Vol49_1_2006def 71 ANNALS OF GEOPHYSICS, VOL. 49, N. 1, February 2006 Key words UV-VIS instruments – remote-sensing – actinic measurements – satellite data validation 1. Introduction The current state of the Earth’s climate and its future development under different scenarios has been a subject of increasing interest in the international scientific community over the last decade. Many studies, based on experimental data obtained using a large variety of instru- ments, indicate the existence of strong links be- tween anthropogenic activity and a number of chemical processes taking place in the atmos- phere (IPCC, 2001). One of the major questions nowadays is to discover the critical margins of the anthropogenic impact on atmospheric pa- rameters, beyond which climate-controlling fac- tors become irreversible thus affecting the bios- phere in an uncontrolled manner. In general, two approaches are adopted to improve knowledge in this field: a) the use of models to simulate possible scenarios and assess the impact of a large range of factors on climate; b) the collec- tion of experimental data and the establishment of long-time data series necessary for the evalu- ation of ongoing climate changes. Both ap- proaches have shortcomings. In case a), the lack of data needed for the initialization of the mod- els can create difficulties in reproducing the ob- served processes, thus preventing reliable pre- dictions. Case b) requires specific procedures for maintaining the technical characteristics of already deployed instruments. If new instru- ments are introduced into the monitoring net- A multi-input UV-VIS airborne GASCOD/A4r spectroradiometer for the validation of satellite remote sensing measurements Ivan Kostadinov (1)(3), Giorgio Giovanelli (1), Daniele Bortoli (1)(2), Andrea Petritoli (1), Fabrizio Ravegnani (1), Giandomenico Pace (4) and Elisa Palazzi (1) (1) Istituto di Scienze dell’Atmosfera e del Clima (ISAC), CNR, Bologna, Italy (2) Geophysics Centre of Évora - University of Évora (CGE-UE), Évora, Portugal (3) Solar-Terrestrial Influences Laboratory (STIL), Bulgarian Academy of Science, Stara Zagora, Bulgaria (4) Università degli Studi di Roma «La Sapienza», Roma, Italy Abstract The present paper describes a UV-VIS spectroradiometer named GASCOD/A4r developed at ISAC-CNR for remote sensing measurements aboard stratospheric M55-Geophysica aircraft, flying up to 21 km. Obtained ex- perimental data are used for retrieving of NO2, O3 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 air- craft missions in different geographical areas have already been utilized for ENVISAT validation. Mailing address: Dr. Ivan Kostadinov, Istituto di Scien- ze dell’Atmosfera e del Clima (ISAC), CNR, Area della Ri- cerca CNR, Via Gobetti 101, 40129 Bologna, Italy; e-mail: I.Kostadinov@isac.cnr.it 72 Ivan Kostadinov et al. works it should be guaranteed that the measure- ment accuracy already established in the long- time data series will not be reduced, in order to not affect the trends of the studied parameters. The above-mentioned aspects are of particu- lar importance regarding satellite remote sensing instruments, contributing to b). Instruments in- stalled aboard environmental satellites, such as ODIN, ERS-1, ERS-2, ILAS, ACE, etc. supply a large variety of data, e.g., gas concentration, tem- perature, water vapour, cloud top height. These data are derived from measurements carried out under very different environmental conditions and observational geometries. To overcome prac- tical problems related to b) specific intercompar- isons and validation campaigns are carried out. Satellites, with payloads dedicated to explore the Earth’s atmosphere are usually launched in polar orbit to allow coverage of all geographical regions. Although the repetition rate of these satellites is of the order of a few days, ground- based measurements alone are not sufficient to fully validate satellite data. On the other hand the satellite overpass does not always coincide with the most appropriate conditions for ground- based observations. Changes in the atmosphere, due to local, regional or large-scale dynamic ef- fects can take place during the period between two overpasses for a given location, so different air masses would be observed from the space and from the ground, leading finally to difficulties in the assessment of satellite data quality. Ground-based instruments, distributed all over the world, as uniformly as possible, would provide an opportunity to control the satellite data capability to reproduce the actual atmos- pheric parameters correctly. However, there are large unpopulated geographical areas, where ground-based