cases for future RTM developments.

The present paper describes a UV-VIS spectroradiometer named GASCOD/A4 pi developed at ISAC-CNR for remote sensing measurements aboard stratospheric M55-Geophysica aircraft, flying up to 21 km. Obtained experimental data are used for retrieving of NO2, O-3 and of other minor gases atmospheric content, applying the DOAS (Differential Optical Absorption Spectroscopy) method. UV actinic flux and J(NO2) are also derived. All these parameters are used for satellite data validation tasks. The specific results obtained during dedicated aircraft missions in different geographical areas have already been utilized for ENVISAT validation.

We present the results of an intercomparison exercise between six different radiative transfer(RT) models carried out in the framework of QUILT, an EU funded project based on the exploitation of the Network for the Detection of Stratospheric Change (NDSC). RT modeling is an important step in the interpretation of Differential Optical Absorption Spectroscopy (DOAS) observations. It allows the conversion of the slant column densities (SCDs) into vertical column densities (VCDs) using calculated air mass factors (AMFs). The originality of our study resides in comparing SCD simulations in multi-axis (MAX) geometry (trace gases: NO2 and HCHO) and in taking into account the photochemical enhancement for calculating SCDs of rapidly photolysing species (BrO, NO , and OClO) in zenith-sky geometry. Concerning the MAX simulations, good agreement is observed between the different models with the calculated NO2 and HCHO SCDs differing by no more than 5% in the elevation and solar zenith angles (SZA) ranges investigated (5°-20° and 35°-85°, respectively). The impacts of aerosol scattering, ground albedo, and relative azimuth on MAX simulations have also been tested. Large discrepancies appear for the aerosol effect, suggesting differences between models in the treatment of the aerosol scattering. A better agreement is obtained in the case of the ground albedo and relative azimuth effects. In zenith-sky geometry, the different models agree generally well, especially below 90° SZA. At higher SZA, larger discrepancies are observed with relative difference values between 2% and 14% in some cases. All the initialization data and results have been made publicly available through the QUILT project web site (http://nadir.nilu.no/quilt/), enabling the testing of other RT codes designed for the calculation of SCDs/AMFs.

The work presents measurements obtained with the multi-input spectroradiometer GASCOD/A4p, the Fourier transform far infrared interferometer SAFIRE-A and the two-channel chromatograph HAGAR, on board the M-55 Geophysica stratospheric aircraft during the APE-GAIA measurement campaign, held in September-October 1999 over the Antarctic Peninsula. The paper aims to investigate the gas mixing ratio distributions at the flight altitude, during a recovery phase of the reservoir species in the lower stratosphere in the Antarctic spring. Attention is focused on the daytime flight of 23 September, when the aircraft flew inside the vortex at two different altitudes (about 16.5 and 18.5 km). Concentration distributions were examined for NO2, N2O, O3, BrO, ClO, HCl, H2O and HNO3, during flight inside and outside the vortex. Simultaneous concentration measurements of HNO3 and H2O showed a denitrification but not dehydration inside the vortex core. The reformation of chlorine reservoirs is investigated. It was evident that the re-conversion of HCl was favoured in the Cl+CH4 branches, and its high mixing ratio up to 1 ppbv at 425 K surface confirms that low ozone air masses inside the vortex and strong sunlight greatly favour a more rapid formation of passive chlorine reservoirs.

