Solar radiation takes in today's world, an increasing importance. Different devices are used to carry out spectral and integrated measurements of solar radiation. Thus the sensors can be divided into the fallow types: Calorimetric, Thermomechanical, Thermoelectric and Photoelectric. The first three categories are based on components converting the radiation to temperature (or heat) and then into electrical quantity. On the other hand, the photoelectric sensors are based on semiconductor or optoelectronic elements that when irradiated change their impedance or generate a measurable electric signal. The response function of the sensor element depends not only on the intensity of the radiation but also on its wavelengths. The radiation sensors most widely used fit in the first categories, but thanks to the reduction in manufacturing costs and to the increased integration of electronic systems, the use of the photoelectric-type sensors became more interesting. In this work we present a study of the behavior of different optoelectronic sensor elements. It is intended to verify the static response of the elements to the incident radiation. We study the optoelectronic elements using mathematical models that best fit their response as a function of wavelength. As an input to the model, the solar radiation values are generated with a radiative transfer model. We present a modeling of the spectral response sensors of other types in order to compare the behavior of optoelectronic elements with other sensors currently in use.
Monitoring of atmospheric compounds at high latitudes is a key factor for a better understanding of the processes driving
the chemical cycles of ozone and related chemical species. In this frame, the GASCOD (Gas Analizer Spectrometer
Correlating Optical Differences) equipment is installed at the Mario Zucchelli Station (MZS - 74.69S, 164.12E) since
December 1995, carrying out observations of nitrogen dioxide (NO2) and ozone (O3). The recent advances in sensor technologies and processor capabilities, suggested the setup of a new equipment, based on the same optical layout of the 'old' GASCOD , with enhanced performances and improved capabilities for the measurements of solar radiation in the
UV-visible spectral range (300-700nm). The efforts accomplished, allowed for the increase of the investigated tracers.
Actually, mainly due to the enlargement of the covered spectral range and to the adoption of a CCD sensor, in addition to
the NO2 and O3 compounds, others species can be monitored with the new instrumental setup such as bromine, chlorine and iodine oxides (BrO, OClO and IO). The innovative equipment called GASCODNG (GASCOD New Generation)
was installed at MZS during the 2012/2013 Italian Antarctic expedition, in the framework of the research projects
SAMOA (Automatic Station Monitoring Antarctic Ozonosphere) and MATAGRO (Monitoring Atmospheric Tracers in
Antarctica with Ground Based Observations) funded by the Italian and Portuguese Antarctic programs respectively. In
this paper a brief description of the new equipment is provided, highlighting the main improvements with regard to the
'old' one. Furthermore the full dataset (1996 - 2012) of NO2 total columns, obtained with the GASCOD installed at MZS,
is compared with the data obtained with satellite borne equipments (GOME, SCIAMACHY, OMI and GOME2) and the
main statistical parameters are analyzed and discussed in detail.
The present work deals with UV/Vis up-welling and down-welling irradiation measurements carried out in the lower
Antarctic stratosphere by means of GASCOD-A/4pi spectroradiometer on board the M55-Geophysica aircraft during the
APE-GAIA campaign. Very few such measurements have been performed in the lower stratosphere. The experimental
data are used for the calculation of NO2 photodissociation rate coefficients in the upper troposphere and lower
stratosphere along the altitude of the flight. A detailed description of the measurement method, instrumentation and
calibration procedures is presented. Experimental results are presented and discussed too.
KEYWORDS: NOx, Atmospheric modeling, Spectroscopy, Data modeling, Single crystal X-ray diffraction, Aerosols, Lamps, Polonium, Systems modeling, 3D modeling
A simple method to determine the vertical distribution of a pollutant gas, namely NO2, by means of the spectral
measurements obtained with a scan-DOAS spectrometer, is presented. The developed technique can be summarized as
follows: i) a series of quasi simultaneous measurements in the zenith and in others directions allowing for the
determination of the Slant Column Density of NO2 for different elevation angles; ii) an active DOAS measurement for
the determination of the NO2 concentration at the ground; iii) a set of Radiative Transfer Model (RTM) calculation of the
scattering distance from the spectrometer, for a set of visibility values; iv) a recursive procedure of profile calculation
starting from the first measurement and subtracting the value of NO2 Slant Column Density (SCD) retrieved from the
measurement taken at the previous angle of sight. Measurements are carried out during summer 2007 in S. Pietro
Capofiume (Bologna-Italy). The vertical distribution for NO2 obtained with the above described method has been
compared with the profiles calculated with the GAMES (Gas Aerosol Modelling Evaluation System) model. The results
of this comparison show some differences between the modelled and the measured profiles, probably due to box
approximations in RTM calculation for measured profiles and to the large pixel grid (about 10x10 km2), for model evaluation.
The emissions of the cruise ships, in terms of nitrogen dioxide (NO2) and sulphur dioxide (SO2), 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 NO2 and
SO2 are presented and discussed. Their mean values are about 12g/s and 4 g/s for NO2 and SO2 respectively.
