Performance of a FieldSpec spectroradiometer foraerosol optical depth retrieval: method and
preliminary results
Cristiana Bassani,1,* Víctor Estellés,2 Monica Campanelli,3 Rosa Maria Cavalli,1
and José Antonio Martínez-Lozano2
1Institute for Atmospheric Pollution (IIA), Italian National Research Council (CNR), Via Fosso del Cavaliere 100,00133 Rome, Italy
2Solar Radiation Group, University of Valencia, Dr. Moliner 50, 46100 Burjassot, Valencia, Spain3Institute of Atmosphere Sciences and Climate (ISAC), Italian National Research Council,
Aerosols are an essential atmospheric constituent inatmospheric radiative forcing and global climate stu-dies [1,2]. The direct component of solar radiationmeasured at ground level is commonly used forstudying the variation of atmospheric transmittancedue to aerosol burdens [3,4]. Data acquired by
ground-based instruments can be employed for aero-sol monitoring and modeling, as well as for calibra-tion and validation of satellite retrievals [5,6]. In thelatter case, during the satellite acquisition, simulta-neous ground-based observations can help to im-prove the knowledge of the atmospheric radiativefield by the estimation of atmospheric gaseous andaerosol optical parameters such as gas columnarcontent, aerosol optical depth (AOD), aerosol sizedistribution, and single scattering albedo [7]. Themost common optical parameter retrieved from
measurements of direct solar irradiance at a speci-fied wavelength λ is the AOD, δaðλÞ. Currently, inter-national networks of ground-based Sun photometersroutinely measure AOD, with sites located through-out the world. Among the mostly widespread net-works are the Aerosol Robotic Network (AERONET)based on CIMEL CE318 Sun–sky photometers [8],the Skyrad Network (SKYNET) using PREDE POMSun–sky photometers [9], and the Global Atmo-spheric Watch (GAW) network managed bythe Physikalisch-Meteorologisches ObservatoriumDavos/World Radiation Center (PMOD/WRC), Swit-zerland, using Precision Filter Radiometers Sunphotometers [10]. The AERONET sites are mainlyestablished in North America and Europe, whereasthe SKYNET is well represented in Asia. The mea-surements from both networks are used to study glo-bal patterns of atmospheric aerosol properties. Localanalysis permits one to characterize the temporalvariation of aerosol properties over each site[11,12]. The Sun photometers can be easily deployedduring field campaigns, where they are very usefulfor estimating both the AOD and the columnaramount of some gases such as water vapor and ozone,depending on the wavelengths used [8]. This work isaimed at assessing the performance of the FieldSpecFull-Range spectrometer [13] used for what is to ourknowledge the first time here to estimate the AOD.The instrument measures the direct solar irradiancein the spectral domain from 350nm to 2500nm, andthe AOD can be retrieved later in the atmosphericwindows within this spectral domain. The FieldSpecis a highly portable instrument that so far has beendevoted for characterizing surface reflectance[14,15]. The reflectance of a surface target is ob-tained by applying the reference panel method basedon the ratio between the nadir radiance reflectedfrom the target and from a reference panel with aknown reflectance. “The radiometric calibrationand the correction of the dark current have turnedthe spectrometer suitable as a spectroradiometer”(e.g., see [3]). The instrument measures the directcomponent of solar irradiance in radiance (W=m2=sr=nm) by using a cosine receptor and a telescopeplaced upon the fiber optics of the instrument [3].The AOD is then retrieved by the straightforward ap-plication of the Beer–Lambert–Bouguer law describ-ing the attenuation of solar radiation beam in theatmospheric medium [4,16,17]. The assessment ofthe FieldSpec spectroradiometer performance ismade by an intercomparison of simultaneous data ta-ken by a Sun–sky Sun photometer, Cimel CE318.
2. Instrument and Data Acquisition
During the years 2004 and 2005, the European SpaceAgency (ESA) carried out two field campaignssupported by ground-based and satellite/airborne co-ordinated acquisitions. Radio soundings and ground-based instruments such as Sun photometers, spec-troradiometers, and atmospheric particle counterswere required mainly to accurately characterize
the atmospheric condition and therefore help to cali-brate and validate the acquired remote sensing data.