measurements for validation tasks cannot be performed and only balloon and aircraft measurements can provide the possibil- ity of enlarging the validation areas. In this re- gard the validation campaigns of satellite re- mote sensors, with the participation of airborne instruments, would appear to be essential for continuing satellite payload characterization. As a matter of fact the characterization of the scientific payload begins with the design and as- sembly of individual instruments and includes calibration and test procedures, carried out dur- ing the pre-flight phase. The aim of these proce- dures is to simulate the space conditions under which the instruments will operate in orbit. So- phisticated laboratory equipment and models are included in these pre-flight operations in order to check the behaviour of spaceborne sensors under circumstances expected in space, e.g., sharp vari- ations in temperature (sunlight-darklight parts of the orbit), different illumination levels within in- strumental Field of View (FoV), etc. Notwith- standing the efforts of scientists to reproduce space conditions in the laboratory during pre- flight tests, there are still factors which cannot be forecast and/or simulated correctly. For this rea- son it is necessary to perform specific tests after the launch to supply a complete characterization of the behaviour of the satellite instruments. The aim of the validation procedures also includes optimising methods for satellite data processing, improving the subroutines for better treatment of the observational scenarios. Some of the reasons for discrepancies between ground-based and satellite data can arise from the fact that exactly the same air masses are not observed and hence the output results will be different. In order to evaluate these differences additional informa- tion, useful for better interpretation of derived at- mospheric parameters, is indispensable. Of course, part of the information can be obtained from models, but in order to adapt such models to any given scenario it is preferable to initialize them with experimental data obtained as near in the time as possible to the moment of interest. For this reason the development of instru- ments, which perform simultaneous measure- ments within spaceborne instrumental FoVs, is considered as an advanced approach towards this end. 2. Methods of measurements The first airborne version of GASCOD fam- ily instruments during the APE-1 mission in Rovaniemi, operated only in DOAS mode. Be- fore the tropical APE-THESEO campaign (http://ape.ifac.cnr.it) the instrument was up- graded by adding two 2r sr FoV receivers for collecting of solar direct and diffuse radiation arriving at the flight level. After this modifica- 73 A multi-input UV-VIS airborne GASCOD/A4r spectroradiometer for the validation of satellite remote sensing measurements tion the instrument became known as GAS- COD/A4r providing additional measurements necessary for a better understanding of the at- mospheric photochemistry. 2.1. DOAS mode GASCOD/A4r was initially developed as a ground-based instrument (Bonasoni et al.,1993; Evangelisti et al., 1995; Giovanelli et al., 1998), implementing the DOAS measurement tech- nique. It relies on the retrieval of the column con- tent of minor atmospheric gases such as O3, NO2, BrO, OClO, NO3, SO2, etc. from their character- istic narrow-band absorption spectra, by apply- ing the Beer-Lambert Law in its differential form. Brewer et al. (1973) first attempted to ap- ply this method. Since then, many authors con- tributed to improve the technique (e.g., Noxon, 1975; Noxon et al., 1979; Platt and Perner, 1983; Johnston and McKenzie, 1984; Solomon et al., 1987; Roscoe et al., 1994; Roscoe et al., 1996). 2.2. Spectroradiometric mode Many photochemical reactions take place in the atmosphere, contributing to the variations of the local concentration of minor gases. There- fore, by knowing the incoming UV radiation flux at flight level provides an opportunity to better distinguish the role of photochemistry in overall atmospheric processes. To this end, GASCOD/A4r can also operate as a spectrora- diometer and supply spectrally resolved up- welling and down-welling radiance (Kostadi- nov et al., 1999). The channels with 2r sr FoV used for this purpose are calibrated in absolute terms allowing to convert measured spectral ra- diation through these channels into photon flux. 3. Instrumental design Space restrictions aboard M55-Geophysica aircraft and requirements for spectrometric in- struments have been taken into account and GASCOD/A4r has been designed as a two-unit device. It includes an Optical Unit (OU) and an Electronic Unit (EU). The OU is built-up as a multi-input device probing the atmosphere re- motely in different directions. Such configura- tion provides possibility to perform additional measurements necessary for better understand- ing of the atmospheric photochemistry, under- lying spectral reflectivity etc., widely used for improvement of the satellite data validation. 