Clouds are the major factor regulating the Earth radiation budget, therefore their detection and characterization is of
major importance. Cloud detection and classification is a requirement in order to allow for accurate studies of cloud
microphysical and optical properties, as well as subsequent assessment of their radiative effects. In the present work, a
method for the detection and classification of clouds, over the Iberian Peninsula, is presented. The methodology
developed relies on the use of Meteosat-8 satellite images in different spectral bands combined to form different color
composites, which are then analyzed, in an unsupervised way. The results show that more features are distinguished in
the cloud mask, with respect to the use of traditional methods.
In the frame of DOAS, a Monte Carlo code has been developed, to calculate, for a given detector with assigned diameter
and field of view, the single and multiple scattering radiance. Very general 3-D geometry is foreseen. Spatial distribution
along the detector axis for the single and total scattering radiance are computed. Ground reflected contributions to the
solar radiance are estimated. Differential effects due to small perturbations in physical parameters, such as ozone density,
can simultaneously be taken into account in the same calculation. The code has been applied to ToTaL-DOAS
(Topographic Target Light scattering-Differential Optical Absorption Spectroscopy) measurements.
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 the current study, the cloud base height obtained from the ceilometer measurements, in Evora (south of Portugal), are
compared with the results obtained from atmospheric modelling. The atmospheric model adopted is the nonhydrostatic
MesoNH model, initiated and forced by ECMWF (European Centre for Medium-Range weather Forecasts) analyses.
Also the simulated cloud depth results are presented. The availability of mesoscale modelling for the region, as well as
the cloud local vertical distributions obtained from the ceilometer, provide a good opportunity to compare cloud base
height and estimate the errors associated. From the obtained results it is possible to observe that the simulated cloud base
height values are in good agreement with the correspondent values obtained from the ceilometer measurements.
Since the recognition during the seventies of the importance played by the minor atmospheric compounds in the climate
system (gases and aerosols), the studies regarding the chemistry and dynamical processes of Ozone (O3) and of nitrogen
dioxide (NO2) at high and mid-latitudes, became a fruitful field of research.
This work deals mainly with the retrieval and analysis of O3 and NO2 total columns and vertical profiles over the Evora
Observatory (South of Portugal) for the period 2007-2008. The products presented in this paper are obtained from
spectral measurements carried out with the UV-Vis. Spectrometer for Atmospheric Tracers Monitoring - SPATRAM,
installed at the Observatory of the Geophysics Centre of Evora (CGE) since 2004. The application of the Differential
Optical Absorption Spectroscopy (DOAS) algorithms to the spectral zenith-sky measurements is presented and
discussed. The inversion technique applied to the output of the DOAS procedures (the trace gases content along the
optical path of measurements: the slant column densities -SCD- of the analyzed absorber) are examined. The first
observations obtained with the SPATRAM instrument regarding stratospheric bromine oxide (BrO) are shown. In
addition, the comparison of the ground-based measurements with data derived from satellite equipments (OMI and
SCIAMACHY), are discussed.
The multi purpose UV-Vis. Spectrometer for Atmospheric Tracers Measurement (SPATRAM) is installed at the
Observatory of the Geophysics Centre of Evora (38.5º N, 7.9º W) - Portugal, since 2004, measuring the zenith scattered
radiation in the 300-550 nm spectral range. The main products are the total column and the vertical profiles of NO2 and
O3 obtained with the application of the Differential Optical Absorption Spectroscopy (DOAS) algorithms and with
inversion schemes based on the Optimal Estimation methods respectively. Recently (February 2009), the MIGE
(Multiple Input Geometry Equipment) was coupled to the SPATRAM instrument allowing for the measurements of the
diffused radiation in directions away from the zenith one (Off-Axis). MIGE is an alt-azimuth platform based on a very
simple optical layout, using an optical fibre to transmit the radiation inside the monochromator of the SPATRAM
equipment. Thanks to the solution adopted in the developing phase, MIGE is able to scan the whole hemisphere. In this
work, after a brief description of the MIGE, the first and preliminary results for vertical profiles of NO2 in the Planetary
Boundary Layer (PBL), and the values of Slant Column Densities (SCD) of O3 and SO2 measured in Off-Axis
configuration at Evora Station, are presented and discussed.
The present work aims to investigate the effects of Saharan desert dust storms on cloud properties and respective
radiative forcing over the South of Portugal and nearby Atlantic Ocean, for a case study that occurred between 26 and 29
May 2006, using the MODerate Resolution Imaging Spectroradiometer (MODIS) satellite data.
The determination of cloud properties in different regions subject to the presence of dust aerosols provides information
on the possible alterations that these clouds may suffer due to the presence of an aerosol layer. Cloud effective radius,
optical depth and water path retrieved values are considerably smaller for clouds developing in a dusty atmosphere than
for clouds in a dust-free atmosphere. Due to these changes it was also possible to observe that the instantaneous cloud
radiative forcing (ICRF) values at the top of the atmosphere (TOA) and at the surface are lower when dust aerosols are
not present.
