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This paper presents an evaluation of the nitrogen dioxide (NO 2 ) trend inferred from UV-visible zenith-sky measurements performed at the 'Ottavio Vittori' ground-based research station (www.isac.cnr.it/cimone) at Monte Cimone (44.18 o N, 10.7 o E, 2165 m above sea level (a.s.l.)). Experimental data series are obtained from the measured spectra adopting a retrieval procedure according to differential optical absorption spectroscopy (DOAS) methodology. Data series cover the period from 1993 to 2009, with limited interruptions due to technical maintenance of the instrument or poor meteorological conditions. The NO 2 trends are calculated with a linear regression model considering different factors assumed to contribute to NO 2 atmospheric content. Before running the model, a procedure for data quality control was applied to reduce or exclude the influence of measurements affected by poor meteorological conditions or the vertical transport of polluted air masses from the lower troposphere. The negative trends obtained for sunrise (a.m.) and sunset (p.m.) NO 2 measurements are in good agreement with previous studies accomplished for ground-based stations in the northern hemisphere.

Areas with dimensions of 1000–3000 km in which the total ozone content (TOC) decreases fast are called ozone mini-holes. They are generated mainly dynamically in two ways, either by poor-ozone air mass transport from the tropics to higher latitudes by planetary wave activity or, they are connected with strong adiabatic uplifting of the tropopause height. An ozone mini-hole, generated by the second mechanism, was observed over the Balkan Peninsula on 19/21 March 2005. In the middle of March, the polar vortex was strongly disturbed by Rossby waves, reaching up to the lower stratosphere. Warming episodes over a geographical area, covering the Barents Sea and the Polar Sea north from Central Siberia, displaced a polar vortex fragment extremely southwards. However, the vorticity was weak and the stratospheric temperatures did not reach low values, providing conditions for ozone chemical destruction via heterogenic reactions. At the same time, a Rossby wave ridge was located below the European polar fragment. In the period from 13 to 19 March, the thermal tropopause over Sofia was uplifted almost by 3 km. Ozone distributions observed by the SCIAMACHY instrument on 18–21 March show a fast TOC decrease westwards from Ireland, which was moving eastwards during the next days, increasing the area in which the ozone content decreased. On 20/21 March low ozone content was observed above the Stara Zagora (42°N, 25°E) ground-based station by means of the GASCOD instrument, using DOAS technique. The TOMS Earth probe instrument detected 237 DU over Sofia. This is a record low March value from the beginning of the TOMS instrument measurements in 1978. In March/April the ozone distribution was characterized by its mean annual maximum of 360 DU at 42°N. Using the TUV model, the UV-index was calculated for regions of 2000 m height, typical for the mountains southwards of Sofia. The UV-index was very high (almost 10). This is a result not only of the low TOC but also of the relatively high zenith angle at middle latitudes in March.

This paper presents a methodology for the retrieval of the vertical profile of atmospheric gas pollutants in the boundary layer from ground-based remote sensing measurements. Nitrogen dioxide (NO2) and ozone (O3) slant column amounts were obtained with the differential optical absorption spectroscopy (DOAS) technique used in the multiple-axis configuration (referred to as multiaxis DOAS). The measurements were carried out in the Presidential Estate at Castel Porziano (near Rome) from September to November 2006, within a program started in 1994 for studying and monitoring the Estate's environment. The retrieval of information on trace gas vertical profiles from the slant column amounts requires as follows: (1) the simulation of the radiative transfer in the atmosphere for air mass factor calculation and (2) the application of inversion schemes. This paper illustrates and discusses the vertical profiles of NO2 and O3 obtained from multiaxis DOAS measurements and their daily evolution on October 29, 2006.

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 NO 2 . 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 NO 2 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 NO 2 and up to 0.5 for the ozone variation.

Several UV-Visible spectrometers have been developed at the ISAC-CNR Institute. Differential Optical Absorption Spectroscopy (DOAS) methodology is applied to their measurements to monitor the amount of stratospheric trace gases: mainly ozone (O3) and nitrogen dioxide (NO2) deeply involved in the ozone cycle. Observations of the scattered zenith-sky light were performed with this instrumentation at the Terra Nova Bay station (TNB) – Antarctica. The GASCOD (Gas Analyzer Spectrometer Correlating Optical Differences), is briefly described and a method for data analysis and validation of the results is introduced. Some aspects of the DOAS technique are presented: the algorithm allowing the best spectral alignment between spectra obtained with GASCOD and a high-resolution wavelength calibrated spectrum, is explained. Simple considerations allow for the calculation of the NO2 concentration in the background spectrum used in DOAS analysis. For the period of activity of the GASCOD at TNB (1996-2003), the results of NO2 vertical column density (VCD) at twilight show the maximum in the summer and the minimum during the winter. Three years of measurements (2001-2003) are analyzed in terms of stratospheric temperature and Potential Vorticity, to have information about the stratospheric warming occurred in 2002 over Antarctica. The correlation between NO2 atmospheric contents and stratospheric temperature is highlighted. The diurnal variations of NO2, which are controlled by the photochemistry, show a particular behaviour at high latitudes. The analysis of the AM/PM ratios - the sunrise NO2 VC (AM) over the sunset VC (PM) - at different seasons and various Solar Zenith Angles (SZA) are presented and discussed.

