GAW Report No Instruments to Measure Solar Ultraviolet Radiation Part 3: Multi-channel filter instruments

Transcription

1 GAW Report No. 190 Instruments to Measure Solar Ultravolet Radaton Part 3: Mult-channel flter nstruments For more nformaton, please contact: World Meteorologcal Organzaton Research Department Atmospherc Research and Envronment Branch 7 bs, avenue de la Pax P.O. Box 2300 CH 1211 Geneva 2 Swtzerland Tel.: +41 (0) Fax: +41 (0) E-mal: AREP-MAIL@wmo.nt Webste: WMO/TD - No. 1537

2 World Meteorologcal Organzaton, 2010 The rght of publcaton n prnt, electronc and any other form and n any language s reserved by WMO. Short extracts from WMO publcatons may be reproduced wthout authorzaton, provded that the complete source s clearly ndcated. Edtoral correspondence and requests to publsh, reproduce or translate these publcaton n part or n whole should be addressed to: Charperson, Publcatons Board World Meteorologcal Organzaton (WMO) 7 bs, avenue de la Pax Tel.: +41 (0) P.O. Box 2300 Fax: +41 (0) CH-1211 Geneva 2, Swtzerland E-mal: Publcatons@wmo.nt NOTE The desgnatons employed n WMO publcatons and the presentaton of materal n ths publcaton do not mply the expresson of any opnon whatsoever on the part of the Secretarat of WMO concernng the legal status of any country, terrtory, cty or area, or of ts authortes, or concernng the delmtaton of ts fronters or boundares. Opnons expressed n WMO publcatons are those of the authors and do not necessarly reflect those of WMO. The menton of specfc companes or products does not mply that they are endorsed or recommended by WMO n preference to others of a smlar nature whch are not mentoned or advertsed. Ths document (or report) s not an offcal publcaton of WMO and has not been subjected to ts standard edtoral procedures. The vews expressed heren do not necessarly have the endorsement of the Organzaton.

3 WORLD METEOROLOGICAL ORGANIZATION GLOBAL ATMOSPHERE WATCH No. 190 Instruments to Measure Solar Ultravolet Radaton Part 3: Mult-channel flter nstruments Lead Author: G. Seckmeyer Co-authors: A. Bas, G. Bernhard, M. Blumthaler, B. Johnsen, K. Lantz and R. McKenze Contrbutors: S. Daz, P. Dsterhoft, L. Jalkanen, A. Kazantzds, P. Kedron, B. Petkov, C. Snclar and C. Wlson WMO/TD-No November 2010

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5 TABLE OF CONTENTS 1. INTRODUCTION OBJECTIVES SPECIFICATIONS CALIBRATION Calbraton procedures based on Approach Comparson wth a spectroradometer (spectral response functons NOT requred) Comparson wth a spectroradometer (spectral response functons requred) Transfer from standard of spectral rradance Emprcal calbraton approaches Comparson wth a reference nstrument Calbraton procedures based on Approach CHARACTERIZATION OF MULTI-CHANNEL FILTER INSTRUMENTS Characterzaton of spectral response functons Angular response Stablty tests Comparson wth spectroradometers Comparson wth a reference mult-flter radometer Calbratons wth standards of spectral rradance Repeated spectral response measurements Langley Method Vsble and nfrared leakage test Cosne error correcton APPLICATIONS Bologcally effectve rradance Calculaton of bologcally effectve rradance va regresson analyss Method suggested by Dahlback [1996] Calculaton of bologcally effectve rradance from reconstructed spectra Calculaton of hgh-resoluton spectra Method suggested by Dahlback [1996] and Booth [1997] Method suggested by Mn and Harrson [1998] Method suggested by Fuenzalda [1998] Method suggested by Thorseth and Kjeldstad [1999], and Thorseth et al., [2000] Spectral reconstructon wth neural networks algorthm Calculaton of total column ozone Method suggested by Dahlback [1996] Method suggested by Slusser [1999] Calculatons of cloud optcal depth [Dahlback, 1996] Qualty control of spectroradometers Langley Method...20 Glossary...22 References Annex A - Centre Wavelengths of some Avalable Mult-Flter Instruments...29 Annex B - References of Freely Avalable Radatve Transfer Programmes...30 Annex C - Calculatons n Support of Specfcatons Provded n Secton Annex D - Maxmum Irradance at the Earth s Surface...37