3.1. Optical Unit OU consists of five optical inputs named i0, i1...i4. Three of them have Narrow Fields of View (NFoV) of 1.1E-5 sr and point towards the nadir (i0), zenith (i2) and, horizontally, to the left, perpendicular to the flight direction (i4). These three inputs are dedicated to DOAS measurements. The combination of (i2) and (i4) measurements is used to derive the so-called NO2 and O3 ACILA (Average gas Concentra- tion Inside the Layer near the Aircraft) values (Petritoli et al., 2002). The third NFoV (i0) channel is used to evaluate the underlying spec- tral reflectivity, or combining with (i2) measure- ments to derive the tropospheric and stratos- pheric part of the total vertical column of the gases under interest. Such distinction appears a task of remote-sensing satellite measurements too, aimed to evaluate tropospheric pollution from space (Giovanelli et al., 2006). The other two channels having 2r sr FoV are dedicated to collect diffused up-welling ra- diance, (i1) and diffuse + direct down-welling radiance (i3). Measured spectra through these channels are subsequently used to obtain spec- trally resolved actinic flux need for calculation of photodissociation rate coefficients. The OU consists custom-built monocroma- tor based on a Jobin-Yvon holographic spheri- cal grating with N = 1200 grooves/mm with blaze maximised at 320 nm and a focal length of 300 mm and entrance slit 100 nm × 8.0 mm. The linear dispersion and resolution are about 2.4 nm/mm and 0.7 nm @ 350 nm, respectively, satisfying requirements for resolving power of DOAS type instruments. These two parameters depend upon N and any factor changing N will affect the linear dispersion and resolution. Tem- perature variations appear to be the most impor- 74 Ivan Kostadinov et al. tant factor in this regard. It was found that if dT ± 0.2°C the changes of instrumental spectro- scopic characteristics introduce negligible er- rors on the measured physical parameters. The maintenance of the required temperature regime is guaranteed by means of special internal insu- lating layer of the OU and thermo-electric cool- ing system. The full spectral range (295 nm ÷ 1100 nm), which can be examined by GASCOD/A4r is sampled within single sub-intervals of 60 nm. Depending upon the task of any given mission, sets of sub-intervals can be previously selected, although spectral intervals up to 460 nm are al- ways examined to retrieve O3, NO2 and BrO con- centration, allowing UV actinic flux measure- ments and hence the calculation of J(NO2) too. 3.1.1. CCD sensor The sensor appears 2D (1100 × 330 pixels) SITe CCD based on Back Illuminated technol- ogy. A Peltier cooling system ensures a con- stant operating temperature of −30°C ± 0.1°C. The measured signal is binned into a matrix of 1092 columns and 11 rows. Even though the rows are reduced from 300 to 11, the curvature of the spectral lines still remains. Applying a «shift» procedure further improvement of the instrumental spectral resolution is achieved. The exposure time needed to achieve suffi- cient S/N ranges from 0.1-1.0s, under high sun elevation, to 15 s at a solar zenith angle of about 94°. The most appropriate exposure time which should be applied for single measurements dur- ing flight is calculated by means of an au- toranging procedure included in the measure- ment duty cycle. However in certain situations, due to fast changes of environment conditions (e.g., broken clouds), CCD saturation can take place. In order to decrease the CCD memory ef- fects to negligible levels wiping procedure is applied at the beginning of each measurement. 3.1.2. Spectral and Absolute Calibration A small integrating sphere with diameter of 60 mm, equipped with Hg and QJ tungsten lamps, is incorporated into OU for in-flight con- trol of CCD performance and overall instrumen- tal spectral characteristics and spectral calibra- tions. For each spectral sub-interval there are previously selected spectral lines used for cor- rect positioning of the diffraction grating within ± 1pixel (wavelength equivalent ≈ 0.05nm). Shift and stretch procedures are applied during post-flight data processing to introduce the cor- rect wavelength scale in the measured spectra. The channels with 2r sr FoV used for cal- culation of J-values are calibrated in an ab- solute manner. This includes specific laborato- ry measurements to find the relationship be- tween instrumental output signal and measured radiation. A method, based on use of an inte- grating sphere has been developed for this pur- pose (Kostadinov et al., 1999). The advantage of this method arises from the possibility to ir- radiate simultaneously the whole FoV of the 2r sr receivers as it take place during field meas- urements during the flights. 