The Airborne Laboratory for EnvironmentalResearch (LARA) group of the Italian NationalResearch Council (CNR) participated in these twoESA field campaigns in July 2004 (Spectra BaraxCampaign, SPARC2004) and in July 2005 (Sentinel-2 and Fluorescence Experiment, SEN2FLEX) beinginvolved in ground-based radiative measurementsusing the FieldSpec spectroradiometer. Both cam-paigns took place in Barrax (Spain) located on a pla-teau at 700m above sea level in the La Mancharegion (30°30 N, 2°60 W). Barrax is located in an agri-cultural area, far from important urban aerosolsources and not influenced by near industrial activ-ities. Usually the local aerosol production prevailsover transported aerosol, although an advection ofSaharan mineral dust was also detected during thecampaign [18]. The FieldSpec spectroradiometer andthe CE318 Sun photometer were colocated duringboth campaigns, and the measurements of FieldSpecwere scheduled simultaneously with the CE318 Sunphotometer in order to be compared later.
A. FieldSpec Configured for Direct Solar IrradianceMeasurement
The FieldSpec Full-Range spectroradiometer coversthe spectral range from 350nm to 2500nm. Its mod-ular design is equipped with a fiber optic cable, a ho-lographic diffraction grate (where the radiation beamis spectrally discriminated), and a charge-coupled de-vice (CCD) array on which the beam is projected. Thespectral components of sunlight are in this way de-tected by three different spectrometers: a silicon (Si)photodiode array in the visible to near infrared(VNIR, 350–1050nm) with a software-tunable inte-gration time, and two fast indium gallium arsenide(InGaAs) scanning detectors in the shortwave infra-red (SWIR, 1000–2500nm) with automatically tunedgains to amplify the signal, which was weaker thanin the visible range.
The Si photodiode array, overlaid with an order se-paration filter operating into the VNIR spectral do-main, has 512 contiguous channels. Each one isdetected with a narrow nominal spectral samplingof 1:4nm (Fig. 1), resulting in high spectral reso-lution (full width at half-maximum of 3nm) thatdefines the sensitivity of the detector. The uniformspectral bandwidth as shown in Fig. 1, satisfies theminimum discretization for spectral resolutionrequired to recover spectral features of measure-ments without signal degradation according to [14]:approximately two times the spectral sampling, thatis, 3nm at around 700nm for the FieldSpec spectro-radiometer. The radiation of the SWIR spectraldomain is acquired sequentially by two fast-scanningspectrometers. Each spectrometer is composed ofa concave holographic grating to ensure high effi-ciency and extremely low stray radiation and asingle thermoelectrically cooled detector to stabilizeits temperature. Unlike the VNIR, each SWIR
spectrometer has only one detector, which is exposedto different wavelengths of radiation. The first SWIRspectrometer covers the spectral domain 900–1850nm, whereas the second covers the region 1700–2500nm. The narrow nominal sampling for eachSWIR band is about 2nm resulting from a high spec-tral resolution, 10nm.The splice between the output of the three spectro-
meters is automatically controlled by the overlap ofthe detector spectral subsets. Therefore, the modulardesign of the FieldSpec detection satisfies the instru-mental requirements for hyperspectral remote sen-sing applications: high spectral resolution and finespectral sampling [19]. The most common applica-tion of the spectrometers in the remote sensing fieldis the measurement of surface spectral reflectancedefined by means of normal upwelling radiance ofthe surface normalized to the radiance coming froma reference white panel of known reflectance. Basi-cally, the reflectance is a relative radiative measure-ment achieved by a fast double acquisition (surfaceand reference panel) fixed in a nadir-pointing config-uration, [14,15,20]. In order to use a spectrometer forabsolute radiative measurements (i.e., a spectroradi-ometer) a radiometric calibration of long-term stabi-lity is required.For the FieldSpec, the standard radiometric cali-
bration was performed by Analytical Spectral De-vice, Inc., Company (ASD Inc., Boulder, Colorado,USA) using a lamp powered by a stable direct currentregulated supply. The radiometric performance ofFieldSpec is guaranteed by periodic checks of theradiometric calibration and the noise-equivalent del-ta radiance (NeDL). For example, in Table 1 we showthe specifications of our unit, checked by ASD Inc. inJune 2004 [13] and used later for the analysis re-ported here.Once the radiometric calibration of the FieldSpec
is established, the long-term stability of the absoluteradiative measurements is obtained by automaticdark current control correction during each acquisi-tion. As a result the spectroradiometer provides a
random error lower than 0.5% over the spectralregion from 300 to 2200nm and a random error of2% over the interval from 2200 to 2500nm. TheFieldSpec uncertainties are low enough to be em-ployed as a field spectroradiometer for Sun irradi-ance measurement.