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7 1. INTRODUCTION Ultravolet radaton from the sun causes a consderable global dsease burden ncludng acute and chronc health effects on the skn, eye and mmune system. Worldwde up to 60,000 deaths a year are estmated to be caused by ultravolet radaton, most of whch are due to malgnant melanoma (Lucas et.al., 2008). Much of the UV-related llness and death can be avoded through a seres of smple preventon measures. On the other hand, some UV s essental for the producton of vtamn D n people. Emergng evdence suggests an assocaton between vtamn D levels as an ndcator of health rsk [WHO, 2008] relatng to some cancers, cardovascular dsease and multple scleross among others, along wth the establshed lnk wth musculo-skeletal health. Ths gude s part three of a seres of documents dedcated to nstruments for the measurement of solar ultravolet radaton. These documents have been drawn up by the WMO Scentfc Advsory Group on UV Montorng and ts UV Instrumentaton Subgroup. The am of the seres s to defne nstrument specfcatons and gudelnes for nstrument characterzaton that are needed for relable UV measurements. Part 1 of ths seres [Seckmeyer et al., 2001] descrbes scannng spectroradometers that are able to separate the radaton n small wavelength bands wth a typcal resoluton of 1 nm or less. Broadband nstruments to measure erythemally-weghted ( sunburnng ) UV radaton are descrbed n Part 2 [Seckmeyer et al., 2005]. The mult-channel flter radometers (MCFRs) descrbed here make measurements n several dscrete wavelength bands wth bandwdths of typcally 1 to 10 nm fwhm (full wdth at half maxmum). These nstruments can be used to reconstruct spectra of solar global rradance, to derve specfc products such as erythemally weghted rradance, or to determne total column ozone. Compared to spectroradometers and broadband nstruments, nterpretaton of data of these nstruments s more complex and the separaton of nstrument characterstcs and data products s not straghtforward. There s a dverse range of nstruments that fall wthn ths category. Ther specfcatons must therefore be more flexble, whle ther detaled characterzaton becomes more mportant. At a mnmum, the nstruments must be capable of measurng global rradances n at least two channels. The fore-optc generally conssts of a dffuser, the angular response of whch should deally be proportonal to the cosne of the zenth angle. The wavelength selecton s typcally acheved by narrow to moderate band nterference flters, and the sgnal s detected wth a photodode or a phototube (wthout dynodes for multplcaton). Data acquston and loggng are automated, and software s sometmes provded by the manufacturer to produce standardzed products. Typcally, the number of channels s larger than two, and some examples of these nstruments nclude shadowbands whch enable the near-smultaneous measurement of dffuse and drect rradances, n addton to global rradance. Applcatons of such nstruments are qute varable. For comparson wth other nstruments, the data processng usually requres some sort of normalzaton, convoluton, or deconvoluton. Examples of standard data products are dscussed n Secton 6. The ntended audence for ths document ncludes scentsts, nstrument manufacturers, governmental organzatons, and fundng agences dealng wth UV montorng and research. The document should serve as a gude and s based on the current experence and scentfc knowledge about the measurement of UV radaton wth mult-channel flter radometers. Advantages of mult-channel flter nstruments These nstruments allow the determnaton of one or more of the followng, normally by emprcal or modelled relatonshps: Bologcally effectve doses for a varety of acton spectra wthout the requrement of supplementary ozone data. Total column ozone amount. Cloud attenuaton, especally at hgh temporal resoluton. Reconstructon of solar spectra at arbtrary wavelength resoluton. 1

