diurnal discrepancies in spectral solar uv radiation measurements

Diurnal discrepancies in spectral solar UVradiation measurements

O. Meinander, S. Kazadzis, M. Blumthaler, L. Ylianttila, B. Johnsen, K. Lakkala,T. Koskela, and W. Josefsson

Unexpected diurnal discrepancies between high-quality spectroradiometers were observed during the2000 Nordic Ozone Group Intercomparison campaign. The spectral ratios of the irradiances showed adiurnal variation of �2–9%. This cannot be explained by the nonideal angular response of the instru-ments’ input optics in one plane (cosine effect). Instead, by using a radiative transfer model, we show thatdifferences in the angular response in four azimuth planes have the potential to bias the measured databy up to 4.4% (azimuth effect). Other relevant factors are also discussed and quantified and are shownto be significant when diurnal changes in radiation are explained by environmental factors, or whenmeasured data are compared with model or satellite data. Again, intercomparison campaigns have thepotential to reveal errors that would otherwise remain undetected. © 2006 Optical Society of America

OCIS codes: 260.7190, 120.6200, 120.5630.

1. Introduction

Solar UV radiation, reaching the Earth, has harm-ful effects on both living organisms and the nonor-ganic environment, and it is important to theatmospheric chemistry and air quality as well. Dur-ing the past decades, anthropogenic emissions intothe atmosphere have not only changed the local airquality, but due to emissions of greenhouse gases,global climate change has developed into one ofthe most debated and challenging environmentalproblems.1–3 Climate change and surface UV radi-

ation interact via changes in the total ozone, clouds,aerosols, and ice and snow cover. Stratosphericozone is a direct contributor to the surface UV ra-diation as the shielding ozone layer absorbs theharmful UV-B radiation.

Because human-produced chemicals are capable ofdestroying the ozone, stratospheric ozone loss hasbeen observed at high latitudes, especially above An-tarctica, since the early 1980s.4,5 This has led to adramatic change in the natural cycle of the UV radi-ation penetrating the atmosphere,6 where the spec-tral irradiance reaching the Earth’s surface E�� isregulated by the product of the extraterrestrial spec-tral irradiance EET�� and a wavelength-dependenteffective transmission function T, which is a functionof the solar zenith angle (SZA) �, the ozone opticaldepth �O3

, the Rayleigh-scattering optical depth �r,the optical depth ��a� and the optical properties ofaerosols (single-scattering albedo � and assymetryfactor g), the optical depth of clouds �c, and reflectionfrom the surface with the albedo A:

E�� EET��T��, �, �O3, �r, �a, �c, �, g, A�. (1)

Changes in the UV radiation due to alteration ofthe values of these parameters in the transmissionfunction T can be studied by in situ measurements, orby using satellite data or radiation model calcula-tions. From these, the measurements on surface UVradiation play a key role and form the basis of our

O. Meinander ([email protected]) and T. Koskela are withthe Finnish Meteorological Institute, Earth Observation, UV Re-search, P.O. Box 503, FIN-00101 Helsinki, Finland. S. Kazadzis iswith the Laboratory of Atmospheric Physics, Aristotle Universityof Thessaloniki, P.O. Box 149, 54124, Thessaloniki, Greece. M.Blumthaler is with the Institut für Mediniche Fysik, Universityof Innsbruck, Müllerstrasse 44, A-4060 Innsbruck, Austria. L.Ylianttila is with the Non-ionizing Radiation Laboratory, Radi-ation and Nuclear Safety Authority, P.O. Box 14, FIN-00881Helsinki, Finland. B. Johnsen is with the Norwegian Radia-tion Protection Authority, Box 55, N-1345 Osterås, Norway. K.Lakkala, Arctic Research Center, Finnish Meteorological Institute,P.O. Box 8178, FIN-96101 Rovaniemi, Finland. W. Josefsson iswith the Swedish Meteorological and Hydrological Institute, Folk-borgsvägen, S-10176 Norrköping, Sweden.

Received 4 November 2005; revised 16 February 2006; accepted16 February 2006; posted 15 March 2006 (Doc. ID 65827).

0003-6935/06/215346-12$15.00/0© 2006 Optical Society of America

5346 APPLIED OPTICS � Vol. 45, No. 21 � 20 July 2006

knowledge, to which the satellite and model data arevalidated and verified.

When we compare model or satellite UV radiationdata with measurements,7,8 or one instrument withanother,9 it is essential to include all the errorsources of the measurements in the comparison. Forexample, if we find a difference between the mea-sured and the modeled UV, we need to know theeffect of the errors and uncertainties in the measure-ments, as well as the assumptions of the model inputparameter values.10 In this paper, we concentrate onthe UV measurements. The need for accurate mea-surements of spectral solar irradiance leads to con-tinuous efforts to reduce errors and uncertaintiesassociated with the spectroradiometers. In spectrora-diometry, the measured spectral irradiance EM�� iscommonly expressed as

EM�� SM��r��

, (2)

where SM�� is the signal of the radiometer whenmeasuring the source, and r�� is the spectral respon-sivity of the instrument. In practice, the measuredEM�� varies from the true spectral irradiance E��due to various errors that may be related to the in-strumental characteristics, like the calibration pro-cedures11 or the operational procedures.12,13

Currently, a lot of scientific work has been devotedto develop methods to correct errors in the measure-ment data. For example, errors due to imperfectangular response,14–18 temperature dependency,19,20

wavelength shifts,21 and spikes22 have been reported.The magnitude of the angular cosine error has beenfound to vary from a few percent to 10–20% depend-ing on the measurement conditions and the char-acteristics of each instrument setup.17 Ideally theangular dependence of the response of a diffuser fol-lows the cosine of the zenith angle and is independentof the azimuthal angle, but in practice all diffusersdeviate from this ideal response in the UV range.14

By the term azimuth error we mean the difference ofthe angular response of the input optics when mea-suring its different azimuth planes. When in situmeasurements are concerned, the effect of the azi-muth error, evident and known from a characteriza-tion in the darkroom, depends on the azimuthalorientation of the detector. The magnitude of thiscomplex azimuth error is then linked to the instru-ment’s angular response in combination with the con-tribution of the direct Sun to the total irradiancemeasured, and thereby also to all the factors thataffect these proportions, such as aerosols and clouds.