Thereby, a cosine receptor and a telescope placedupon the fiber optic of the spectroradiometer canbe adapted to measure the direct component of solarirradiance within its spectral domain, from 350nm to2500nm [3]. The study in [3] shows the excellentradiometric stability of the FieldSpec for a periodof one year (outside the water vapor absorptionbands), which means that the method explained inthis paper can be applied at the absolute radiativemeasurements acquired in the summer campaignsheld in July 2004 and July 2005, once the FieldSpecchecking was performed in June 2004.
Figure 2 shows the device deployed and configuredfor direct irradiance measurements, with the new op-tical system mounted at the end of the fiber opticprobe. This probe, denoted by (a) in Fig. 2, is suppliedby the manufacturer for surface reflectance measure-ments. It must be inserted into the back of the fullsky remote cosine receptor (b). A directional tube isused to measure the direct Sun irradiance, and it ismounted upon the cosine receptor (c). Finally, a re-strictor of the view angle (d) is attached at the endof the tube to limit the energy flux into the fiber op-tics. The design of the tube and diaphragms wasmade keeping in mind other devices such as theCE318. A view angle of 1° was chosen, in front of the1:2° from the CE318. This value permits us to per-form accurate manual pointing without vignettingeffects at the time most of the circumsolar radiationis screened.
On the field, the optical system for Sun directmeasurements is connected to the FieldSpec withthe fiber optic probe, and it is mounted on a tripodequipped with a three rotational axis joint, so thesystem can be manually aimed at the Sun [3]. Inorder to get accurate pointing of the system, we vi-sually check the shadow of a mark in the tip extremethat is projected on the tube base. The correct align-ment of the tube was previously checked by theauthors in the laboratory.
Figure 3 shows a spectrum of the Sun direct irra-diance as acquired by the FieldSpec during one of thefield campaigns. The absorption bands of the atmo-spheric gases in the VNIR region are shadowed inthe plot. In the visible band, this spectrum shows
Fig. 1. Spectral resolution and spectral sampling.
Table 1. Characteristics of the FieldSpec Spectroradiometer
Spectral range 350–2500nmSpectral resolution 3nm@700nm, 10nm@1400nm=2100nmSpectral sampling 1:4nm@350–1050nm,
a partial extinction of the direct irradiance for fivemain spectral regions. The estimation of the AODin these regions cannot be performed because thetransmittance depends more on the complex gas ab-sorption features than on the aerosol extinction. Theusual procedure in Sun photometry is to estimate theaerosol extinction out of these gas absorption bands[17] to minimize uncertainties coming from the gasburden determination and their absorption features.Usually, only a few atmospheric transmission win-dows are defined for aerosol determination. The2000–2220nm region contains two low absorptionbands of CO2 that could be used for our purpose,but in this spectral region, the long-term radiometricstability is strongly affected by the combination ofhigh NeDL (see Table 1) and a lower signal fromthe Sun. In addition, in this band the aerosol extinc-tion is much lower than in the other atmospheric
windows at shorter wavelengths, so its determina-tion would be highly uncertain. For this reason theAOD was not retrieved in these atmospheric win-dows. As a conclusion, the estimation of AOD wasonly performed for 370 of the 2151 available channels(18%) of the FieldSpec spectroradiometer [3], corre-sponding to wavelengths outside the gas absorptionbands.
The uncertainty of direct solar irradiance acquiredby the FieldSpec is related to shown in Table 1. Theextremely low NeDL in combination with high speedof data collection (0:1 s) are optimum specificationsthat make the FieldSpec an optimum spectroradi-ometer for AOD determination [3].