8 Compared to spectroradometers (Part 1 of ths seres), they typcally have a hgher temporal resoluton, a lower prce, and are much smpler to operate. Compared to sngle-channel broadband nstruments or broadband nstruments measurng erythemally weghted solar radaton (Part 2 of ths seres) they allow separaton of the nfluence of dfferent atmospherc parameters affectng UV rradance (e.g., total column ozone, cloud attenuaton, aerosol effects, etc.). Furthermore, they are not restrcted to only one acton spectrum. If equpped wth a solar tracker or a shadowband they have the capablty to determne drect solar spectral rradance, both at the centre wavelength of the ndvdual channels or at all wavelengths n the UV by means of spectral reconstructon. In ths case, the Langley method may also be used as a calbraton tool, to determne aerosol optcal depths, or the extraterrestral solar rradance [Slusser et al., 2000]. Dsadvantages of mult-channel flter nstruments Compared to spectroradometers, whch delver spectra, the sgnal output of mult-flter nstruments usually requres post processng and the development of algorthms to gan meanngful results or measurement quanttes. Although the stablty of these nstruments may match or exceed the stablty of spectroradometers, the absolute calbraton s usually acheved by calbraton aganst the latter. When calbrated aganst spectroradometers, the uncertanty of mult-channel nstruments s usually hgher than that of spectroradometers due to the addtonal transfer uncertanty. Mult-flter nstruments can also be calbrated wth Standards of Spectral Irradance rather than by comparson wth a spectroradometer under the Sun. However, ths calbraton method requres laboratory characterzaton work, ncludng the accurate characterzaton of the flter s response functons and angular response functons. Ths s partcularly the case for channels n the UV-B, due to the great dfference of lamp and the sun spectra. To obtan a small calbraton uncertanty, accurate determnaton of the spectral responsvty s requred. Ths n turn requres a tunable, small-bandwdth monochromatc lght source of suffcent radatve power. Flters that are used for wavelength separaton may be subject to drfts, both absolute and spectral, whch are dffcult to detect durng operaton. 2. OBJECTIVES Mult-channel flter nstruments can be employed for a varety of scentfc applcatons. In a strct sense these nstruments are capable of measurng only the global rradance n the UV at several wavelength channels weghted wth the response functons of the respectve channel. However, the major advantage and present applcaton for these nstruments are the derved data products. In ths respect these nstruments dffer from the other nstruments descrbed n the seres (Part 1: Spectroradometers; Part 2: Broadband Instruments Measurng Erythemal Irradance; and Part 4: Array Spectroradometers). The objectves for employng these nstruments may be summarzed as follows: 1. To derve data products such as erythemal rradance wth hgh temporal resoluton. 2. To provde nformaton on the varablty of solar UV rradance partcularly due to clouds. 3. To contrbute to determnng geographc dfferences n UV and understandng ther causes. 4. To derve spectral global rradance n the UV at the nstruments nomnal wavelengths. These data can be used to calculate spectral rradance at other wavelengths. 5. To help n qualty control (QC) of spectroradometrc UV measurements. 6. To support ground truthng of satellte estmates of UV. 7. To measure global response-weghted-rradance n the UV, whch s the solar spectral rradance weghted by the spectral response functon of each channel. As wth spectroradometers, multflter-nstruments may also be used to derve total column ozone and, n combnaton wth a solar tracker or a shadowband, aerosol optcal depth at varous wavelengths. 2

9 3. SPECIFICATIONS Quantty Qualty Cosne error (a) < ±5% for ncdence angles <60 (b) < ±5% to ntegrated sotropc radance (c) < 3% azmuthal error at 60 ncdence angle Mnmum spectral range nm Wavelength stablty < 0.03 nm for centre wavelength Wavelength accuracy Not applcable (see remark) Bandwdth (fwhm) < 10 nm Bandwdth stablty < 0.04 * fwhm Stray lght ncludng senstvty to < 1 % contrbuton to the sgnal of wavelengths outsde 2.5 fwhm vsble and IR radaton for SZA less than 70 Stablty n tme on tme scales up to a year Sgnal change Currently n use: better than 5% Desred: 2% Mnmum number of channels At least one channel wth centre wavelength < 310 nm and at least one wth centre wavelength > 330 nm Maxmum rradance Sgnal of the Instruments must not saturate at radaton levels encountered on the Earth s surface. Detecton threshold SNR = 3 for rradance at SZA=80 and total ozone column of 300 DU. Instrument temperature Montored and suffcently stable to mantan overall nstrument stablty Response tme < 1 s Multplexng tme < 1 s Accuracy of tme Better than ±10 s Samplng frequency < 1 mnute Levellng < 0.2 Calbraton uncertanty < 10 % (unless lmted by detecton threshold) Remarks on specfcatons: Cosne error Smaller cosne errors would be desrable. Defntons of cosne and azmuthal error are gven n the Glossary. Mnmum spectral range The mnmum spectral range should be large enough to allow calculaton of bologcally effectve rradance (e.g., erythemal rradance). Instrument channels should deally cover the complete UV range as most bologcal systems respond to wavelengths n both the UV-B and UV-A regons. Wavelength stablty In prncple, wavelength stablty has to be wthn the gven range for all observng condtons. Ths specfcaton s hard to verfy n the feld, but can be approxmately verfed n laboratory experments by characterzng the spectral response functon as descrbed n Secton The specfcaton of 0.03 nm was chosen, because calculatons show that a shft of ths magntude leads to a change of up to 2% of the sgnal for a channel of 10 nm bandwdth centred at 305 nm for solar zenth angles between 0-80 degrees and ozone amount less than 500 DU. Calculatons for smaller bandwdths do not change the concluson apprecably. For more detals see Annex C.1. Wavelength accuracy Ths specfcaton s not applcable for multflter nstruments because for meetng the objectves t s suffcent that the nstrument s flter functons are characterzed accurately. For further detals see gudelnes for nstrument characterzaton n Secton 5. 3