The aim of this work was to study and discuss thediurnal discrepancies on UV data, to quantify thepotential factors to explain the observations, and topresent methods to reduce possible uncertainties ofthe measurements. The results are applicable toother types of solar radiometers as well, and themethods to reduce the uncertainties are described ina general way. A more comprehensive evaluation of

the overall errors and uncertainties associated withmeasurements of spectral global UV irradiance hasbeen presented earlier.13 The factors addressed inthat paper include radiometric calibration, cosine er-ror, spectral resolution, wavelength misalignment,stability, noise, stray light, and timing errors. Theseare not included in our analysis unless considered tocause diurnal differences. The factors addressed hereinclude azimuth error, misalignment, intensity hys-teresis, temperature effects of Teflon, and nonperfecttemperature correction. We are unaware of any pre-vious scientific articles on the azimuthal error evi-dent in the in situ UV data. Our work aims to be anadditional tool to the work presented in Ref. 13 toexplain deviations of UV measurements in the field.

2. Solar UV Campaign

The Nordic Ozone Group Intercomparison campaign(NOGIC) took place in Tylösand �56°38�N, 12°43�E�,Sweden, in June 2000. Fourteen instruments fromfour manufacturers represented research groupsfrom three Nordic countries, Finland, Norway andSweden, and from Austria, Netherlands, and Po-land.23

The common measuring sequence consisted of scansinitiated at every full and half-hour at the wave-length of 290 nm with a wavelength step of 0.5 nmsynchronized at 3 intervals up to 400 nm, or the high-est reachable wavelength of each instrument. All theinstruments recorded the solar spectral irradianceusing horizontal receivers (diffusers). A fundamentaldifference among some of the instruments was theirorientation to the azimuth of the Sun. Five of theinstruments were capable of following the actual so-lar azimuth. Here they will be called sun-tracking(ST) instruments. They were different models of theBrewer spectrophotometer, manufactured by Kipp &Zonen B.V.24 The other ones we will call non-sun-tracking (NST) instruments, most of which weremanufactured by Bentham Instruments Ltd.25

For the calculation of spectral ratios of irradiancefrom the NOGIC data, we selected three ST instru-ments and three NST instruments (Table 1). Theselected six instruments had performed well in theNOGIC-2000, producing reliable good-quality data.Under clear conditions the average deviation variedbetween �1.3% and 5.2% from the NOGIC-2000 ref-erence spectrum, depending on the instrument andwavelength.23 In general, these six selected high-quality instruments serve as reference instrumentsfor calibrating lower-quality instruments (e.g., Ref.26). All data were corrected for possible wavelengthshifts using the SHICRIVM algorithm.21 Because ofthe differences in the instruments’ slit functions, thesame algorithm was used to standardize the mea-sured spectra to a fixed slit with 1 nm full width athalf-maximum.

The data comparison consists of synchronized spec-tral ratios calculated as a function of time. Because ofthe other activities of the campaign, not all the in-struments measured every full and half-hour cases;we used all the available cases (the spectra were

20 July 2006 � Vol. 45, No. 21 � APPLIED OPTICS 5347

measured between 6 a.m. and 8 p.m.). Two days withdiffering weather conditions were investigated. Thecampaign’s clearest day (day 162) was an almostcloudless day, i.e., a day during which the wholemorning before noon approximately six to seventenths of the sky was covered by cirrus clouds,whereas in the afternoon only one tenth was coveredby cirrus. We used ancillary broadband data of theFinnish Meteorological Institute (FMI) Solar Light501 erythemal UV detector (SL-501) to study the ef-fect of the cloudiness on the irradiance. The spectraland absolute calibration of this instrument was per-formed during the World Meteorological Organisa-tion and the Finnish Radiation and Nuclear SafetyAuthority (STUK) intercomparison of erythemal de-tectors.27 The second day represents variable cloud-iness conditions (day 166). The spectral UV datawere used as high-spectral-resolution measurements�0.5 nm step�, since integrated values over largerwavelength ranges might block out the sensitivewavelength-dependent information.

3. Observed Discrepancies

The spectral ratios of irradiance of one instrument toanother were expected to include variability due tovarious sources of uncertainties. For example, a dif-ference in the calibration lamp source could lead to

an absolute wavelength-dependent difference whencomparing two instruments. This difference wouldnot be SZA dependent. In addition, differences in thecosine responses of the instruments could lead toSZA-dependent differences, but these differenceswere expected to be symmetric according to the noon.Within the half-hour measurement time step, theSun was at its highest at 11.00 UTC (local noon).However, unknown diurnal discrepancies of �2–9%could be observed in the instrument-to-instrumentratios of the ST�NST from the clearest day 162. Ingeneral, the ratio ST�NST declined during day 162from morning at a SZA of 60° toward evening at aSZA of 60° by up to 8%. More specifically, the ratiosfirst dropped by �4–9% and in the afternoon slightlyclimbed up by �1–3%, a behavior that could possiblyindicate cosine angle dependency. The difference in-creased as a function of wavelength (Fig. 1). In thedata, the morning time 6.00 UTC corresponds to theSZA of 64.0°, and the time 16.00 UTC corresponds tothe SZA of 61.3°.