3.1.3. Electronic Unit This is a pressurised unit installed inside the aircraft and consists of a DC/DC converter pro- viding the power, a computer based on a PC104 motherboard, sensors for internal and external pressure and temperature controls. A driving program guards the execution of a duty meas- urement cycle. This includes home positioning of the diffraction grating, scanning mirror, band-pass filters wheel, CCD wiping and dark current measurements, etc. 4. Data processing The acquired data from the narrow FoV hor- izontal channel (i4) are used to derive average concentration of NO2 and O3 along the instru- mental optical axis at distances of about 50-70 km from the aircraft. This approach is actually based on DOAS methodology combined with a 2D single-scattering radiation transfer model (rtm) (Solomon et al., 1987), adapted to the specificity of the aircraft measurements (Petritoli et al., 2000). The model uses ray-tracing in a 75 A multi-input UV-VIS airborne GASCOD/A4r spectroradiometer for the validation of satellite remote sensing measurements spherical atmosphere, with optical paths inte- grated over individual 1-km shells, within which the atmosphere is divided for the model calcula- tions. The rtm has been adapted in order to eval- uate the probability density function (for scatter- ing towards the instrument) and to calculate the effective absorber optical path under off-axis measurement geometry (Slusser et al., 1996). Such an approach reveals the possibility of con- verting the derived slant columns detected at the aircraft altitude into corresponding ACILA val- ues and to use them for satellite data validation. While DOAS requires an appropriate refer- ence spectrum, the calculation of actinic flux and subsequently deriving rate coefficients (J- values) of photochemical reactions strongly de- pends upon the accurate correction of all types of sensor noise and dark current. For this pur- pose two particular measurements – one with 0.0 s and 2.0 s integration times, together with previously mapped CCD pixel properties, cor- responding to different integration times, allow one to extract the pure output signal generated by the incoming radiation. The signal is con- verted into instrumental units versus time, (i.u./s), allowing signals detected under differ- ent environmental and navigation conditions to be meaningfully compared. Next measured spectra are converted from (i.u./s) units into (mW/m2.nm) units by applying the results from laboratory, pre-flight and post-flight calibra- tions. All detected spectra through the upwards- facing and downwards-facing 2r sr FoV re- ceivers, undergo a cubic-spline interpolation procedure, allowing to obtain equidistant time grids for all actinic measurements during a giv- en flight. Derived actinic flux is converted into photon fluxes used for calculation of the J-val- ues for a number of photochemical reactions. For example, using the NO2 absorption cross section (Harder et al., 1997) and recommended data for primary NO2 quantum yield (Gardner et al., 1987), we can calculate J(NO2). 5. Satellite measurements requisites and aircraft measurements Over the last few decades, spatial and time resolution of satellite remote sensing measure- ments have been substantially improved. How- ever, the resolution depends upon the type of observation – nadir or limb and ranges from a few kilometres up to hundreds of kilometres. Assuming M55-Geophysica aircraft cruise speed of 10 km/min it means that a long time is needed to examine in situ airborne instruments air masses observed from space. Of course in situ measurements have the ad- vantage of providing results with high time reso- lution, but they are not sufficient for validation procedures, so airborne remote sensing measure- ments appear essential for the validation tasks. Each of these two measurement approaches has its advantages and shortcomings, and only their simultaneous deployment helps to achieve scien- tifically valuable information for satellite data validation (Kostadinov et al., 1999). GASCOD/ /A4r forms part of the remote-sensing chemical payload of the M55-Geophysica together with two FTIR instruments SAFIRE/A (O3, HNO3, N2O and ClO) and MIPAS-A (O3, HNO3, N2O, CH4, H2O and T profiles), (http://ape.ifac.cnr.it/). By means of vertical scanning to the right, perpendicular to the flight direction, both FTIR instruments probe atmospheric air masses ob- served from space and profiles of the aforemen- tioned gases are retrieved. These profiles corre- spond to areas located at ∼150 ÷ 300 km away Fig. 1. Observational directions of MIPAS/A, GAS- COD/A4r and SAFIRE /A instruments aboard M55- Geophysica aircraft. Measured parameters are as- signed to areas of ∼150 km÷300 km away the aircraft (MIPAS/A and SAFIRE/A) and ~50 km÷70 km (GASCOD/A4r). 