10 Bandwdth The bandwdth of flters used n currently avalable nstruments ranges between 1 and 10 nm. Instruments wth small bandwdth typcally requre smaller correctons to convert ther raw data to spectral rradance (Secton 4.1). For example, calculatons presented n Annex C.3 show that transfer of calbratons from a lamp standard wll result n an error of 200% n solar measurements at 305 nm for a bandwdth of 10 nm fwhm, SZA less than 80º, and total ozone between 250 and 450 DU, f no correctons are appled. The correspondng number for a bandwdth of 1.0 nm fwhm s 0.5 %. On the other hand a small bandwdth reduces the nstrument s senstvty, whch makes t dffcult to detect low-ntensty radaton n the UV-B. The bandwdth s therefore a compromse between the competng requrements of beng able to calbrate nstruments accurately and to detect solar rradance n the UV-B. Smulatons provded n Annexes C.1 and C.2 show that accurate characterzatons of the nstrument channels spectral response functons (ncludng the accurate determnaton of the channel s centre wavelength) are more mportant than the bandwdth or the shape of response functons. These calculatons ndcate that data products such as total ozone and bologcally effectve rradance can be calculated wth smlar accuracy from raw data of nstruments wth 1-nm or 10-nm wde channels. Bandwdth stablty Calculatons presented n Annex C.2 show that small changes n the bandwdth can have a sgnfcant effect on measurements of mult-flter nstruments n the ozone cut-off regon (wavelengths below 315 nm). For an nstrument wth a bandwdth of nomnally 10 nm fwhm, a 0.2 nm broadenng n bandwdth wll result n up to 3% ncrease n the sgnal at 305 nm for SZA between 0 and 80, and ozone amount between 250 and 450 DU. For narrower flters proportonal changes n bandwdth are less of a concern. Stray lght, ncludng senstvty to vsble and IR radaton Multflter nstruments use nterference flters to realze ther spectral response functons. These flters may have secondary peaks n the vsble and nfrared, whch may be outsde the spectral range of the apparatus for measurng response functons. These secondary peaks can sgnfcantly contrbute to the nstrument s sgnal. Ths senstvty should be checked wth cut-off flters, (e.g., wth WG or GG Schott longpass flters) usng both Sun and calbraton lamp as lght source. A descrpton of ths technque can be found n Secton 5.4. Mnmum number and wavelength of channels By defnton the nstrument must have at least two channels. Normally these nclude one n the UV-B that s senstve to total column ozone, and one n the UV-A. Dependng on the applcaton, e.g., dervng of bologcally effectve rradance or aerosol parameters, addtonal channels are usually necessary. Centre wavelengths of exstng nstruments or other wavelengths relevant for specfc applcatons can be found n Annex A. Maxmum rradance A complaton of the maxmum UV rradance to be expected at the Earth s surface s provded n Annex D. Detecton threshold A low detecton threshold s partcularly necessary at locatons where rradance s low, e.g. at hgh-lattude stes or n wnter. Instrument temperature Instrument temperature should be montored and stablzed. Operatng condtons logged should nclude the nternal nstrument temperature (specfed by the manufacturer) and the effect of heatng by solar radaton, whch may warm the nstrument by a consderable margn above the ambent temperature. Temperature stablzaton s requred for accurate measurements snce both nterference flters and photododes are temperature senstve. If the correlaton between temperature and senstvty s well establshed the nstrument may be used wthout stablzaton by applyng a temperature correcton to the data. Temperature stablzaton s preferable, snce temperature effects on flter-functons are dffcult to correct. 4