International audience; We present the results of an intercomparison exercise between six different radiative transfer (RT) models carried out in the framework of QUILT, an EU funded project based on the exploitation of the Network for the Detection of Stratospheric Change (NDSC). RT modeling is an important step in the interpretation of Differential Optical Absorption Spectroscopy (DOAS) observations. It allows the conversion of the slant column densities (SCDs) into vertical column densities (VCDs) using calculated air mass factors (AMFs). The originality of our study resides in comparing SCD simulations in multi-axis (MAX) geometry (trace gases: NO2 and HCHO) and in taking into account the photochemical enhancement for calculating SCDs of rapidly photolysing species (BrO, NO2, and OClO) in zenith-sky geometry. Concerning the MAX simulations, good agreement is observed between the different models with the calculated NO2 and HCHO SCDs differing by no more than 5% in the elevation and solar zenith angles (SZA) ranges investigated (5°?20° and 35°?85°, respectively). The impacts of aerosol scattering, ground albedo, and relative azimuth on MAX simulations have also been tested. Large discrepancies appear for the aerosol effect, suggesting differences between models in the treatment of the aerosol scattering. A better agreement is obtained in the case of the ground albedo and relative azimuth effects. In zenith-sky geometry, the different models agree generally well, especially below 90° SZA. At higher SZA, larger discrepancies are observed with relative difference values between 2% and 14% in some cases. All the initialization data and results have been made publicly available through the QUILT project web site (http://nadir.nilu.no/quilt/), enabling the testing of other RT codes designed for the calculation of SCDs/AMFs.

International audience; High tropospheric NO2 amounts are occasionally detected by space-borne spectrometers above cloudy scenes. For monitoring of near-ground air pollution such data are not directly applicable because clouds shield the highly polluted planetary boundary layer (PBL). We present a method based on trajectories which implicitly estimates the additional sub-cloud NO2 distribution in order to model concentrations at ground stations. The method is applied to a transboundary pollution transport episode which led to high NO2 vertical tropospheric column densities (VTCs) over middle Europe observed by the Global Ozone Monitoring Experiment (GOME) instrument above clouds on 17 February 2001. The case study shows that pollution originally residing near the ground in central Germany, the Ruhr area and adjacent parts of the Netherlands and Belgium has been advected to higher tropospheric levels by a passing weather front. Combining the above-cloud NO2 VTCs with trajectory information covering the GOME columns and including their sub-cloud part yields an estimate of the total NO2 distribution within the tropospheric columns. The highly polluted air masses are then traced by forward trajectories starting from the GOME columns to move further to the Alpine region and their impact there is assessed. Considering ground-based in-situ measurements in the Alpine region, we conclude that for this episode, at least 50% of the NO2 concentration recorded at the sites can be attributed to transboundary transport during the frontal passage. This study demonstrates the potential of using NO2 VTCs from GOME detected above clouds when combined with transport modelling.

The satellite remote-sensing instruments for climatic studies are required to have: i) fast time sampling, ii) high spectral resolution, iii) high space resolution, iv) wide field of view, v) broad spectral range, vi) simultaneous measurements in different spectral intervals and/or type of measurements vii) lack of movable mechanical parts, etc. Here it is described the main idea of an input optic of remote sensing UV-VIS-IR instrument aimed for climatic studies form the space. It is attempted with the proposed optical system to satisfy requirements ii), vi), v), vii) and, at least partially, the rest ones.

In this paper we report a climatological analysis of nitrogen dioxide total column amounts at Mt. Cimone station 44/sup /spl deg// 11' N, 10/sup /spl deg// 42' E, 2165 m asl). Eight years of measurements obtained with the Gas Absorption Spectrometer Correlating Optical Differences (GASCOD) UV-Vis spectrometer are presented and a method based on the power spectrum analysis is proposed to investigate the nitrogen dioxide long term trend. This kind of diagnostic approach uses the geophysical phenomena (predictor) Identification in the frequency space to filter the time series from the natural induced oscillations. The advantage with respect to a statistical approach based on the correlation study between measurements and geophysical predictors is that only the identified phenomena are removed from the trend. On the other hand the effect of data missing in the filtering should be discussed and quantified in order to avoid misunderstanding in the results interpretation. Results show that a 7% increase per decade in the total column NO/sub 2/ is present at northern mid-latitudes.