B. Sun Photometer CIMEL CE318-NE
The CIMEL CE318-NE Sun photometer measuresdirect solar irradiance and sky diffuse radiance innine channels in the UV, VIS, and NIR ranges (340,380, 440, 500, 670, 870, 940, 1020, and 1610nm) [8].The bandwidth is about 2nm (at 340nm), 4nm(at 380nm), 10nm (440 to 1020nm), and 40nm(1610nm). When used for monitoring the atmo-sphere in a key site, it automatically performs directSunmeasurements at least every 15 min and sky dif-fuse measurements at least once every hour. In thecase of field campaigns, the basic measurement pro-tocol can be conveniently adapted, and thereforemore frequent measurements are performed for bet-ter temporal resolution. AOD, Ångström wavelengthexponent α (a parameter that gives us informationabout the size of the optically active aerosols), andcolumnar water vapor are also derived from the di-rect Sun measurements. A complete description ofthe employed methodology is given in [21]. Otheraerosol properties can be retrieved from the radiancemeasurements if an inversion code is applied. In ourcase, SKYRAD.PACK code version 4.2 was used inthis field campaign [6]. It allowed us to retrievethe aerosol size distribution, complex refractive in-dex, single scattering albedo, and asymmetry para-meter. From these results, other secondary derivedcolumnar properties as effective radius, columnaraerosol mass, volume, and surface can be obtained.More details on the application of the code to thesedata are given in [6,18] together with an extensiveanalysis of the atmospheric situation in these fieldcampaigns.
For AOD determination a direct Sun calibration isneeded. This calibration can be performed by stan-dard Langley plots (e.g., see [17]), improved Langleyplots [22], or calibration transfers from a master in-strument [21]. For this Sun photometer unit, a com-bination of standard Langley plots and calibrationtransfers was performed from the years 2004 to2006. The calibration factors at a given date arecomputed by interpolation between the previousand next available calibration set. The calibrationand the interpolation procedures include the estima-tion of the associated uncertainty, which later wasemployed for deriving the uncertainty on the AOD
Fig. 3. Example of a spectrum measured by the FieldSpec spec-troradiometer in direct irradiance configuration. The shaded areasrepresent the absorption bands of atmospheric gases.
Fig. 2. FieldSpec deployed in the field: (a) fiber optic probe, (b) re-mote cosine receptor, (c) limiting tube, (d) field of view restrictor.
[21]. The resulting nominal value for the uncertaintywas 0:01–0:02, dependent on channel and opticalair mass. This estimation is of the same order of theuncertainties found for field AERONET instru-ments [23].
3. Methodology
The purpose of this work is to retrieve the AOD fromthe FieldSpec direct Sun irradiance measurementsand to validate the results by comparing them withthe Cimel Sun photometer values.The proposed method is a straightforward applica-
tion of the Beer–Lambert–Bouguer law, also used forthe Langley calibration of the FieldSpec. The irradi-ance is acquired after pointing the system at the Sunin a cloudless sky (at least around the Sun) in orderto avoid direct sunbeams to interact with the clouddroplets or ice crystals. The Beer–Lambert–Bouguerattenuation law describes the relationship betweenthe measured irradiance at ground level and theproperties of the medium through which theradiation travels (the atmosphere) by the followingequation:
Iλdir ¼ Iλ0dirR−2es Tλ; ð1Þ
where Iλ0dir is the spectral direct solar irradiance,R−2es
is the actual Earth–Sun distance in astronomicalunits, Tλ is the spectral atmospheric transmittanceand Iλ0dir represents the direct solar irradiance out-side the atmosphere, also called the solar calibrationconstant. The transmittance Tλ is a physical quan-tity that depends not only on the wavelength but alsoon the amount and properties of the atmospheric con-stituents. The general expression of Tλ is
Tλ ¼ e−mδðλÞ; ð2Þwherem is the optical air mass (function of the cosineof solar zenith angle) and δðλÞ represents the totaloptical depth. The transmittance in Eq. (2) is the ex-ponential sum of different extinction processes: Ray-leigh scattering TλRy, gas absorption Tλg, and aerosolextinction Tλa:
Tλ ¼ TλrTλgTλa: ð3ÞIn the spectral regions specifically selected in this
work, the main gas absorption feature is due to theozone TλO, and nitrogen dioxide Tλn:
Tλg ¼ TλOTλn: ð4ÞMost simplified models use a single optical air
mass, the inverse of cosine solar zenith angle, to es-timate the total slant path for all the extinction pro-cesses in the atmosphere. By contrast, differentoptical masses for different atmospheric componentsare considered in the present work because eachextinction process corresponds to a particular consti-tuent with a different distribution profile in the at-mospheric column. Separating the different opticalair masses improves the accuracy of the retrievedAOD. Each optical air mass was parameterized by
fitting the data rigorously calculated in [24]. Thefit function proposed by [25], is represented by thefollowing common equation:
mi ¼ ½cos θ þ ai1ðθÞai2ðai3 − θÞai4 �−1; ð5Þwhere mi means mRy (Rayleigh), ma (aerosol), mNO2
(nitrogen dioxide), or mO (ozone) optical masses. Thesolar zenith angle is θ, and the coefficients aij are fit-ting coefficients, according to [25]. These coefficientsare listed in Table 2.