11 Response tme A tme constant of one second s suffcent for most applcatons and s easly achevable wth the exstng nstrumentaton. In some specalzed applcatons, e.g., statstcs for clear sky determnaton or the nvestgaton of transent cloud effects, shorter tme constants may be desrable. Accuracy n tme Tme errors of 10 s can lead to measurable dfferences as SZA and cloud condtons change. Tme-keepng of better than 10 s s requred f an nstrument s to be compared to other nstruments, n partcular durng cloudy stuatons. Uncertantes of less than one second are readly achevable wth current technology (e.g., Internet tme server, GPS). Levellng Levellng to better than ±0.2 can be acheved wth a smple bubble level. Care should be taken that the reference plane used for levellng s parallel to the nstrument s collector. Samplng frequency Less than 1 mnute for most applcatons. However, for specfc cloud studes, a much hgher frequency (e.g., 1 s) may be necessary. Calbraton uncertanty The calbraton uncertanty ncludes all uncertantes assocated wth the rradance calbraton procedures. Qualty Assurance and Qualty Control (QA/QC) To obtan data of hgh qualty t s not suffcent that nstruments meet the basc specfcatons dscussed above. In addton, measurements of ancllary data for nterpretng the measurements should be avalable, nstruments have to be well mantaned, and a Qualty Assurance and Qualty Control (QA/QC) plan has to be followed [Webb et al., 1998; 2003]. Recommended ancllary data and QA/QC procedures are compled below. Recommended ancllary data Total ozone column, from ndependent nstruments or satelltes to establsh correcton factors (Secton 4) or to check for bas n ozone retrevals of mult-channel nstrument data (Secton 6.3). Data from ndependent radometers such as pyranometers, broadband UV sensors or spectroradometers to help to valdate the nstrument s stablty n tme (Secton 5.3). Meteorologcal data. Mantenance and QA/QC 1. Daly: Checkng of nput optcs (rradance collector), and cleanng f necessary. Determnaton of offset (Most nstruments provde an automated offsetdetermnaton durng the dark hours. Offset checks may have to be done manually n polar regons durng perods wth 24 hours of sunlght). 2. Weekly: Checkng of the effectveness of temperature stablzaton, tme-keepng, levellng, and data loggng. 3. At least once per year (every sx months s desrable): Checkng of nstrument s stablty by comparson to a reference nstrument, lamp, or spectroradometer. Checkng of the correct operaton and calbraton of electronc supportng devces (data loggers, A/D boards, sgnal amplfers, cables, computers etc.). 5