The variation in the irradiance for day 162 accord-ing to ancillary data of the FMI’s SL-501 filter radi-ometer is shown in Fig. 2. On the basis of theseancillary measurements, throughout the day only mi-nor variations in the irradiance due to cloudinesscould be observed. No obvious changes between irra-diance during noon versus afternoon took place. Yet

Fig. 1. Ratio ST�NST declining from the morning toward theevening at 310 nm (circles), 325 nm (squares), and 340 nm (trian-gles). NOGIC-2000 data during mostly clear skies, Julian day 162in 2000 at Tylösand.

Table 1. Instrumentation of the NOGIC-2000 Used in this Study

Institute InstrumentSun

Tracking Diffuser

Finnish Meteorological Institute (FMI), Jokioinen Observatory Brewer MKIII 107 double monochromator Yes TeflonArctic Research Centre (ARC) of the FMI Brewer MKII 037 single monochromator Yes TeflonSwedish Meteorological and Hydrological Institute (SMHI) Brewer MKIII 128 double monochromator Yes TeflonUniversity of Innsbruck Austria (ATI) Bentham DTM300 No J1002a

Norwegian Radiation Protection Authority (NRPA) Bentham DM150 NoFinnish Radiation and Nuclear Safety Authority (STUK) Bentham DM150 No J1002a

aShaped cosine head diffuser.

Fig. 2. Erythemal irradiance (W�m2) during measurement days162 (mostly clear skies) and 166 (variable cloudiness), according tothe FMI SL-501 filter radiometer. The values are presented as 1min averages.


we are not interested in the asymmetry in the irra-diance, but in knowing why various instrumentswould measure the prevailing irradiance in a differ-ent way.

Contrary to ratios of ST�NST, the ratios of one STdivided by data of another ST (ST1�ST2 and ST1�ST3) were approximately symmetrical to the noon(Fig. 3). The ratio of one of the ST instruments to eachof the three NST instruments (ST1�NSTx) in Fig. 4shows the differences between the various NST in-struments, one with less diurnal variation than theothers. At SZAs larger than 60° the measured ir-radiances became smaller. These larger-angle in-strument-to-instrument ratios contain cases whenthe radiation is nearly totally diffuse, which mini-mizes the angle-related differences. When the data ofthe morning hours are compared with the eveninghours, it is obvious that the values of the ratios are atdifferent levels. This gives evidence that the diurnal

discrepancy is not entirely connected to the contribu-tion of direct or diffuse radiation but exists also athigher SZAs ��60°�, when most of the radiation isdiffuse. To have some empirical comparison of mate-rial from the same campaign, we will next study datafrom day 166 with variable cloudiness.

During day 166 with variable cloudiness, a diurnaldiscrepancy in the ratios of the ST�NST data was alsodetectable (Fig. 5). A graph showing the variation inirradiance for day 166 according to the ancillarybroadband SL-501 radiometer is shown in Fig. 2. Themagnitude of the variability of the ratios increasedcompared with day 162 (Figs. 1 and 4). The ratios ofthe ST instruments were symmetrical to the noonsimilar to the clear-sky day 162.

The most interesting feature in the data of day 166occurred with the STUK NST instrument, as the ra-tio ST�NST was nonsymmetrical according to thenoon on day 162 (Fig. 4), but on day 166 the sameratio was symmetrical. Between the two measure-ment days, the diffuser had been turned by 180°. Wehave no additional proof that there would be a con-nection between the turning of the diffuser and thediurnal discrepancy. Using the notes made duringthe campaign, it was identified that the diffuser dur-ing day 162 versus 166 had been changed from themaximum response plane to the east during day 162to the minimum response in that direction during therest of the campaign (high values in the morning ofday 162). From the point of view of the azimuth effect,we would have expected exactly the opposite. As aresult, we now know that the diurnal discrepancycould have been even larger on day 162 if the diffuserhad been at 180°. The maximum difference betweendifferent azimuthal orientations was later measuredto be below 5%. The alignment of the diffuser’s bubblewas also checked; the possible alignment error is be-low 0.4°.

To understand the reasons for the observed diurnaldiscrepancies, the effect of the azimuth error of thecosine response on these data was studied next. Fol-lowing that, the other possible factors that cause theobserved differences will be discussed and quantified.

Fig. 3. Ratios ST�ST at 310 (circles) and 325 (squares) nm withalmost no azimuthal differences in the morning and afternoonlevels during the mostly clear-skies Julian day 162 at Tylösand in2000. ST1�ST2 is shown by open symbols and ST1�ST3 by closedsymbols.

Fig. 4. Data of one of the ST instruments divided by the data ofthe NST instruments. ST1�NST1 is shown by circles, ST1�NST2by squares, and ST1�NST3 by triangles. Day 162; open symbolsused for 310 nm and closed symbols for 325 nm.

Fig. 5. Data of ST�NST for day 166 at 310 nm (circles) and 325(squares) nm.