The application of Differential Optical Absorption Spectroscopy (DOAS) methodology to the zenith scattered light data collected with the GASCOD spectrometer developed at the ISAC Institute allow for the detection of stratospheric trace gases involved in the ozone cycle such as NO2, OCIO, BrO. The instrument was installed in December 1995 in the Italian Antarctic station at Terra Nova Bay (74degrees26'S, 164degrees03E', Ross Sea), after several tests both in laboratory and in Antarctic region, for unattended and continuous measurement in extreme high-latitude environment. The GASCOD is, still working and producing very interesting data for the study of the. denitrification processes during the formation of the so-called ozone hole over the Antarctic region. For the continuous NO2 monitoring for whole the year, also during winter when the station is unmanned, the [407-460] nm spectral region is investigated. The results for Nitrogen Dioxide, obtained by application of DOAS algorithms to the data recorded during the year 2001, are presented. ERS-2 was launched in April 1995 into a near-polar sun-synchronous orbit at a mean altitude of 795 km. The descending node crosses the equator at 10:30 local time. GOME is a nadir-scanning double monochromator covering the 237 nm to 794 nm wavelength range with a spectral resolution of 0.17-0.33 run. The spectrum is split into four spectral channels, each recorded quasi-simultaneously by a 1024-pixel photodiode array. The global spatial coverage is obtained within 3 days at the equator by a 960 Km across-track swath (4.5 s forward scan, 1.5 s back scan). The ground pixel size of the measurements is 320 X 40 km(2). A comparison of GASCOD and GOME results for NO2 total column is performed.

The scientific payload aboard the stratospheric aircraft M55 Geophysica consists of both in-situ and remote sensing instruments deployed to validate the ENVISAT chemical payload - SCIAMACHY, MIPAS-E and GOMOS during dedicated field campaigns: July and October 2002, Forli (Italy) and February-March 2003, Kiruna (Sweden). Along with the precise measurements required for correct validation procedures, it is necessary to provide additional information related to certain relationships between the available geophysical parameters in order to allow us to better interpret retrieved results, both from the space and from the aircraft measurements. In this regard NO2/NO ratio is inferred/tested along the flight tracks and used to verify the existence of a steady state photochemical equilibrium, using the data obtained by GASCOD-A/4pi, FOZAN and SIOUX instruments. The obtained experimental ratio is compared to that derived from model calculations. A short description of the instruments and flight conditions are described also.

Nitrogen dioxide (NO 2 ) is a representative of a family of chemically very active radicals NO x =NO+NO 2 . These radicals catalytically destroy the ozone and play a significant role in other ozone-destroying catalytic cycles. In the polar night NO 2 can deactivate ClO by a termolecular reaction to form the reservoir gas Cl0NO 2 . In this way NO 2 restricts the ozone destruction in the polar vortex. NO 2 also takes part in heterogeneous reactions, which occur on the surface of polar stratospheric clouds. For the trace gases study and particularly for the nitrogen dioxide a spectrometric instrument GASCOD-BG was installed at the Solar Terrestrial Influences Laboratory - Bulgarian Academy of Sciences , Stara Zagora Department in August 1999. The instrument was developed at the Institute of Atmospheric and Oceanic Sciences at the Council of National Research, Italy, Bologna. The results of the NO 2 observation for more than two years are presented in this paper.

International audience; This paper introduces the preparations, data collection and initial validation activities performed by the team. The Campaign Database objectives, development and initial performance are addressed in a separate papers [1,2,3,4]. The subsequent papers consist of a set of overview papers concentrating on validation analysis results for specific products, and a set of individual papers outlining in detail the

Summary form only given. NO/sub 2/ is a member of the nitrogen oxides family which takes part in a large number of homogeneous chemical and photochemical reactions. They are very important for the distribution of the stratospheric ozone and the ozone decline. So, it is clear that NO/sub 2/ plays a significant role in understanding the climate change processes. The study of the photochemistry and transport phenomena of NO/sub 2/ requires long-time measurements. For some years at Mt.Cimone station near Bologna, Italy, a UV-VIS spectrometer, GASCOD has operated, developed at the Institute for Atmospheric and Oceanic Sciences (ISAO) for detection of atmospheric trace gases-ozone, NO/sub 2/ etc. Since August 1999 a similar instrument, developed by the ISAO too, has been installed at the observatory in Stara Zagora (42/spl deg/25'N, 25/spl deg/38'E), Bulgaria, at the Department of the Solar-Terrestrial Influences Laboratory for diurnal and long-time studies, simultaneously at Mt.Cimone and Stara Zagora. The Stara Zagora GASCOD type instrument is placed on the roof of one of the observatory buildings and the diffused zenith skylight by means of a 45/spl deg/ mirror is directed to the spectrometer and by the help of a f/5 telescope it is focused on the entrance slit. The instrument records the spectra in the range of 410 nm to 460 nm. DOAS methodology is applied for retrieving the NO/sub 2/ slant column. The preliminary results are shown and a comparison is made between the measurements carried out at Mt.Cimone station and Stara Zagora station.