For computing δRy (Rayleigh), δNO2(nitrogen diox-
ide) and δO (ozone) the method explained in [26] andimplemented in the SMARTS2 model has been ap-plied. Consequently, the spectral AOD (or δa has beenretrieved from FieldSpec measurements of direct so-lar irradiance by the following equation:
δaðλÞ ¼1ma
�ln�Iλ0dirR−2
es
Iλdir
�−
Xi¼r;n;O
miδiðλÞ�: ð6Þ
In order to solve the Eq. (6), the calibration coeffi-cients for each FieldSpec channel, lnðIλ0dirR−2
es Þ, arerequired.
A. Retrieval of Calibration Coefficients
The solar calibration constant represents the extra-terrestrial direct solar irradiance by assumingthe validity of the Beer–Lambert–Bouguer law, ex-pressed by Eq. (1). Taking logarithms in both mem-bers of Eq. (1) and rearranging,
lnðIλdirÞ ¼ lnðIλ0dirR−2es Þ þ lnðTλÞ: ð7Þ
The calibration coefficients lnðIλ0dirR−2es Þ are the di-
rect solar irradiance that the instrument would mea-sure if there were no atmosphere (Tλ ¼ 1) when thedistance between the Sun and the Earth was 1 A.U.Substituting Eq. (2) into Eq. (7),
lnðIλdirÞ ¼ lnðIλ0dirR−2es Þ −mδðλÞ: ð8Þ
The calibration coefficients can be found for the at-mospheric windows by means of the Langley calibra-tion method, that is, by fitting a series of solarirradiance measurements acquired during the sun-rise or sunset hours under a cloudless and clearstable atmosphere. These conditions are needed inorder to apply a linear fit to the previous equation,taking the optical depth as the slope. The best placeto find these conditions is high sites in the moun-tains, well over the boundary layer if possible, to
Table 2. Coefficients for Optical Masses Reported in [25]
minimize the influence of aerosols and atmosphericchanges [17]. Therefore, the optimum Langley cali-bration assumptions are (a) considering the opticaldepth δðλÞ constant during the series acquisition timeand (b) evely sampling the measurements over an op-tical air mass range of 2 to 4, generally.A calibration campaign was carried out for the
FieldSpec spectroradiometer. It consisted of two dif-ferent series of measurements in summer, on 17 and25 June 2004. The sampling was performed with atemporal interval of 5 min for lower zenithal angles(from sunrise to 1000 UTC) and 10 min from 10:00 to12:00 UTC. In this way the optical air mass rangerequired for the Langley calibration (2–4) was com-pletely covered. The measurements were acquiredover a time span long enough to provide a sufficientnumber of sample points for a robust least-squaresfitting determination [3] and an accurate estimationof calibration uncertainty. The calibration campaignwas performed in a high-altitude site with a clearand stable atmosphere: the Mount Pollino massif inthe southern Italy Apennines Range (altitude2500m, 45°330 N, 7°50 E).Figure 4 shows the logarithm of the direct solar ir-
radiance measured by the FieldSpec at channel500nm as a function of the optical air mass. Thenumber of samples and the total absence of outliersmade both plots suitable for the calibration. A linearleast-squares fit was applied to the data followingEq. (8). In the plot the intercept represents thedistance corrected logarithm of the calibrationirradiance, lnðI0dirR−2
es ðλ ¼ 500nmÞÞ. The calibrationis finally obtained from the average of the aboveintercepts.The least-squares fit procedure is performed for all
the available channels. Fig. 5 shows the values of thefitting intercepts for the 370 instrument channelslocated within the atmospheric windows. The stan-dard deviation of the data from the retrieved calibra-tion coefficient, lower than 1% throughout the
spectral regions considered in this analysis, confirmsthe robustness of the results obtained from the cali-bration campaign.