4. Discussion on Sources of the Discrepancies

In this section error sources causing diurnal dis-crepancies in UV measurements are discussed.These include errors that arise from instrumentcharacteristics and also from data postprocessingmethods. The factors addressed will also be quantifiedin the instrument-type specific uncertainty table(Table 2) using a statistical approach to uncertaintyaccording to the International Organization for Stan-dardization Guide to the Expression of Uncertainty inMeasurement.28 We are able to provide each instru-ment with a single value of the uncertainty that can beassociated with the measurement result at the givenwavelength and SZA. After the uncertainties of theindividual instruments were quantified, the uncertain-ties for all the instruments were gathered into onetable specifying the instrument type (NST or ST).

A. Effect of the Asymmetry of the Angular Response

If we suspect that one source of data deviations isthe effect of the azimuth error of the angular re-sponse, we expect the error to become larger withwavelength, as in our data. This is due to the factthat the contribution of the direct radiation is pro-portional to the wavelength at a constant zenithangle; the smaller the wavelength, the larger therelative contribution of the diffuse radiation to theirradiance.

On the other hand, we assume that the Sun trackerdata are not biased by the azimuth error, as the sameside of the diffuser is always toward the Sun. There-fore the azimuth error of a ST instrument evidentunder laboratory conditions has only minor effects onthe solar measurement data. This fact was supported

by the data because the ratios ST�ST were symmet-rical to noon (Fig. 3).

Furthermore, if we calculate a theoretical diurnalratio for an instrument with �3% azimuth error be-tween morning and evening to an instrument with noazimuth error, we find compatibility in the shape ofthe theoretical curves (Fig. 6) and the experimentaldata (Fig. 4).

B. Evaluation of the Magnitude of the Azimuth Effect

For the evaluation of the magnitude of the complexwavelength-dependent azimuth effect, in one casestudy we utilized the four-plane angular response ofModel J1002 input optics, manufactured by Ing. Dr.Schreder-CMS and used in the Bentham DTM300spectroradiometer of the University of Innsbruck,Austria (ATI). The response data were combined withdirect and global irradiances estimated by the UVSPEC

radiative transfer (RT) model, version libradtran 1.0-beta2-disort2.29 The model input was specified to rep-resent the conditions corresponding to NOGIC-2000.Aerosol input parameter values � �1.4� and �0.1� inthe Ångström formula �� of the aerosol opticaldepth were determined by tuning the model results tomatch with direct spectral irradiance measurements.

The characterization of the ATI instrument indi-cates only a small asymmetry of the cosine response.The azimuth error is minimal ��1%� at small zenithangles up to 30°. The error increases slightly as afunction of the zenith angle. At the zenith angle of 60°it is approximately �2%. This would lead to an azi-muthal difference of up to 1% at 350 nm. Hence, thiswould explain only �15% of the differences in theobservations. The case shows that the azimuth errorof the cosine response was not the major contributorof the asymmetry in these data.

We also calculated more examples using variouspossible theoretical azimuthal errors, using back-ground aerosols and clear-sky cases with wavelengthsup to 400 nm. For this purpose, we also utilized the

Table 2. Uncertainty Budgets for Diurnal Discrepancies under ClearSkies and with Realistic Aerosols at 310 and 350 nm at a SZA of

60° with Uncertainty Ranges for the ST Brewer and NSTBentham Instruments Used in this Studya

UncertaintyComponent u(xi)

Source ofUncertainty

STui(y) (%)

NSTui(y) (%)

Az (wl, Idir) Azimuth error at310 nm

0 0.25–0.8

Azimuth error at350 nm

0 0.45–0.9

CCF (Idir�Idiff) Cosine correction 0.5 0–1.1L (wl, Idir) Leveling 0.5–1 0.5H (t, T, ?) Intensity hysteresis 0 0–0.6TTef Teflon temperature 1.2 0–1.2T Temperature effects 0.5–1 0uc(y) Combined uncertainty

at 310 nm1.5–1.7 1.3–1.5

Combined uncertaintyat 350 nm

1.5–1.7 1.4–1.6

U Expanded uncertaintyat 310 nm

3.0–3.4 2.6–3.0

Expanded uncertaintyat 350 nm

3.0–3.4 2.8–3.2

aThe uncertainties differed due to wavelength only in the case ofazimuth error. The expanded uncertainty U is comparable to un-certainties at the 2� level.

Fig. 6. Theoretically calculated diurnal ratio of an instrumentwith �3% azimuthal error between morning (negative SZA values)and evening (positive SZA values) to an instrument with no azi-muth error calculated for 310 nm (circles), 350 nm (squares), and400 nm (triangles).


results of an old diffuser of the NST double-monochromator spectroradiometer Bentham DM150,participating in the testing phase (year 2002) of theQuality Assurance of Spectral Ultraviolet Measure-ments in Europe (QASUME) project.30,31 The angularcharacterization results showed a difference betweenthe east plane and the west plane at a SZA of 60°, adifference of 5% (Fig. 7, left side). This would lead toan underestimation of the afternoon irradiance directcomponent.

Using calculations presented in Ref. 32, for exam-ple, for the contributions of diffuse radiation to thespectral irradiance at the surface under cloud- andaerosol-free conditions, the total irradiance would be5% in the case of asymmetry, underestimated by�4% at 400 nm and 2% at 310 nm. The character-ization of only one plane for the cosine correctionwould also affect the correction of the diffuse irradi-ance. For correcting measurements where the Sun iscovered by clouds, the difference could be �3%.

