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Page 1: Absolute spectral measurements of direct solar ultraviolet irradiance with a Brewer spectrophotometer

Absolute spectral measurements of direct solarultraviolet irradiance with a Brewer spectrophotometer

Alkiviadis F. Bais

A methodology for the absolute calibration of spectral measurements of direct solar ultraviolet radiation,performed with a Brewer spectrophotometer is presented. The method uses absolute measurements ofglobal and diffuse solar irradiance obtained practically simultaneously at each wavelength with thedirect-Sun component. On the basis of this calibration, direct-Sun spectra, measured over a wide rangeof solar zenith angles at a high altitude site, were used to determine the extraterrestrial solar spectrumby applying the Langley extrapolation method. Finally this spectrum is compared with a solar spectrumderived from the airborne tunable laser absorption spectrometer 3 Space Shuttle mission, showing anagreement of better than 63%. © 1997 Optical Society of America

Key words: Solar ultraviolet, direct-Sun spectra, extraterrestrial spectrum.

1. Introduction

Measurements of the solar ultraviolet radiationreaching the Earth’s surface became important dur-ing the last few years, as both observations and pre-dictions suggest the tendency to an increase of UVlevels, following observed ozone decreases.1–3 Fourdifferent types of solar UV measurement are cur-rently available at various monitoring stations, eachneeded for different purposes. Measurements ofglobal UV irradiance are the most commonly used,and they are useful in studying the effects of ozone,clouds, and aerosols on solar UV radiation and itschanges with time and for the formation of suffi-ciently long data records that could be used in thefuture to determine long-term UV trends.2–5 To alesser extent these measurements can also be usedfor biological studies. Actinic flux measurementsare suitable for photochemical as well as for biologicalstudies.6,7 Measurements of the diffuse componentcan be used, together with global and radiance mea-surements, to study the effects of atmospheric aero-sols. Finally direct-Sun measurements are useful todetermine the aerosol optical depth as well as thecolumn abundance of atmospheric species that ab-

The author is with the Department of Physics, Laboratory ofAtmospheric Physics, Aristotle University of Thessaloniki, Cam-pus Box 149, Thessaloniki 54006, Greece.

Received 19 December 1995; revised manuscript received 7 No-vember 1996.

0003-6935y97y215199-06$10.00y0© 1997 Optical Society of America

sorb solar radiation in the ultraviolet.8,9 Modelingstudies can benefit greatly from measurements of thedirect-Sun component, since its parameterization ismuch easier than that of global or diffuse compo-nents. Despite their usefulness, spectral measure-ments of the direct component of solar ultravioletradiation have been included only recently in theobservational programs of various UV monitoringstations. The first reason for this delay was theneed for a pointing system to direct the beam light ofthe Sun into the spectrometer, which would also beadaptable to existing instruments. This problemwas finally solved by the wide spread use of fibers inUV spectroradiometry. Another reason for the de-lay was the difficulty to establish a reliable method-ology for the absolute calibration of direct solarirradiance. Despite the great improvements madein recent years, some of the uncertainties in themethodology still have not been eliminated.

The Brewer spectrophotometer is among the in-struments capable mechanically, from the firstmoment, of conducting direct-Sun spectral measure-ments. I describe the methodology used for calibrat-ing such measurements taken with the Brewerspectrophotometer MKIII of the Laboratory of Atmo-spheric Physics, University of Thessaloniki. The ab-solutely calibrated direct-Sun spectra were then usedto determine the extraterrestrial solar spectrum fromground-based measurements made at a high-altitudesite through application of the Langley extrapolationmethod. Since it is not within the objectives of thispaper to discuss in detail the accuracy of the Brewer-derived solar spectra, only a preliminary comparison

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with an existing extraterrestrial solar spectrum ispresented. Of all such spectra reported in theliterature,10–12 one of the most recent spectra wasselected, which was obtained by the airborne tunablelaser absorption spectrometer ~ATLAS 3! mission ofthe Space Shuttle.