The emissions of the cruise ships, in terms of nitrogen dioxide (NO 2 ) and sulphur dioxide (SO 2 ), are evaluated with the DOAS scanning spectrometer TropoGAS (Tropospheric Gas Analyser Spectrometer) developed at ISAC CNR in close collaboration with the CGE-UE. The slant columns amounts of the above mentioned compounds are obtained with the application of the Differential Optical Absorption Spectroscopy (DOAS) technique to the spectral measurements carried out with the TropoGAS instrument. This last is linked with an optical fibre to a simple scanning optical system allowing for measurements in multiple axis configurations. The measurements are carried out across the Giudecca Channel in Venice, during two field campaigns performed in July and in October 2007. The instrumental setup, the DOAS method and the technique for the evaluation of the ships emissions, are described. The results of flow rate emissions for NO 2 and SO2 are presented and discussed. Their mean values are about 12g/s and 4 g/s for NO 2 and SO 2 respectively.

The GASCOD (Gas Analyzer Spectrometer Correlating Optical Differences) has been installed at the 'Mario Zucchelli' Antarctic station since 1996. It measures the zenith sky radiation in the 405-465 nm spectral range in unattended and automatic mode. The application to the spectral data of the DOAS (Differential Optical Absorption Spectroscopy) algorithms coupled with a Radiative Transfer Model (RTM) for the computation of the Air Mass Factor (AMF), allows for the retrieval of the total content of the main absorber in this spectral range, namely nitrogen dioxide (NO2). Moreover, the application of sophisticated inversion schemes to the output of the DOAS program, using the AMF matrix as the kernel of the inversion algorithm, permits the determination of the vertical distribution of the above mentioned compound. The full dataset of the spectral data obtained with GASCOD during the period 1996-2008, was re-analyzed with a modified version of the software tool previously utilized. Even if the spectral range examined with GASCOD is not the most favorable for the ozone total column and vertical profile retrieval, the re-processing of the spectral data allowed for the determination of the total ozone columns (TOC). The uncertainties range from 4% to 8% for ozone and 3% to 6% for NO2. The peculiar features of the seasonal variation of NO2 total columns (i.e. the normal decreasing during the austral fall and the irregular growing towards the summer month) are presented and discussed. The confirmations of the significant declining of the ozone total columns during the 'Ozone Hole' periods (mid-August to mid-October) are reported. The vertical distributions obtained for the preceding atmospheric compounds are shown and examined.

In this paper we present a methodology for the retrieval of the vertical profile of atmospheric gas pollutants in the boundary layer from ground based remote sensing measurements. Nitrogen dioxide (NO 2 ) and ozone (O 3 ) slant column amounts have been obtained with the Differential Optical Absorption Spectroscopy (DOAS) technique used in the multiple axis configuration (the so called MAX-DOAS). The measurements have been carried out in the Presidential Estate at Castel Porziano (Rome) in the period from September to November 2006 in the frame of a programme started in 1994 for studying and monitoring the Estate's environment. The retrieval of information on the vertical profile of trace gases from their slant column amounts requires: (1) the simulation of the radiative transfer in the atmosphere for Air Mass Factor (AMF) calculation; (2) the application of inversion schemes. In this paper the vertical profiles of NO 2 and O 3 obtained from multiple axis DOAS measurements and their daily evolution are presented and discussed. The day under study is the 29th of October, 2006.

In this work we present a tomographic-like methodology for the retrieval of the vertical profile of nitrogen dioxide (NO2) and ozone (O3) in the boundary layer from ground based remote sensing measurements. A set of NO2 and O3 slant column densities at different elevation angles can be obtained with the Differential Optical Absorption Spectroscopy (DOAS) technique used in the multiple axis configuration. Measurements have been carried out in the Presidential Estate at Castel Porziano (Rome) using this technique, in the frame of a programme started in 1994 for studying and monitoring the Estate's environment, in the period from September to November 2006. The retrieval of information on the vertical profile of a trace gas obtained from slant column density DOAS measurements requires: (1) the simulation of the radiative transfer in the atmosphere for Air Mass Factor (AMF) calculation concerning the target trace gases; (2) the application of inversion schemes to derive the vertical profile of the gas pollutants under interest using the AMF matrix as the kernel of the inversion algorithm. In this paper the first retrieved vertical profiles of NO2 and O3 and their time evolution during the day are presented and discussed.

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.

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.

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.

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.

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 spectrometer (named GASCOD) for Differentiated Optical Absorption Spectroscopy (DOAS) has been developed at ISAO Institute and deployed for ground based measurements of stratospheric trace gases for several years at mid-latitudes and the Antarctic region. An airborne version, called GASCOD/A has been installed on board a M55-Geophysica airplane, a stratospheric research platform, capable of flying at an altitude of up to 20 Km. After a test campaign in Italy, the GASCOD/A performed successfully during the Airborne Polar Experiment in the winter 95/96. More recently, the instrument was upgraded to achieve higher sensitivity and reliability. Two additional radiometric channels were added. The input optics can turn in order to collect solar radiation from five different channels: one for detection of the zenith scattered radiation through the roof window (for DOAS measurement), two for direct and diffused radiation through two lateral windows and two for radiometric measurements through two 2(pi) optical heads mounted on the upper and bottom part of the aircraft and linked to the instrument by means of optical guides. The radiometric channels give us the possibility of calculating the photodissociation rate coefficients (J-values) of photochemical reactions involving ozone and nitrogen dioxides. The mechanical and optical layout of the instrument are presented and discussed, as well as laboratory tests and preliminary results obtained during flights onboard the M55- Geophysica.