GASCODs are UV-Visible ground-based spectrometers developed at the ISAO Institute and used to detect stratospheric trace gases involved in the ozone cycle such as NO2, OClO, BrO, by application of Differential Optical Absorption Spectroscopy (DOAS) methodology to the zenith scattered light collected data. After several tests both in laboratory and in Antarctic region, one of the spectrometers was modified for unattended and continuous measurement in extreme high-latitude environment. The instrument was installed in December 1995 in the Italian Station at Terra Nova Bay (74 degree(s)26'S, 164 degree(s)03E', Ross Sea). The GASCOD is still working and causing very interesting data for the study of the denitrification processes during the formation of the so-called ozone hole over the Antarctic region. When the station is unmanned, to allow for the continuous NO2 monitoring for whole the year without mechanical problems, the fixed [407 - 460] nm spectral region is investigated. The results for Nitrogen Dioxide, obtained by application of DOAS algorithms to the data recorded during the year 2000, are presented. During a leg (December 2000 - January 2001) of the 16th Italian Antarctic Expedition, after the usual instrument check, many measurements were carried out in other spectral regions, with the aim to obtain information about the stratospheric tracers contents. The results obtained for Ozone, Nitrogen dioxide and Formaldehyde at different Solar Zenith Angle are presented.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The UV-Vis DOAS spectrometer GASCOD/A4p (Gas Analyzer Spectrometer Correlating Optical Differences, Airborne version) was installed on board the stratospheric Geophysica aircraft during the APE-THESEO and APE-GAIA campaign in February-March and September-October 1999 respectively. The instrument is provided by five input windows, three of which measure scattered solar radiation from the zenith and from two horizontal windows, 90 degree(s) away from the zenith to perform limb-absorption measurements. Spectra from 290 to 700 nm were processed through DOAS technique to obtain trace gases column amounts. Data from horizontal windows, which are performed for the first time from an airborne spectrometer, are used to retrieve an average concentration of the gases along a characteristic length of the line of sight. An atmospheric Air Mass Factor model (AMEFCO) is used to calculate the probability density function and the characteristic length used to reduce the slant column amounts to in-situ concentration values. The validation of the method is performed through a comparison of the values obtained, with a in-situ chemiluminescent ozone analyzer (FOZAN) which performed synchronous measurements on board Geophysica aircraft. Data from the APE-GAIA campaign was presented and discussed.

The measurements of HCl and CH4 atmospheric total abundance is very important, because these minor gases play a fundamental role in the stratospheric ozone cycle and in the climatic change. In fact, the first is considered source and sink for chlorine compounds; the latter is a greenhouse gas (26%) and can contribute to the hydrochloric acid formation. HCl and CH4 present a vibrorotational absorption spectrum in the near infrared (3-4 micron). For this reason it is possible to use a Fabry-Perot interferometer (FPI) as a multiple narrow band filter with an appropriate free spectral range (FSR), so its transmission bands overlap the absorption lines of gas under observation. A remote sensor called NISES (Near Infrared Single-Etalon Sensor) and based on a plane FPI with a dynamic control of the etalon gap, is developing. It utilizes the direct sun radiation collected by a solar tracker to detect atmospheric HCl and CH4 slant columns and its suitable for both ground based and airborne applications. The model MAES (Mathematical Algorithm for Etalon Sensor) has been used to study the main optical characteristics of the FPI (free spectral range, finesse, transmission lines number) and to optimize the instrumental response over a wide range of atmospheric conditions. Moreover, line by line computations of atmospheric layer optical depths and radiances are performed, using HARTCODE (R. Rizzi et al. submitted to Applied optics), so a sequence of different Free spectral Range (FSRs), during the measurements itself, is proposed to minimize the water vapor and gases interfering contribution. The main optical characteristics of an FPI and its deployment for atmospheric sensing are discussed; the result of model simulation and the layout of the HCl sensor are presented as well as some preliminary tests.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An airborne UV-visible spectrometer, the Gas Analyzer Spectrometer Correlating Optical Differences, airborne version (GASCOD/A4?) was successfully operated during the Airborne Polar Experiment, Geophysica Aircraft in Antarctica airborne campaign from Ushuaia (54°49?S, 68°18?W), Argentina in southern spring 1999. The instrument measured scattered solar radiation through three optical windows with a narrow field of view (FOV), one from the zenith, two from the horizontal, as well as actinic fluxes through 2? FOV radiometric heads. Only a few airborne measurements of scattered solar radiation at different angles from the zenith are available in the literature. With our configuration we attempted to obtain the average line-of-sight concentrations of detectable trace gases. The retrieval method, based on differential optical absorption spectroscopy, is described and results for ozone are shown and compared with measurements from an in situ instrument as the first method of validation. © 2002 Optical Society of America.