12 Checkng of dark stablty durng the year. Instablty may suggest temperature dependence of the electroncs or other problems. 4. At deployment and f qualty checks above ndcate a problem: Verfcaton of the spectral and angular response. Checkng of the accuracy of the nstrument s level ndcator. (The optcal plane of an nstrument s sometmes not consstent wth the reference plane that s used for checkng whether the nstrument s level). 4. CALIBRATION There are several fundamentally dfferent approaches to calbratng mult-channel flter nstruments, some resultng n dfferent radometrc quanttes. Each approach can also be mplemented n dfferent ways. Several methods and ther advantages and dsadvantages are dscussed below. Some mplementatons requre that the spectral response functons of all channels are accurately known. Other methods are based on a comparson wth a spectroradometer under varyng atmospherc condtons. In ths case, measurements of spectral response functons may not be necessary. Approach 3 s applcable only to nstruments equpped wth shadowbands. Approach 1 Spectral Irradance The objectve of Approach 1 s to establsh a calbraton functon for each channel of the nstrument whch, when appled to the raw sgnal, returns spectral rradance at the nomnal wavelength of the channel. For example, a calbraton functon may be defned such that the calbrated measurement of each channel approxmates spectral rradance, measured by a spectroradometer wth a 1-nm bandpass at the channel s nomnal wavelength. (Ths example assumes that spectroradometer and flter nstruments are exposed to the same radaton feld.) The advantage of Approach 1 s that the measurement spectral rradance s a common radometrc quantty. The dsadvantage s that calbraton functons usually depend on the radaton source measured. Ths s partcularly a problem for measurements n the UV-B part of the solar spectrum due to the rapd change of the Sun s spectrum wth wavelength n ths regon. In ths case, calbraton functons wll depend on solar zenth angle and other atmospherc parameters affectng the shape of the solar spectrum, such as total column ozone. Approach 2 Response-weghted rradance The objectve of Approach 2 s to apply a calbraton factor to each channel of the nstrument such that the calbrated measurement of a gven channel s response-weghted rradance. Ths quantty s defned as the wavelength ntegral of the product of spectral rradance and spectral response functon of the channel (see Glossary). For example, f measurements from a spectroradometer are weghted wth the spectral response functons of a collocated flter nstrument, the resultng response-weghted-rradance wll be dentcal wth the calbrated measurement of the flter nstrument. The advantage of ths calbraton approach s that the calbraton functon smplfes to a factor, whch s ndependent of the radaton source. When measurng solar radaton, ths factor does not depend on solar zenth angle and other atmospherc parameters. If the calbraton factor was establshed wth a standard lamp, t can be appled to solar measurements wthout correctons. The dsadvantage of the approach s that the quantty measured response-weghted rradance s nstrument dependent. Comparng the results of dfferent nstruments s therefore dffcult. However, standardzed data products such as erythemal rradance can be calculated wth good accuracy from calbrated values wthout the need of consderng the atmospherc condtons durng the tme of the measurement (Secton 6.1). 6

13 Approach 3 Langley Method Ths approach s based on the Langley method and requres that nstruments are equpped wth shadowbands (Secton 6.6). From consecutve measurements of global and dffuse rradance (the latter determned by blockng the Sun wth the shadowband), drect rradance s calculated. Measurements of drect rradance are performed at dfferent armasses and extrapolated to armass zero to derve the nstrument s sgnal that would be expected outsde the Earth s atmosphere. The sgnals of the dfferent channels at armass zero are then compared wth a reference extraterrestral spectrum, whch s weghted wth the response functon of each channel to establsh calbraton factors as n approach 2. These factors are fnally appled to measurements of global rradance. The method has been descrbed by Slusser et al. [2000], and s not dscussed n more detal here. Implementatons of Approach 1 and 2 are descrbed n the followng sectons. 4.1 Calbraton procedures based on Approach Comparson wth a spectroradometer (spectral response functons NOT requred) For ths method, the mult-channel nstrument s operated next to a hgh-resoluton (1) spectroradometer, both of whch are exposed to sunlght. A calbraton factor C s establshed by dvdng the net sgnal measured by channel of the mult-channel radometer, wth spectral solar rradance E S ( λ ), measured by the spectroradometer at nomnal wavelength λ : C (1) = E V S S, ( λ ) ( V = S,, G E S V ( λ ) S,,0 ). Here V S,, G s the lght sgnal (e.g., measured n volts) of channel when exposed to the radaton of the Sun, V S,, 0 s the dark sgnal, obtaned by coverng the collector, and V S, s the net sgnal, calculated as the dfference of V S,, G and V S,, 0. Measurements of the spectroradometer have to be corrected for all systematc errors, such as the (1) cosne error, pror to the comparson. Once C has been derved, solar spectral rradance at the nomnal wavelength λ of the mult-channel radometer s calculated wth: V E S ( λ ) = C S, (1) Due to the msmatch of the spectral response functons and the slt functon of the (1) spectroradometer, C wll usually depend on solar zenth angle (SZA), total ozone (O 3 ), and (1) other atmospherc parameters x. For ths reason, C s a functon dependng on these parameters: C (1) = C (1) ( SZA, O3, x) The argument ( SZA, O 3, x) s omtted n the followng for better readablty. The comparson of the mult-channel nstrument and the spectroradometer has to be performed 7