4. Results
The following results refer to the data from both fieldcampaigns (summer 2004 and 2005) when the Field-Spec calibration coefficients were valid. Figure 6shows two instances of AOD spectra for field cam-paigns SPARC2004 (15–18 July 2004) and SEN2-FLEX (12–14 July 2005). These spectra exhibit thewell-known spectral behavior of the AOD [27], de-creasing with wavelength. The shadowed areas inthe IR region hide the spectral intervals in whichthe solar radiation is completely absorbed by gases,and consequently the AOD cannot be retrieved. Inthe VIS region these gray areas also cover the spec-tral intervals where the radiation is not completelyabsorbed by the atmospheric gases.
Regarding the AOD uncertainty, the error propa-gation method has been applied to Eq. (6) after ne-glecting the relative uncertainties of δiðλÞ (i ¼ Ry,NO2, O) according to [21]
ΔδaðλÞ ¼1ma
��ΔI0I0
�2þ�ΔI
I
�2�1=2
: ð9Þ
The ratio ΔI0=I0 is less than 1% for the available370 channels. ΔI represents the uncertainty in theirradiance measurement related to the instrumentaccuracy.
Figure 7 shows the AOD uncertainty orΔδaðλÞ as afunction of time for two different FieldSpec channels,related to the first and second spectrometer of the in-strument, respectively. For the first spectrometerchannel, the central wavelength is 500nm; the sec-ond channel is centered on 1020nm. As a conclusion,the uncertainty from our retrievals has been esti-mated to be lower than 0.02 for all the available
Fig. 4. Two Langley plots acquired during 17 and 25 June 2004 for 500nm.
channels. Therefore, the accuracy of the FieldSpecretrievals is comparable to the nominal uncertaintyof the CE318 field Sun photometers in AERONET(0:01–0:02) [21,23].In order to obtain comparable AOD values, every
FieldSpec spectral AOD (sampled at 1nm) has beenconvoluted with the CE18 filter transmission pro-files, characterized by a Gaussian function, centeredon 440, 500, 670, 870, and 1020nm (λCi with i ¼ 1, 5)and with a bandwidth ΔλCi ¼ Δ ¼ 10nm for eachchannel. The five FieldSpec spectra δFSa ðλCi Þ are con-voluted with the following equation:
δFSa ðλCi Þ ¼PλiþΔ
λ¼λi−Δ δaðλÞe−ðλ−λC
iÞ2
Δ2
PλiþΔλ¼λi−Δ e−
ðλ−λCiÞ2
Δ2
with i ¼ 1; ::; 5: ð10Þ
Figure 8 represents the evolution of the AOD at500nm, retrieved by the FieldSpec, δFSa ðλCi Þ, andthe CE318 Sun photometer AOD, δCa ðλCi Þ, for all thecampaign days. Figure 8 shows that a general lowdaily variation of AOD, as well as a steep increaseduring the afternoon of 14 July 2005, can be observed
Fig. 5. FieldSpec calibration coefficients within the atmosphericwindows. The gray areas hide the absorption bands of atmosphericgases.
Fig. 6. Different examples of AOD spectra, each one representedby a different gray tone, during the campaigns SPARC2004 (left)and SEN2FLEX (right). The shaded areas are the absorptionbands of atmospheric gases.
Fig. 7. Estimation of AOD uncertainty for two different channelsin an example day.
Fig. 8. Evolution of the AOD at 500nm during the SPARC2004(above) and SEN2FLEX (below) campaigns. The solid black trian-gles are the FieldSpec results convoluted in the CE318 profiles; thesolid gray circles are the CE318 values.
in the data from both instruments. This AOD tem-poral evolution is thought to be related to an intru-sion of Saharan mineral dust, as described in [18].In order to validate the AOD retrievals obtained
from the FieldSpec, the AOD obtained from theCE318, is considered as a reference. Figure 9 shows
δFSa ðλCi Þ versus δCa ðλCi Þ for all the simultaneous mea-surements from both campaigns. The results of thelinear regression analysis between the two seriesof data can be considered statistically significant be-cause the dataset is composed of a large number ofsamples.
Fig. 9. Linear regressions of FieldSpec against CE318 AODs for the SPARC2004 and SEN2FLEX campaigns.