For the purposes of the project, the angular re-sponse of the diffuser of the instrument was rechar-acterized after technical corrections on its shape,resulting in the response presented in Fig. 7 (rightside), where maximum azimuthal deviations cannotexceed the 1% level. Such small systematical errorsare hard to detect in spectral UV measurements.

During the European traveling schedule of the in-strument, the angular responses of various fieldinstruments were characterized.33 In some cases dif-ferences of up to 10% were found for measurementsfor two different planes. This is the case for theBrewer 107 spectroradiometer. The results of its an-gular characterization are shown in Fig. 8. There, thecharacterizations in 1996 and in 2003 showed differ-ences of more than 10% between two planes at highincident angles. These deviations could lead to theazimuthal dependence on the incident irradianceeven if they were ST instruments. In this case thecharacterization only made in one plane could lead to

proportional azimuthal errors in cases of cloudy at-mospheric conditions where the diffuse irradiancedominates. This would be a result of an erroneouscosine correction, depending on the angular responseof the plane that was characterized.

C. Quantitative Examples on Azimuthal Asymmetry

The quantitative effect of the azimuthal asymmetryon measured sky irradiance is now demonstratedwith the following examples. First, we assume a dif-ference of �2% or �5% in the angular response be-tween the two planes of a spectroradiometer. Suchvalues are probably realistic, as shown above. Sec-ond, we use RT model calculations for day 162 at aSZA of 60° for cloud-free cases with a surface albedoof 0.05. In the first case we assume default aerosolsaccording to Ref. 34, i.e., rural-type aerosol in theboundary layer, background aerosol above 2 km,spring–summer conditions, and a visibility of 50 km.We also use two subcases for the total ozone: thevalues 300 and 360 Dobson Units (DU) that representthe minimum and maximum daily mean ozone valuesmeasured during the campaign. In the second casewe use realistic ozone (312 DU) and aerosols (� �1.4, � 0.1) according to NOGIC-2000.

Using these values we find that the difference inthe spectral sky irradiance at a SZA of 60° would varyfrom 0.4 to 4.4% (Table 2). The effect of aerosols onthe resulting azimuth error was observed to be largerthan the effect of ozone: At 310 nm and a SZA of59.9°, the proportion of direct radiation of the total,Idir�Itot, was 11.3% when using realistic aerosols,compared with 16.4% for background aerosols. Thiswould also cause the resulting azimuthal errors todiffer from each other by 0.3–0.7 percentage units(case 1 and case 2, Table 3). The precise value of theSZA did not have a large effect on the calculatedazimuth error. At 310 nm the portion Idir�Itot was11.3% for a SZA of 59.9° and 10.0% for a SZA of61.3°. At 350 nm, the corresponding portions were

Fig. 7. Cosine characterization results for the Bentham B5503.The graph on the left shows the azimuth problem, and that on theright shows the corrected results. Solid curves represent the meanof the four planes measured and the dashed curves represent thetwo measured planes with the maximum deviation.

Fig. 8. Cosine characterization results for the Brewer 107 fromtwo cosine characterizations, 1996 and 2003.


22.3% and 20.7%, and at 400 nm it was 36.8% and35.2%, respectively. This SZA difference would intro-duce a difference in the azimuth error of 0.1–0.2 per-centage units only. The effect of total ozone wasobserved to be minimal: (i) at 310 nm and a SZA of59.9°, Idir�Itot was 16.4% for a total ozone of 300 DUand 16.5% for 360 DU; (ii) at 350 and 400 nm, ozonedid not have any effect, as ozone does not absorb UVradiation at these wavelengths; and (iii) the calcu-lated theoretical azimuthal errors were exactly thesame for 300 and 360 DU within the accuracy of onedecimal. Our calculations (Table 3) demonstrate thatthe azimuth effect in general has the potential tocause a bias with a magnitude of several percentage,depending on various atmospheric parameters andon instrument characteristics.

D. Effect of the Cosine Correction and Leveling

As we can consider the irradiance data of the STinstrument as a product of the calibrated signal andthe cosine correction, we investigated the possibleerror introduced by the applied cosine correction.

Cirrus clouds may have affected the distribution ofthe direct or diffuse radiation during the morninghours of day 162. This could cause the cosine correc-tion of the ST Brewers to be different in the morningcompared with the afternoon. However, the value ofthe cosine correction factor (CCF) differed by no morethan 1% at a SZA 60° for all wavelengths. On thisbasis, we can assume that the cosine correction (orthe cirrus clouds) cannot introduce an asymmetryerror greater than 1% in the morning. From this itwould follow that the diurnal discrepancies due toother factors would be up to 5% at 340 nm.

Diurnal discrepancies could also basically be due toimperfect cosine correction of the ST Brewer data.These instruments have typically a large cosine er-ror. The Brewer cosine correction could be either toosmall be or overcorrective.

During the QASUME project, the measured mag-nitude varied between 5 and 11% with the worstcases for the single Brewers.35 During a clear day theimperfections should be similar to the correspondingsolar angles in the morning and in the evening with-out a diurnal change, assuming constant conditions:cloudless or full cloudiness, homogeneous clouds, and

no significant change of aerosols due to transporta-tion by wind.

In the cosine correction, the sky radiance is as-sumed isotropical. In reality it is distributed noniso-tropically both under clear and cloudy skies, whichhas an effect on the cosine correction algorithm. Forcloudy cases, it would be possible to include thenonisotropical distribution of the diffuse radiationinto the cosine correction method, and to prove anunderestimation by �1–3% (Ref. 15) in the cosinecorrection due to the use of isotropy assumption. Theuncertainty in cosine corrections, especially undervariable cloudiness, would be its own subject for anuncertainty study. However, for the problem we dis-cuss here, only the symmetry between morning andevening is important and this depends only on theconditions of cloudiness.