14 over a perod suffcent to nclude a large set of envronmental condtons. Snce the calbraton functon can be establshed only for condtons that occurred durng the comparson perod, deployment of the flter nstrument at a locaton wth dfferent condtons may be problematc. (1) In practce, the dependence of C on SZA, O 3, and other factors can be descrbed wth a lookup table or a numerc parameterzaton. For example, Díaz et al. [2005] used a mult-regressve model to parameterze the relatonshp between V S,, E S ( λ ), SZA and O 3 : ln( ES ( λ )) = a1 ln( VS, ) + a2o3 + a3 f (90 SZA) + b, o o where f ( 90 SZA) s a functon of SZA, and a 1, a 2, a 3, and b are coeffcents determned by regresson. Although parameterzatons such as the one suggested by Díaz et al. [2005] may delver suffcently accurate results, great care must be appled when extrapolatng parameterzatons to condtons for whch they were not desgned. For example, a parameterzaton derved from measurements at a hgh-lattude locaton may lead to systematc errors f the nstrument s deployed at a low-lattude ste Comparson wth a spectroradometer (spectral response functons requred) As an alternatve to the method descrbed n Secton 4.1.1, a calbraton functon may be establshed from a sngle reference solar spectrum that s measured by a mult-channel nstrument and a spectroradometer for well known SZA and atmospherc condtons. The (2) (2) alternatve calbraton functon s denoted C and ncludes a correcton term K, whch depends on SZA and O 3, and possbly other atmospherc parameters x. (2) VR, (2) C = K = C E ( λ ) R ( R) K (2). Here E R ( λ ) s solar spectral rradance of the reference spectrum, V R, s the net sgnal of (R) channel when measurng ths spectrum, and C s the calbraton factor for the reference spectrum, calculated as rato of V R, and E R ( λ ). Solar spectral rradance E S ( λ ) for arbtrary condtons s then calculated wth: V V E S ( λ ) = = C C S, ( R) (2) K S, (2) Note that ths formula does not account for the cosne error of the nstrument or any other systematc errors. In practce, t s usually necessary to correct for these errors (Secton 5.5), leadng to a further modfcaton of the measurement equaton: VS, E S ( λ ) = X ( 1, 2,..., ) (2) p p pn, C where X ( p1, p2,..., pn) s a correcton term dependng on n parameters such as SZA, O 3, and cloud condton. The correcton functon (2) K s defned by: 8

15 K (2) = E E S R ( λ' ) R ( λ' ) dλ' E ( λ' ) R ( λ' ) dλ' E R S ( λ ) ( λ ) where R (λ) s the spectral response functon of channel. Systems for measurng the spectral (2) response of flter radometers are descrbed n Secton 5.1. Determnaton of K requres the knowledge of the solar spectrum E S ( λ ), the quantty to be measured by the nstrument. An exact (2) (2) determnaton of K s therefore not possble. However, K can be estmated from model calculaton usng SZA, O 3, and x as nput parameters. For most applcatons knowledge of SZA and O 3 s suffcent. O 3 can be taken from satellte measurements. A complaton of radatve (2) transfer models that can be used for the calculaton of K s provded n Annex B Transfer from standard of spectral rradance In ths mplementaton, the radometer s set up n front of a standard lamp. The spectral rradance produced by the lamp at the place of the nstrument s collector s denoted E L ( λ ) ; the (L) assocated net sgnal measured by channel of the radometer s V L,. The calbraton factor C s defned as the rato of V L, and spectral rradance at the nomnal wavelength λ of channel : ( L) VL, C = EL ( λ ) Wth ths defnton, solar spectral rradance at wavelength λ may be approxmated wth: E S S, λ ), ( L) C ( V where V S, s agan the net sgnal of channel when measurng the Sun. Solar spectral rradance calculated wth ths approach wll have a large systematc error n the ozone cut-off regon of the solar spectrum. The error ncreases wth decreasng wavelength and ncreasng bandwdth of the radometer s channel. It s caused by the dfference of lamp and solar spectra at short wavelengths, where the second dervatves of both sources devate consderably (Fgure 1). For a hypothetcal nstrument wth a centre wavelength of 305 nm, the error can be as large as 200% for an nstrument wth a bandwdth of 10-nm for SZA 0 80 and total ozone DU. It s less than 1% for an nstrument wth a bandwdth of 1-nm. 9