76 Ivan Kostadinov et al. from the aircraft, where the strongest contribu- tions of detected emissions arise. If we image a virtual scale defining the distances D, from the M55-Geophysica aircraft to the zone with max- imum contribution to the retrieved parameters, fig. 1, it can be concluded that the zone lying a few tens of kilometers away from the aircraft will remain unexplored without ACILA values supplied by ASCOD/A4r instrument. It took part in several validations operating successfully more than 95% of the flight time. However, the still limited quantity of available MIPAS-E and SCIAMASCHY data at the time of the preparation of this work prevent wider use of obtained by GASCOD/A4r NO2 and O3 quasi in situ measurements and hence it is hard to evaluate the ability of these satellite instru- ments to reproduce the atmospheric content of gases under interest. Here we would like to em- phasize that before assessment of satellite data a preliminary internal quality control of the da- ta obtained by means of different in situ and re- mote sensing instruments aboard M55-Geo- physica be carried out (Heland et al., 2002). We report below some examples of the re- sults obtained by means of GASCOD/A4r in- strument during campaigns dedicated for vali- dation of MIPAS-E and SCIAMASCHY data. The flight route of flight of 12 March 2003 during ENVISAT High Latitudes Campaign (http://ape.ifac.cnr.it) is shown in fig. 2, while in fig. 3 MIPAS-E O3 profile sampled within 69.3N-71.9N and GASCOD/A4r O3 ACILA values are plotted. During this flight M55 air- craft flew up to 19.6 km, so MIPAS-E data on- ly below this altitude can be compared directly to the aircraft data. In this case satellite meas- urements reproduce the general shape of verti- Fig. 2. M55-Geophysica aircraft flight route (12/03/2003). MIPAS-E scan 21, orbit 5386 (08:49:37UTC) is compared to GASCOD/A4r measurements. Closest correlation corresponds to EA back leg around 69N÷71N (see fig. 3). Color bar indicates the tangent height of MIPAS-E line of sight. 77 A multi-input UV-VIS airborne GASCOD/A4r spectroradiometer for the validation of satellite remote sensing measurements cal distribution of O3 mixing ratio obtained by GASCOD/A4r. An overall reasonable coinci- dence between satellite and aircraft data sets is evident in this example. In general, the discrep- ancies of the results are due mainly to differ- ences in time and spatial probing from both platforms. Better consistency is evident for the data (filled diamonds in fig. 3) corresponding to the descent of M55 aircraft, when GASCOD/ /A4r probe almost the same area examined ap- proximately two hours before by MIPAS-E. A similar comparison regarding results ob- tained during APE-ENVISAT Mid-Latitude Campaign are shown in fig. 4. Here only the low- Fig. 3. Ozone mixing ratio derived in 12 March 2003 by GASCOD/A4r (diamonds) and MIPAS-E (black tri- angles) during APE-ENVISAT High-Latitude Validation Campaign. MIPAS-E profile corresponds to scan 21tak- en within 69.3 N÷71.9N latitude interval. Filled diamonds correspond to EA back leg. Fig. 4. Ozone mixing ratio measured by MIPAS-E (v.4.61) at 09:19:25 UTC, (45.7° N, 12.9°E), error = ± 3v (black diamonds) and GASCOD/A4r (open diamonds) around (45.2°N, 10.2E) zone. Data are obtained during APE-ENVISAT Mid-Latitude Campaign held at Forlì airport, flight 22 July 2003. 78 Ivan Kostadinov et al. Fig. 6. ABLE narid background signal @355 nm and GASCOD/A4r signal in 333 nm ± 30 nm spectral band obtained through nadir faced NFoV channel during 28/02/2003 flight from Kiruna within APE-ENVISAT High- Latitude Validation campaign. Due to lower time resolution of GASCOD/A4r some fine spatial structures of the underlying reflectivity are omitted. However for each GASCOD/A4r point detailed spectral properties of the scattering or reflecting objects under the aircraft is available. The axes of the nested figure represent up-welling radiance: abscise – wanelength, Å; ordinate – normalised instrumental signal, a.u./s. Fig. 5. J(NO2) values (open circles) measured during flight of 28 February 2003 as a function of time, solar zenith angle, latitude and pressure. 