2. Instrumentation and Measurements

The Brewer spectrophotometer was originally de-signed to perform total ozone measurements in theUV region by using either direct sunlight or diffuseskylight from the zenith.13 With the addition of anUV diffuser the Brewer also became capable of takingspectral measurements of the global solar irradiance.These scans cover the spectral region 290–325 nm forconventional single-monochromator instruments~types MKII and MKIV! and 285–365 nm for adouble-monochromator ~MKIII! Brewer instrument.Since the instrument was designed to include a Sun-tracking system for total ozone measurements, at-tempts have also been made for the last several yearsto obtain spectra of the direct component of solarultraviolet radiation.

To obtain direct-Sun spectra the pointing system ofthe Brewer instrument is directed toward the Sun’sdisk, which is viewed under an angle of approxi-mately 1.5°, determined by the aperture of theBrewer input optics. Of course the viewing angle islarger than the Sun’s angle ~'0.5°! and therefore theBrewer instrument senses a fraction of the skylight,which could introduce a small error in the measure-ments. The direct sunlight is then collimated anddirected onto a ground quartz diffuser plate beforeentering the spectrometer. From then on the proce-dure is similar to that followed for gathering globalUV irradiance spectra.1,14,15 The quality of a mea-sured direct-Sun spectrum depends strongly on theeffectiveness of the instrument’s pointing system intracking the Sun continuously. Failure to achieveaccurate tracking can result in measuring only partof the direct components of solar radiation. Anotherproblem that might be encountered during such mea-surements is the increased signal of the direct beamwhen one samples the long UV wavelengths. Thiscan happen particularly at sites with aerosol-free airand low ozone content as well as at high solar eleva-tions. Under such conditions the signal can becomelarger than the saturation limit of the detector, re-quiring the use of attenuation filters. Although theneutral density filters of the Brewer instrument areconsidered spectrally flat, it is necessary to measuretheir spectral response or at least to take their pres-ence into account during the calibration procedure.

For this study I used a double-monochromatorBrewer MKIII, which has operated at the Laboratoryof Atmospheric Physics since 1993, producing bothglobal and direct-Sun UV spectral measurementsregularly. The absolute calibration of global UVspectra is maintained through comparisons with1000-W reference sources traceable to National Insti-tute of Standards and Technology standards. Theoverall uncertainty associated with the global irradi-

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ance measurement is within 64%. This estimatedoes not include the systematic error induced by thecosine response of the input optics, which by labora-tory measurements was found to be ;25% for a 60°zenith angle, increasing to 230% at 75°. These er-rors of course apply only to the direct component thatbecomes less significant at higher solar zenith angles.An internal mercury discharge lamp is used for thewavelength calibration of the instrument, which onecan do by aligning the spectrometer with the302.1-nm line with a precision of 0.007 nm.16 Thiscontrol test is always done before each scan to ensureproper wavelength calibration.

3. Calibration of Direct-Sun Spectra

Calibration of direct-Sun spectra is not as easy ascalibration of global spectra. The former requires apoint source of known characteristics, and a cali-brated lamp at a 50-cm distance could by no means beconsidered as a point source. This could be doneonly if the lamp were moved far from the instrument,e.g., at a distance of 5 m. Although the absoluteirradiance of the lamp at such a distance could becalculated by the inverse square law, a 100-fold de-crease of its signal would occur, which consequentlywould increase appreciably the uncertainty of thecalibration. The sensitivity of the specific instru-ment is such that, when one uses a 1000-W quartz-halogen reference lamp, the uncertainty in themeasured irradiances, induced only by the photon-counting statistics,17 ranges from ;1.5% at 290 nm to;0.5% at 360 nm. If the signal were reduced 100times, these uncertainties would increase to 15% and5%, respectively.

Indirectly, calibration of direct-Sun spectra couldbe done by using simultaneous absolute spectralmeasurements of global and diffuse solar irradiance.The difference between the two measurements D~l!,corrected by the cosine of the solar zenith angle z,gives a good estimate of the direct component of solarirradiance. Then the uncalibrated direct-Sun irra-diance B~l!, also recorded simultaneously, can becompared with that derived indirectly from the globaland diffuse irradiances, thus determining the re-quired calibration factor F~l! as

F~l! 5B~l!