On a cloudless day, the diurnal effects due to cosinecorrections with ST Brewers are assumed to be min-imal, and the uncertainty is evaluated to be less than1% (Table 2). Because of the better cosine response ofthe NST Benthams, cosine corrections are not com-monly used with these instruments, or if applied, theCCF is very small. For example, in these data, thecosine correction was used for the STUK instrument,and the correction factor varied between 1 and 1.02during the intercomparison. In the case of the dif-fuser of the ATI NST Bentham, the cosine responsemeasurements were carried out at every 60° of azi-muth resulting in six planes, and the largest differ-ence for any orientation results in the final cosineerror of this diffuser of less than 2%, and no cosinecorrection of the measurements during NOGIC-2000were made. The possible diurnal effects in the case ofNST Benthams are considered to be taken totally intoaccount with the uncertainty values of the azimutheffect.

We need to define the additional uncertainty com-ponents for the diurnal discrepancies based on thepractical experience during NOGIC-2000, i.e., type Bevaluation according to Ref. 28. The effects of shad-ows and mirror reflections during the campaign areconsidered negligible and not worthy of further eval-uation. More likely, contributors to diurnal discrep-ancies could be the misalignment effects of theinstruments. Horizontal alignment has a wavelength

Table 3. Example of the Theoretical Azimuth Error for an Instrument with �2% and �5% Azimuth Response Differences between Planes Using theUVSPEC Model at a SZA of 59.9° under Clear Skies for Four Casesa

Wavelength(nm)

Azimuth Error(%)

Portion of DirectRadiation (%)

Case 1

Portion of DirectRadiation (%)

Case 2

TheoreticalAzimuth Error (%)

Case 1

TheoreticalAzimuth Error (%)

Case 2

310 4 16.4 11.3 0.7 0.4310 10 16.4 11.3 1.6 1.1350 4 29.4 22.3 1.2 0.9350 10 29.4 22.3 2.9 2.2400 4 44.3 36.8 1.8 1.5400 10 44.3 36.8 4.4 3.7

aCase 1, background aerosols; case 2, realistic aerosols and total ozone according to NOGIC-2000.


dependency as the calculation of the effect is based onthe direct or diffuse radiation ratio that varies asa function of wavelength. According to Ref. 36, 1° ofmisalignment means �1% error to the measurement.Although all the instruments were carefully leveled,by experience we know it is hard to level the diffusersat an accuracy greater than 1°–2° for Brewers and 1°for Benthams (Table 2).

E. Intensity Hysteresis

Another potentially relevant factor capable of caus-ing differences between morning and afternoon insome cases is the effect of intensity hysteresis of thephotomultiplier tube (PMT). The intensity hysteresiseffect exists more or less for all photomultipliers. Itcan be considered as a sensitivity change or a memoryeffect of the PMT, where the spectral response be-comes significantly higher in the afternoon than inthe morning due to the increasing load to the PMT.As a result, the PMT does not perform stable duringthe day.

In the case of the Norwegian Radiation ProtectionAuthority (NRPA) NST Bentham, the effect wasclearly evident for the original PMT from 1996, butnot for the PMT applied in the NOGIC-2000. In 1996,for sunny days, lamp measurements with the originalPMT showed that the response increased by up to 6%from morning to afternoon and evening, even if theinstrument was temperature stabilized and operat-ing indoors with only the fiber outdoor and with theuse of 10% and 1% transmittance filters. DuringNOGIC-2000 the PMT of the NRPA was found stablein the experiments done. The STUK instrument wasassumed stable with no more than 1% uncertainty byusing 10% and 1% transmission filters in the spec-troradiometer. With one NST a special batterybackup pack was used to keep the high-voltage sup-ply on for up to five days when moving the instru-ment. For any instrument that has an internal lampoperating between some global scans, these datacould state whether the response changes in a diur-nal way. The intensity hysteresis effect should notaffect the Brewer instruments, as they count the in-dividual photon counts whereas the Bentham instru-ments measure the PMT current. Hence none of theinstruments in our study is considered to suffer fromthe intensity hysteresis (Table 2).F. Temperature Effects and Applied Corrections

Temperature effects related to the postprocessingmethods are possible sources for diurnal discrepan-cies. Temperature effects cause an error that containswavelength dependency. According to Ref. 20, thetemperature error in Brewers can be large or negli-gible, and it is instrument related. The authors re-ported that in the part of the spectrum above 325 nm,the temperature dependence is generally indepen-dent of wavelength. Below 325 nm, the temperatureeffects vary as a function of wavelength and are gen-erally largest at the shortest wavelengths. A decreaseof the responsivity of the Brewer MKIV instrumentwith increasing temperature was reported in Ref. 19.

These authors also found the temperature effect tobe greater at low wavelengths. Over the 290–325nm spectral interval, the inaccuracy was up to 8% insummer.

The temperature dependency of two of the threeST Brewer instruments used in our study has beenmeasured. The temperature measured during the tem-perature characterization of the Brewer is the temper-ature of the PMT. Solar data from the Brewer 037 wasnot temperature corrected during the campaign. Inthis case the error in the Brewer 037 data caused by a10 °C change in the temperature is �1%. During day162 the temperature of the Brewer 037 varied from24° to 43 °C. The largest difference from the calibra-tion temperature �34 °C� was at 6 UTC, being 10 °C.Then the difference between the uncorrected andthe temperature-corrected irradiances was 1.2% at305 nm and 0.7% at 318 nm. The other, double-monochromator Brewer 107 has only a small temper-ature error (maximum of 22% depending on thewavelength), and the data used here have been tem-perature corrected. The third ST instrument from the(Swedish Meteorological Hydrological Institute) re-ported here is temperature stabilized.