One of the most important reservoirs of chlorine in the stratosphere is HCl, which sequesters active species and affects the rate of catalytic reactions with ozone. HCl presents a vibro- rotational absorption spectrum, in the near IR; for this reason it is possible to use a Fabry-Perot Interferometer as a multiple narrow band filter with an appropriate free spectral range so its transmission bands overlap the HCl absorption lines.

UV-visible ground-based spectrometers were developed at the ISAO Institute and they are used for application of differential optical absorption spectroscopy (DOAS) methodology to detect stratospheric trace gases involved in the ozone cycle such as NO2, OClO, and BrO. Observations of the light scattered from the zenith-sky were performed with the instrumentation above mentioned, in various stations situated in both the hemispheres. Some problematics connected to data validation and results analysis are introduced. Considerations about the temperature dependency of the cross-section used for the determination of the trace gases slant column are carried out. Results for nitrogen dioxide abundances at different season and various Solar Zenith Angle in their seasonal and diurnal variation are presented and discussed. Finally, the behavior of the sunrise nitrogen dioxide abundance over the sunset slant column is shown and examined.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A UV/Vis DOAS spectrometer (GASCOD, Gas Analyzer Spectrometer Correlating Optical Differences) was installed at Monte Cimone station in 1993 and since then it has been measuring zenith scattered solar radiation at sunset and sunrise. During 1995 it was possible to investigate two spectral regions, about 50 nm width, centered at 365 nm and 436 nm while later we only have measurements at 436 nm available. The spectra obtained during the 1995 - 96 period have been processed with DOAS technique to obtain column amounts of NO2 and O3. The seasonal and diurnal variation of the NO2 column amounts is shown with a summer maximum (about 1.2 X 1017 mol(DOT)cm-2 for p.m. value and 6 X 1016 mol(DOT)cm-2 for a.m.) and winter minimum (about 2 X 1016 mol(DOT)cm-2 for a.m. and 5 X 1016 mol(DOT)cm-2 for p.m.). An anomalous spring increase in p.m. NO2 value during 1995 is investigated through a vertical distribution analysis. The gas profile is retrieved through a Chahine inversion algorithm applied to the slant columns measured at different solar zenith angle. In fact the air mass factor variation with solar zenith angle can be used to extract information about the gas concentration at each atmospheric layers. A consistent and frequent tropospheric increase in NO2 a.m. concentration is evident. The method and the results obtained are discussed.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The present paper deals with the factors influencing the accuracy of the DOAS and in particularly, with the changing of the depolarization ratio of the zenith scattered radiation and related variations of the retrieved NO2 slant column. Ground based measurements carried out during 1997 and 1998 of the polarization state of the zenith- scattered radiation and the NO2 slant columns have been obtained in 4075 angstrom - 4640 angstrom spectral interval. The depolarization ratio is calculated from the spectra registered with a sheet linear polarizer, inserted into the instrument. A linear regression analysis is applied to retrieve NO2 slant columns. The obtained results are analyzed and discussed.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.