The low values of the intercepts shown in Fig. 9 arean estimator for the systematic errors connected di-rectly with the FieldSpec calibration coefficients andtherefore highlight the validity of the calibrationmethod and results. Furthermore, the high correla-tion index shown in Fig. 9 also suggests the goodnessof the applied methodology. In Table 3, the key para-meters to validate the δFSa ðλCi Þ with respect to the re-ference values δCa ðλCi Þ for any channel i are shown.The slopes (close to 1) and the intercepts (close to 0)highlight the goodness of the comparison for all theCE318 channels. The R2 coefficients are close to 0.9and also show the high correlation between both da-tasets. As can be seen, the root mean squared devia-tion (RMSD) between the AOD obtained by using theFieldSpec and the Cimel was typically less than 0.02,that is well within the combined uncertainty fromboth FieldSpec and CE318 retrieved AODs [23].
5. Conclusions
The FieldSpec spectroradiometer is a portable in-strument designed and usually devoted for measure-ments of surface reflectance. In this study, we haveassessed its performance for retrieving atmosphericproperties, such as the aerosol optical depth. TheAOD is an important atmospheric parameter relatedto the burden of particles in the atmospheric columnthat is routinely applied for the correction of remotesensing products. The same portable instrument canbe used now not only for measuring surface reflec-tance but also atmospheric properties, depending onthe field campaign requirements. Our objective wasto assess its performance before it could be used rou-tinely in the field when the automatic Sun photo-meters, such as the Cimel, are not available.For the instrument calibration, a field campaign
was held in a high mountain site under stable weath-er and atmospheric conditions. Measurements wereperformed during two mornings in order to obtain anoptimum span of optical air masses for the applica-tion of the standard Langley plot method. The repe-tition served as a check of the obtained calibrationand also for the uncertainty estimation.After the calibration, a well-established procedure
for retrieving the AOD (previously used in state ofthe art Sun photometers) was adapted to the Field-Spec spectroradiometer. From the whole spectrum,only five channels were selected within atmosphericwindows. The corresponding wavelengths were coin-cident with the Sun photometer used for the valida-tion. These channels were moreover taken from theset recommended for Sun photometry by the WorldMeteorological Organization [28].
For the AOD validation, two series of data fromtwo different field campaigns were used. The datasetwas obtained in days of different atmosphericcharacteristics. Measurements of a colocated CimelCE318 Sun photometer were also taken simulta-neously during these field campaigns for the compar-ison. The FieldSpec spectroradiometer was operatedmanually to take coincident measurements to theCE318 device.
The final comparison has been made by a linearregression between the instantaneous AODs ob-tained with both instruments and by computingthe RMSD. Although care was taken to make simul-taneous measurements, a maximum difference be-tween the two measurements of 1 min was appliedin the automatic processing step. This time stampis actually a very relaxed criterion and does not affectthe results. The temporal evolution of the aerosolburden was not so fast as to be affected by this timeelapse. The FieldSpec AOD spectrum was then con-voluted by a Gaussian curve, with a full width athalf-maximum of about 10 nm, in order to get resultscomparable to the Cimel resolution. The results showthat both datasets are correlated significantly, withR2 of about 0.9 for the five channels. The high corre-lation index, the linear regression parameters (slopeand intercept close to 1 and 0, respectively), and theRMSD are consistent and confirm the applicability ofthe FieldSpec spectroradiometer to retrieve accuratevalues of the AOD. In this way, we have been alsoable to reproduce the evolution of the aerosol burdenduring both field campaigns, finding the passage ofrespective dust intrusions, as previously documentedin [6], with an uncertainty the same as that of theCimel.
We can also identify a few advantages of the cur-rent method over the Cimel: first, the FieldSpec isthermoelectrically cooled, so the temperature effectson the NIR spectral domain are corrected; second,the spectral resolution is higher, so the AOD at a gi-ven wavelength is more accurate; third, you alwayshave the possibility to employ many more channels ifyou so desire; fourth, you do not need to purchase andmaintain a Cimel Sun photometer if you actually areworking with one of these FieldSpec instruments.Moreover, the FieldSpec is easily portable, and youcan carry it easily in the field. Of course, for deeperaerosol monitoring purposes, the Cimel is superior inthe sense that it is an automatic instrument, and itretrieves much more information from the skyradiance measurements (single scattering albedo, re-fractive indices, phase function).
Table 3. Coefficients of the Comparison between the FieldSpec and Cimel AOD
Consequently, the FieldSpec spectroradiometerhas proved to retrieve reliable values of atmosphericparameters such as the AOD, if a proper calibrationand data elaboration procedure is applied to the rawradiometric data. In this sense, the instrumentshould be periodically calibrated if the uncertaintiesare expected to be below the estimations given inthis study.
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