Finally, we assume that the temperature error ofthese instruments is small and there is no clear tem-perature effect in the ST Brewer data. However, wecannot be sure that direct sunshine into the spec-trometer during a campaign would not cause a dif-ferent temperature dependency than that measuredwhile the temperature effect is determined under lab-oratory conditions. In the case of the outdoor mea-surements, the gradient of the temperature alsoaffects the results, but for that purpose further in-vestigation is needed. Therefore, temperature effectscannot be totally excluded as a contributor to thedetected azimuthal errors, and an uncertainty valueof 1% is assumed for the ST Brewers when temper-ature correction has been applied and 2% is assumedwhen the temperature is not corrected. Contrary tothe ST Brewer spectrometers, the NST Bentham in-struments are temperature stabilized and tempera-ture effects are considered to be avoided.

G. Temperature of Teflon

Related to the temperature effect, Ref. 37 reports onthe effect of the changes in Teflon properties at 19 °C.Teflon is used in the diffusers of the entrance optics ofthe spectroradiometers. The effect of changing tem-peratures on the Teflon would be �2–3% in the ob-served direction with an increasing signal toward theafternoon (colder than 15 °C in the morning, warmerthan 19 °C in the afternoon). As the exact tempera-ture of Teflon is unknown, quantitative correction isimpossible. All three NST instruments had similarentrance optics, but one of them (NRPA) had atemperature-stabilized and heated diffuser and thetemperature was assumed to have been above 19 °Cat all times. The magnitude of the effect on the Brew-ers is unknown. Brewers have different diffusers, andbased on the measurements by one of the authors(Ylianttila), different diffusers have different changes


at 19 °C. The effect has only a small wavelength de-pendency.

5. Estimation of the Uncertainties

Uncertainty analysis tables for each individual instru-ment were made on the basis of the instrument char-acterization, as presented earlier. The uncertaintiesdiffered due to wavelength only in the case of azimutherror. In cases where no specific a priori knowledge ofthe possible values of the uncertainty xi, within theinterval a� to a existed, it was assumed that it wasequally probable for xi to lie anywhere within it, anda rectangular a priori probability distribution wasused. These cases were intensity hysteresis andTeflon temperature. A normal distribution was usedwhen enough information was available to choosethis distribution. After that, all these data were com-bined to represent uncertainty for the two groups ofthe NST and ST instruments used in this study. Thefinal probability distribution is therefore a symmetricdistribution, a combination of normal and rectangu-lar. The values of the combined uncertainty uc�y�represent the values first calculated for the individ-ual instruments using the equation28

uc�y� � ��ui2. (3)

By multiplying this result with the coverage factork � 2, the expanded uncertainty U (Table 2) isobtained, providing a level of confidence of approxi-mately 95%. According to the estimation of uncer-tainty presented here, the uncertainty of the NSTBenthams and ST Brewers was dominated by theuncertainty due to temperature effects of Teflon, forwhich a rectangular probability was assumed. Forthe ST Brewers, the uncertainty was greatly influ-enced by misalignment effects, for which a normaldistribution was used. The results of the uncertaintyanalysis indicate that at a SZA of 60°, diurnal dis-crepancy with a magnitude of approximately 3% canbe expected and explained. Hence, in some cases thedetected diurnal discrepancy could fully be explained,but not in all cases.

6. Conclusions and Summary

According to our solar data of the NOGIC-2000, di-urnal differences between instruments with the mag-nitude of 2–9% may currently be related to the UVmeasurement data. The deviation increased as afunction of wavelength. Here, data from six instru-ments were used from two days. Measurements onmany more days with constant weather conditionswould have been necessary for a real quantitativeanalysis.

The detected errors are close to the limit of the in-strument differences. Large uncertainties have beenhistorically part of the outdoor UV measurements.Looking back to the first intercomparisons in the mid-1990s, discrepancies of 50% between instrumentswere common. Currently, the discrepancies are com-monly well below 10%. This paper, which is part of a

process to make improvements, points at some errorsources that should be noted and that hopefully canbe removed in the future. One should also rememberthat precision of individual instruments, used for cli-mate monitoring, can be kept much smaller. How-ever, the goal of this paper was to point out smaller,and not-well defined, errors that come from eitherhardware or postcorrecting methods. According toRef. 38, a useful but ambitious goal is to attempt todetect a change in spectral UV irradiance resultingfrom a 1% change in total ozone column. On the otherhand, the lowest calibration uncertainty is reportedto be currently limited to a few percent (e.g., �5%). Todetect a 1% change in ozone, the wavelength accuracymust be better than �0.05 nm, the detection thresh-old must be �106 Wm2 nm1, and the accuracy ofthe absolute calibration must be at least �5%, valuesthat the instruments we used in our analysis fulfill.These values form the basis of the accuracy of thespecification of type S-2 instruments (highest accu-racy required).

By combining RT model calculations with labora-tory results on the angular response of a ST instru-ment used in the intercomparison, we studied thepotential of the azimuth error of the cosine responseto bias these measurement data. Using realistic aero-sol input in the RT model, it was possible to cause abias of 1%. However, the same analysis for a NSTinstrument gave an error up to 4.4% at high wave-lengths, and a SZA of 60° that were the result ofdeviations up to �5% when its angular response wascompared in two planes.