79 A multi-input UV-VIS airborne GASCOD/A4r spectroradiometer for the validation of satellite remote sensing measurements er part of the O3 profile measured by MIPAS-E, corresponding to GASCOD/A4r measurements around (45.7°N, 12.9°E) is presented. The better spatial-time coincidence of the aircraft and satel- lite measurements in this case leads to a closer consistency of the retrieved ozone mixing ratio by means of both instruments. Despite this, the complete understanding and quantification of satellite measurements need additional 3D CTM modelling, (Giovanelli et al., 2003) in order to account also for the transport and chemical trans- formation of the species under interest. This ap- proach is applied by Wetzel et al. (2004) using KASIMA 3D CTM model output for assessment of MIPAE-E NO2 profiles. Unfortunately at the present time it is not always possible to calculate the NO2 profile from MIPAS-E measurements down to the M55 aircraft flight level, so direct MIPAS-E and GASCOD/A4r NO2 comparison is limited. For instance MIPAS-E measurements do not match air masses probed by GASCOD/ /A4r in the (45.4°N, 11.6°E) region during the flight in 14 October 2002. However, as pointed out in Wetzel et al. (2004), the comparison of KASIMA 3D CTM NO2 profile with GASCOD/ /A4r NO2 ACILA values is qualitatively reason- able. This and another comparisons allow us to state that GASCOD/A4r NO2 and O3 quasi in situ products (ACILA) can be widely deployed for further validation analysis. It was mentioned above that GASCOD/A4r provides other kinds of products appearing im- portant for atmospheric studies. Figure 5 demonstrates the variation of J(NO2) detected through the 2r sr FoV receivers during the flight of 28/02/2003 in the APE-ENVISAT campaign, at Kiruna, Sweden. The three sharp spikes are due to the partial saturation of the measured spectra. Strong increasing of J(NO2) during the return leg of the flight is caused by increasing available solar radiation as the SZA decreases. These measurements, together with NO measurements performed by means of the SIOUX instrument (Ziereis et al., 2000) aboard the same aircraft, have been used to calculate the NO2\NO ratio (Kostdinov et al., 2003). This ratio is one of the controlled parameters during adaptation of the models to the environmental conditions under which dedicated validation measurements have been carried out. Another product supplied by GASCOD/Ar instrument appears to underlie spectral reflec- tivity. This product was derivable also before the installation of the nadir NFoV channel, but with very low spatial resolution, because it was possible only through the nadir 2r sr channel. Adding the NFoV nadir faced channel signifi- cantly improves the spatial resolution of this kind of measurements. We examined the ability of GASCOD/A4r to reproduce spatial patterns of the landscape and/or cloud coverage compar- ing integrated measured spectra within 333 nm ± 30 nm interval to the ABLE (AirBorne Li- dar Experiment) (Fiocco et al., 1999; Pace et al., 2003) background noise measurements @355 nm. The background noise measured by ABLE, towards the nadir, is actually an uncalibrated ra- diometric measurement of the solar radiation re- flected and scattered from the surface and by the atmosphere. Figure 6 plots the data series ob- tained by means of both instruments during the flight of 28/02/2004, Kiruna. It is evident that overall spatial structures detected by ABLE are also reproduced by GASCOD/A4r. The advan- tage of such combined measurements arises from the opportunity to obtain underlying reflec- tivity with high spatial and spectral resolution. 6. Conclusions An airborne version of the GASCOD family instruments has been deployed during several campaigns dedicated to the study of tropical up- per troposphere/lower stratosphere regions, satel- lite data validation and Antarctic ozone depletion studies. It operates successfully under a wide va- riety of environmental conditions. The ACILA values fill the gap in the spatial scale defined by the classical remote sensing and in situ instru- ments. Simultaneous measurements of atmos- pheric constituents and actinic fluxes provided by GASCOD/A4r reveal the possibility of eval- uating the impact of photochemical reactions on tropospheric and stratospheric chemistry. Correl- ative analysis of GASCOD/A4r and ABLE enlarges LIDAR and spectroradiometric meas- urements for a better understanding of cloud processes, underlying reflectivity, radiative trans- fer and other aspects of geophysical science. 80 Ivan Kostadinov et al. 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