A VAISALA Ceilometer CL31 is operating continuously in the Observatory of the Évora Geophysics Centre (CGE)
since May 2006. The CL31 ceilometer provides measurements of the cloud base height up to three simultaneous layers
and of the profile of the backscatter coefficient, which in the absence of clouds gives a good approximation of the
qualitative aerosol boundary layer profile. The ceilometer backscatter measurements are used here to study special
aerosol events that reach Évora (38°34'N, 7°54'W, 300m a.m.s.l.) such as forest fires, desert dust transports originating
from the Sahara desert, which often occur in the south of Portugal and European pollution. The aerosol backscatter
coefficient corresponding to the lowest layers of the atmosphere is also correlated with mass concentration
measurements obtained from a TEOM (Tapered Element Oscillating Microbalance) installed in the same place (CGE
observatory). The TEOM measures the in situ mass concentration of aerosols near the ground, with aerodynamic
diameter lower than 10μm (PM-10), with temporal sampling of 10min. Furthermore, ceilometer measurements and
derived mixing height are also compared with measurements taken with a Lidar during an intensive campaign that took
place at the CGE observatory in Évora, during the first half of June 2006.
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 (NO2) and ozone (O3) 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 NO2 and
O3 obtained from multiple axis DOAS measurements and their daily evolution are presented and discussed. The day
under study is the 29th of October, 2006.
The SPATRAM (Spectrometer for Atmospheric TRAcers Monitoring) instrument has been developed by the
collaboration between CGE-UE, ISAC-CNR and ENEA. SPATRAM is a
multi-purpose UV-Vis-NIR spectrometer
(250-950 nm). It is installed at the Observatory of the CGE since April 2004 and actually it is utilized to carry-out
measurements of the zenith scattered radiation, the
so-called "Passive mode", in order to retrieve-by application of
DOAS (Differential Optical Absorption Spectroscopy) methodology-the vertical content of some atmospheric tracers
such as Ozone (O3) and Nitrogen Dioxide (NO2). For the continuous NO2 monitoring the 425-455 nm spectral region is
investigated. For the Ozone retrieval the spectral interval 320-340 nm is chosen. In this study, after a brief description of
the instrument, a short explanation of the DOAS methodology and of the inversion algorithms used for the determination
of the vertical distribution of the some atmospheric compounds are provided. The obtained results in terms of diurnal and
seasonal variation of O3 and NO2 total column are presented. The measurements are in good agreement with the
photochemical theory of NO2 and O3, showing the maximum values during the summer season and the minimum during
the winter. In addition the application, to the output of the DOAS program, of sophisticated inversion schemes, using the
Air Mass Factor (AMF) matrix as the kernel of the inversion algorithm, allowed for the determination of vertical
distribution of some atmospheric tracers. The results obtained for NO2 and O3 are presented and discussed.
LIDAR (LIght Detection and Ranging) is an optical active remote sensing technology with many applications in
atmospheric physics. Modelling of LIDAR measurements appears useful approach for evaluating the effects of various
environmental variables and scenarios as well as of different measurement geometries and instrumental characteristics.
In this regard a Monte Carlo simulation model can provide a reliable answer to these important requirements. A
semianalytic Monte Carlo code for modelling LIDAR measurements has been developed at ISAC-CNR. The
backscattered laser signal detected by the LIDAR system is calculated in the code taking into account the contributions
due to the main atmospheric molecular constituents and aerosol particles through processes of single and multiple
scattering. The contributions by molecular absorption, ground and clouds reflection are evaluated too. The code can
perform simulations of both monostatic and bistatic LIDAR systems. To enhance the efficiency of the Monte Carlo
simulation, analytical estimates and expected value calculations are performed. Artificial devices (such as forced
collision, local forced collision, splitting and russian roulette) are moreover foreseen by the code, which can enable the
user to drastically reduce the variance of the calculation.
Ivan Kostadinov, L. Grassi, G. Ballista, Giorgio Giovanelli, Rodolfo Guzzi, Daniele Bortoli, W. Di Nicolantonio, C. Lecerf, Andrea Petritoli, Fabrizio Ravegnani, S. Scarpanti
The satellite remote sensing instruments for climatic studies are required to have: (1) fast time sampling, (2) high spectral resolution, (3) high space resolution, (4) wide field of view, (5) broad spectral range, (6) simultaneous measurements in different spectral intervals and/or type of measurements (7) 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 (2), (6), (5), (7) and, at least partially, the rest ones.
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, OClO, BrO. The instrument was installed in December 1995 in the Italian Antarctic station at Terra Nova Bay (74°26'S, 164°03E', 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 nm. 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 km2. A comparison of GASCOD and GOME results for NO2 total column is performed.
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.
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.
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.
Andrea Petritoli, Giorgio Giovanelli, Paolo Bonasoni, Tiziano Colombo, Franco Evangelisti, U. Bonafe, Daniele Bortoli, Ivan Kostadinov, Fabrizio Ravegnani
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.
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.
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.
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