The UV measurement results are a complicatedcombination of many different effects, each one dif-ferent for each instrument and not necessarily con-stant with time. Other possible factors that causediurnal deviations include both measurement andpostprocessing methods, for example, an error eitherleft or introduced due to a nonperfect correction pro-cedure, such as cosine or temperature correction. Al-ternatively, for any other reason, an instrument maynot be capable of measuring the signal equally in themorning and in the afternoon, e.g., due to improperleveling, intensity hysteresis, temperature effects ofTeflon, etc. According to our experience and quanti-tative analysis on the uncertainty, we could explaindiurnal differences with a magnitude of �3%.

To minimize the effect of the possible azimuthalasymmetry, the cosine correction of the UV measure-ments should be applied using the average of themeasurements of the four planes to correct the diffuseirradiance component. For the direct component,the plane that is toward the Sun should be used forthe cosine correction to obtain the best possible cor-rection. In practice, this may not be possible. For STinstruments, the same side of the diffuser is alwaystoward the Sun, and the azimuth error evident underlaboratory conditions has only a minor effect on thesolar measurements. In the case of the azimuth error,periodical measurements on the four-plane cosine re-sponse are needed. When leveling the head for out-


door measurements, a tilt by 0.5° along the plane ofmaximum nonsymmetry is also possible to reduce theazimuthal variation so it is insignificant. Accordingto our experience, a reduction of other asymmetricaluncertainties can be obtained by the following: (i) inthe case of the intensity hysteresis of the NSTBenthams, by use of 10% and 1% transmission filtersin the spectroradiometer or (ii) with a special batterybackup pack to keep the high-voltage supply on whenmoving the instrument; (iii) for an instrument thathas an internal lamp operated between some globalscans, these data could state whether the responsechanges in a diurnal way; (iv) minimizing the tem-perature effects using a big box to stabilize the wholeinstrument at a stable temperature under all condi-tions.

Finally, a comparison of model calculations withspectral UV measurements of the NOGIC-2000 isshown in Fig. 9. If we had assumed corrected mea-surements for known errors, we would have assumedthat the instrument-to-model difference is due tomodel input parameters. Assuming that the effect ofcloudiness is insignificant (day 162 was almost cloudfree), the most likely explanations for the local timevariation of the ratios could have been the modelinput parameters related to aerosol properties. Theeffect of changes in the ozone would not affect thehigher wavelengths. From the NOGIC-2000 data setwe know that the diurnal discrepancies were alsoevident in ratios of instrument to instrument. There-fore, in reality, the diurnal variation in the model-to-

measurement ratios cannot in this case be explainedby the effect of aerosols. On the contrary, the effect isdue to diurnal discrepancies in the measurementdata. Connected to aerosols, in Ref. 8 similar azi-muthal (morning versus afternoon) deviations of5–10% in ratios of modeled UV spectra to measuredspectra were reported. The authors explained this tobe due to aerosol-related model input parametersand reported the ratios to be fairly constant withwavelength. Contrary to their findings, we foundwavelength-dependent diurnal discrepancies (i.e.,azimuthal deviations); it is most likely that in theStandardization of Ultraviolet Spectroradiometry inPreparation of a European Network (SUSPEN) cam-paign data set used in Ref. 8, the aerosol propertiesalso changed.

In addition to factors quantified in this work, dif-ferent calibration traces (with no diurnal effect) cancause a wavelength-dependent difference betweenthe measurements.39 Future work is needed from thenational laboratories to solve this problem. Mean-while, the trace of each instrument should be takeninto account in intercomparison campaigns and inusing any UV data.

Currently, UV radiation measurement data are be-ing gathered into databases, such as, e.g., the WorldOzone and Ultraviolet Radiation Data Center inToronto, Canada,40 and the European UV Databaselocated at the FMI in Helsinki, Finland. In such da-tabases, the quality of the data can be flagged. How-ever, automatic quality control systems are mostuseful to detect only large irradiance scale errors, asthe radiation due to changing cloudiness and otherenvironmental conditions is variable. This leaves theresponsibility to detect and identify asymmetrical er-ror sources to the researcher using the data.

Instrument makers, in collaboration with scientificgroups, need to look at device variation and makechanges that will reduce errors to make the UV mea-surements sensitive to small variations that mightaccumulate over time. For example, according to Ref.38, azimuthal dependencies of the entrance opticsshould be avoided as much as possible because re-sulting errors are difficult to correct. The reduction oferrors is possible, as evidenced here with one exam-ple in Fig. 7. In addition to instrument makers, sci-entific groups may introduce several means to reduceerrors during measurements and data processing, aspresented here.

We thank all those who have enabled the valuableintercomparison data sets to exist, i.e., all the NOGIC-2000 participants; Harry Slaper for correcting all datafor wavelength shifts; the E.C. QASUME project; theAcademy of Finland (FARPOCC project); as well asAntti Arola and Jussi Kaurola, and other collegues notnamed here, for valuable conversations.

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Fig. 9. Spectral ratios of measured (ST�NST) and modeled-to-measured UV on day 162 at the corresponding SZAs near 60°,including morning and afternoon spectral ratios. 6.30 UTC corre-sponds to SZA of 59.9°, 7 UTC to 55.8°, 15.30 UTC to 57.2°, and 16UTC to 61.3°.


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