D~l!@cos~z!#. (1)

Although this method seems quite simple, there areseveral restrictions that require considerable atten-tion.

~1! If the method were applied at moderate solarzenith angles, larger than ;30°, the cosine error ofthe instrument’s diffuser could be inherited by thecalibration factor. Therefore one must either correctfor the cosine error or, preferably, perform such mea-surements when the Sun is close to zenith ~i.e., at lowsolar zenith angles!.

~2! The method for measuring the diffuse compo-nent is to use a shading disk to exclude the direct-Sun

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component from the global measurements. The sizeof the disk in relation to its distance from the diffusercan affect significantly the accuracy of the diffuse-component measurements. A larger than necessarydisk would increase the fraction of the skylight that isobscured, introducing a systematic error in the cali-bration factor. This becomes more important whenthe aerosol optical depth is high, which modifies sig-nificantly the diffuse component. The optimum con-figuration would be such that the solid angledetermined by the size of the disk is equivalent to theviewing angle of the direct-Sun port of the instru-ment.

~3! It was mentioned above that the three types ofmeasurement ~global, diffuse, and direct! must besimultaneous. If one considers using sequentiallyobtained spectra, then the time required to obtain thethree spectra is at least 10 min for the single-monochromator and 20 min for the double-monochromator Brewer instrument.

~4! The shading disk must cover the diffuser platecontinuously in order to obtain consistent measure-ments. However, during a single scan both the ze-nith and the azimuthal angles of the Sun changesignificantly. To compensate for these changes theposition of the disk should be updated regularly tomaintain its relative position to the diffuser.

4. Results and Discussion

Here I describe the methodology followed for the cal-ibration of direct-Sun spectra obtained by the BrewerMKIII of the Laboratory of Atmospheric Physics.The measurements were carried out at the Izana Ob-servatory, Tenerife, during a period of one week inJuly 1995. Izana is a tropical site ~28 °N! located atan altitude of 2367 m. Owing to the high altitude,clouds are constantly below the horizon of the site atthis time of the year, except when high-altitudeclouds are present. In addition, the aerosol-free at-mosphere ~e.g., optical depth at 320 nm usually below0.05! makes the site suitable for actinometric mea-surements and calibrations. The methodology fol-lowed to calibrate the Brewer direct-Sun spectralmeasurements is the one described previously thatutilizes global, diffuse, and direct-Sun spectra. Toobtain the diffuse spectrum a disk of 8 cm diameterwas placed 1 m away from the diffuser, toward theSun, supported by a relatively thin, black metal rod.The rod was mounted on the instrument so that itcould rotate about a horizontal axis, located at theplane of the diffuser plate and perpendicular to theSun’s meridian. In this way the azimuthal angle ofthe disk was always the same as that of the Sun,since the disk’s mount was rotated together with theBrewer. Its zenith angle could be set by rotatingmanually the metal rod about its horizontal axis.The experimental setup is shown in Fig. 1.

To fulfill the fourth requirement described in Sec-tion 3 a special routine was made for the Brewer,allowing the instrument to record in sequence and,within only ;30 s, the global, diffuse, and direct ir-radiances for each wavelength. This sequence was

repeated for the spectral range 290–360 nm, in stepsof 2 nm. The calibration factor is expected to be asmooth function of wavelength, and therefore the2-nm step is sufficient to define this function, reduc-ing at the same time the duration of the measure-ments. The alternation between the diffuse andglobal measurements was made by offsetting auto-matically the azimuth of the Brewer by a few degreesso that the shadow of the disk was moving away fromthe diffuser. In this way the obstructions introducedto the radiation field by the metal rod and its mount-ing were the same for both the global and diffusemeasurements, assuming of course that the diffuseskylight within a few degrees of azimuth does notchange significantly. Finally the direct componentwas measured by pointing the Brewer to the Sun andusing the internal diffuser plate as described previ-ously. The signal was checked continuously, andthe appropriate neutral density filter was inserted inthe light path, if the signal exceeded a predefinedcount level. Occasionally and while the Brewer wasperforming a direct-Sun measurement, the disk wasadjusted manually to account for the small changesin the solar zenith angle during the period of mea-surements.

Several spectra of this type were obtained duringthe campaign, covering different solar angles. Notethat the cosine response of the Brewer input opticscan modify significantly the results as the solar ze-nith angle increases. Therefore the calculated di-rect component D~l! was corrected for the cosineerror according to the zenith angle. As mentionedpreviously, the cosine error of the instrument isknown from laboratory measurements.15 Finallythe calibration factor F~l! was determined separatelyfor each wavelength by using Eq. ~1!.

Fig. 1. Experimental setup used for the calibration of the Brewerdirect-Sun spectra.

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Figure 2 shows the calibration factor as determinedfrom measurements taken at various solar zenith an-gles. It is evident from this figure that measure-ments corrected for the cosine response give similarresults, and therefore the calibration factor F~l! canbe represented by their average, shown in Fig. 2 asfilled circles. From the variance of these measure-ments it was estimated that the uncertainty intro-duced in the calibration factor is ,1%. Sincedifferent neutral density filters were used during themeasurements to adjust the level of the signal, it isnecessary to determine the calibration factor inde-pendently for each filter. In the example of Fig. 2,results for Brewer filter 2, which provides a tenfoldattenuation, are presented. For comparison and todemonstrate the wavelength dependence of the fil-ters, the calibration factor for filter 3 ~attenuationfactor of 101.5! is also shown as diamonds and at adifferent scale ~right ordinate!.

During the period of measurements at Izana inJuly 1995, the aerosol optical depth was very small~ranging between 0.02 and 0.05 at 320 nm!18 andtotal ozone was also stable as indicated by the Brewermeasurements. These two conditions must be metin order to apply the Langley extrapolation meth-od.19,20 The method was applied to the Brewerdirect-Sun spectra obtained at different zenith anglesduring the day, aimed at determining the extrater-restrial solar spectrum. Figure 3 shows examples ofLangley plots for four wavelengths in the UV regionfor four days in July 1995. Apparently there is astrong linear relationship between the logarithm ofthe irradiance and the corresponding air mass for allwavelengths, which verifies that the conditions dur-ing the measurements were stable and that the datagathered are suitable for calculating the extraterres-trial solar flux. This is also supported by the factthat measurements for different days give similarresults ~see corresponding different symbols in Fig.3!. The decreasing slope of the regression line withincreasing wavelength is due to the weaker absorp-tion of ozone at higher wavelengths. The uncer-

Fig. 2. Calibration factor of the Brewer direct-sun spectra asdetermined for various solar zenith angles by using the Brewerneutral density filter 2: F, a quadrature fit of these measuredvalues; {, quadrature corresponding to measurements with filter 3~right scale!.

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tainty in the extraterrestrial flux determination bythis method varies from ;0.1% at the upper wave-length ~360 nm! to ;0.6% at the lower UV-B wave-length ~300 nm!.

Linear regressions of the logarithm of solar irradi-ance versus the corresponding air mass were calcu-lated for each wavelength in the region from 295 to366 nm, in steps of 0.5 nm. The intercept of eachlinear regression ~for an air mass of 0! equals thesolar irradiance outside the atmosphere at the corre-sponding wavelength. The extraterrestrial solarspectrum was calculated separately for four differentdays, and the repeatability between all spectra wasremarkable. Figure 4 shows the mean spectrumfrom these days together with an envelope corre-sponding to 61s ~sample standard deviation!.

It is obvious that several sources of uncertaintiescan be encountered in the methodology describedabove. A summary of these uncertainties is pre-sented in Table 1, together with some estimates of

Fig. 3. Logarithm of solar irradiance versus air mass ~Langleyplots! for four different wavelengths in the UV region as measuredat Izana, Tenerife, in July 1995. Dashed lines represent the least-squares fits on each set of data. Different symbols correspond todata obtained on four different days.

Fig. 4. Extraterrestrial solar spectrum as determined by apply-ing the Langley extrapolation method to measurements of thedirect-sun irradiance obtained at Izana, Tenerife, by a Brewerspectrophotometer. The thick solid line represents the averagespectrum from four different days, and the envelope corresponds to61s limits.

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Table 1. Error Budget of the Brewer Direct-Sun Spectral Measurements

Uncertainty of reference calibration lamp 62.0%Transfer of calibration uncertainty 62.0%Uncertainty of globalydiffuse spectra ~to derive the direct-Sun calibration! 63.0%Uncertainty from the neutral density filters’ calibration 61.0%Uncertainty of direct-sun spectral measurements 62.0%Uncertainty in extraterrestrial flux determination ~Langley method! 60.6%

Total uncertainty ~rms! 64.7%

other errors that are common in UV spectroradiom-etry.

Finally to verify its accuracy the Brewer-derivedsolar spectrum was compared with the one providedby the ATLAS 3 Space Shuttle mission12 that wasextracted from the anonymous ftp site susim.nrl.navy.mil. To account for the effect of the differentslit functions the ATLAS 3 spectrum was first con-volved by the slit function of the Brewer instrument~0.55 nm FWHM!, which was measured to a fine res-olution in the laboratory.15 Before the slit convolu-tion the wavelength scale of ATLAS 3 was shifted by0.1 nm toward lower wavelengths to account for thedifferences between the vacuum and the air wave-lengths. Finally the irradiance at each wavelengthof the ATLAS 3 solar spectrum was reduced by 3.2%to account for the seasonal variation of the Sun–Earth distance. The ratio of the two spectra in0.5-nm steps for two different days is shown in Fig. 5,from which the following are evident.

~1! The ratio is spectrally flat ~within ;62.0%!,which is an encouraging result if one considers thatthe Brewer spectrum is a product of a statisticalmethod ~Langley plot! applied to measurementstaken during a whole day.

~2! The observed structure, which is repeatable forall the calculated solar spectra, is mostly a result ofthe convolution process that cannot be used to emu-late exactly the slit function effect. Additionally AT-LAS 3 was measured at a spectral bandpass of 0.15nm, which was not taken into account ~e.g., by decon-

Fig. 5. Ratio of the extraterrestrial spectrum on two differentdays in July 1995 as determined by the Brewer measurements atIzana, Tenerife, to the ATLAS 3 solar spectrum. The thick linerepresents the ratio smoothed by averaging at 2.5-nm intervals.

volution! before its convolution with the Brewer slitfunction. Finally small differences in the wave-length settings of the two instruments could explainpart of the observed structure. When one smoothsthe two spectra by averaging the irradiances at2.5-nm intervals ~thick curve in Fig. 5!, the markedstructure or their ratio is appreciably attenuated.

~3! On average the ratio between the Brewer-derived and the ATLAS 3 solar spectra is close tounity, and the observed disagreement of ;3% at thespectral intervals around 310 and 360 nm is withinthe error limit usually present in solar UV spectralmeasurements.21 The main source of this disagree-ment is probably a small systematic error in the ab-solute calibration of the Brewer direct-Sun spectra.

5. Conclusions

From this study it appears that the Brewer ozonespectrophotometer can be used to measure the spec-trum of the direct component of solar ultraviolet ra-diation, with an accuracy that falls within the currentlimits of global UV irradiance measurements. Theabsolute calibration of these measurements can bedone by using recorded spectral measurements of theglobal, diffuse, and direct components simulta-neously, also taking into account the cosine error ofthe input optics ~diffuser! and the spectral response ofthe attenuation filters of the instrument.

The spectral measurements obtained under idealconditions at the high-altitude site of Izana, Tenerife,were used successfully to calculate, using the Langleymethod, the extraterrestrial solar spectrum in theregion from 295 to 365 nm, in steps of 0.5 nm. Thesolar spectra obtained on four different days showremarkable repeatability. The comparison of thesespectra with the ATLAS 3 solar spectrum is satisfac-tory, being wavelength independent with an absolutedifference of ,3%.

The author is indebted to the Environment Divi-sion of the World Meteorological Organisation forproviding partial financial support. The author alsothanks the director and staff of the Izana Observatoryand the National Meteorological Institute of Spain forhosting the campaign by providing all the availableinfrastructure and facilities. Finally the author ac-knowledges the help of Andreas Kazandzidis duringthe campaign.

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