16 Spectral rradance n mw/(m² nm) Solar spectral rradance at SZA=30 and total ozone of 300 DU Solar spectral rradance at SZA=60 and total ozone of 300 DU Spectral rradance of 1000W FEL standard lamp n 50 cm dstance Typcal spectral response functons of moderate-bandwdth radometer Wavelength n nm Spectral response functon Fgure 1 - Comparson of solar spectral rradance at SZA=30 and 60, the spectrum of a 1000-Watt FEL calbraton standard at 50 cm dstance, and typcal response functons of a moderate-bandwdth mult-channel flter radometer wth nomnal wavelengths of 305, 320, 340, and 380 nm and a bandwdth of approxmately 10 nm To correct for the error, a correcton functon a modfed calbraton functon (3) VL, (3) C = K = C E ( λ ) L ( L) K (3) C : (3) The correct expresson for calculatng solar spectral rradance s then: V V E S ( λ ) = = C C S, ( L) (3) K S, (3) (3) K has to be appled for each channel, resultng n Smlar to the procedure descrbed n Secton 4.1.2, t s usually necessary to correct for addtonal systematc errors, such as the cosne error, leadng to the followng modfcaton of the measurement equaton: VS, E S ( λ ) = X ( 1, 2,..., ) (3) p p pn, C where X ( p1, p2,..., pn) s agan a correcton term dependng on n parameters such as SZA, O 3, and cloud condton. (3) The correcton functon K of channel s defned as: K (3) = E E S L ( λ' ) R ( λ' ) dλ' E ( λ' ) R ( λ' ) dλ' E L S ( λ ) ( λ ) 10

17 (3) Here R ( λ ) s the spectral response of channel. K s a functon of SZA, O 3, and other parameters affectng the transfer or radaton through the atmosphere, and can be estmated from (2) modelled spectra n a smlar manner as K. For most applcatons, knowledge of SZA and O 3 s suffcent. For lamp-based calbratons, R ( λ ) must be known very accurately. Accordng to Annex C.1, the wavelength applcable to a gven spectral responsvty needs to be known to an accuracy of 0.03 nm to gve an error n the solar rradance of less than 2% for a flter wth centre wavelength at 305 nm. The error n the sgnal s essentally ndependent of the fwhm of the radometer for bandwdths from 1 10 nm and the error s smaller wth ncreasng centre wavelength Emprcal calbraton approaches The dependence of the calbraton functon on SZA and O 3 can partly be accounted for by ncludng measurements of all channels n the calbraton [Díaz et al., 2005]. For example, solar spectral rradance at wavelength λ may be expressed as a lnear combnaton of net sgnals measured by all channels: E S (λ ) = cj VS, j j The coeffcents c j are determned va mult-lnear regresson of solar spectral rradance E S ( λ ), measured wth a spectroradometer under varyng condtons, aganst net sgnalsv S, j, measured wth the mult-channel radometer at channels j. To further mnmze errors caused by the SZAdependence of the calbraton, Díaz et al. [2005] suggested the followng modfcatons to the regresson equaton: o For SZA < 40 : ES ( λ ) = [ cj VS, j ] + c f (90 SZA) j For SZA > 40 : o ln( ES ( λ )) = [ cj ln( VS, j )] + c f (90 SZA) + d j o where f ( 90 SZA) s a polynomal ft-functon, and c and d are ft-coeffcents. Emprcal approaches should be valdated over a large range of condtons wth dfferng SZA, O 3, and other parameters x, and should not be appled to condtons outsde ths range Comparson wth a reference nstrument In ths case, the nstrument to be calbrated should operate alongsde a reference multchannel nstrument ( R ) wth well-establshed calbraton factors c (R), whch are generally dependent on SZA and O 3. If the spectral response functons of the two nstruments are vrtually (T) dentcal for all channels, the calbraton factor c of the nstrument under test ( T ) s: (T) (T) V (R) c = c (R) V By ths comparson, also c (T) wll also become dependent on SZA and O 3. The nstruments should run sde-by-sde for several days coverng a full range of SZA, and deally a wde range of ozone columns. If the spectral response functons of the two nstruments are dfferent, a correcton 11

18 factor must be appled, whch can be calculated n a smlar way as descrbed n Secton Calbraton procedures based on Approach 2 * In ths approach, the calbraton factorc s the rato of the net sgnal of channel, V, to rradance E (λ), weghted wth the relatve spectral response R (λ) of channel : C * = V E( λ) R ( λ) dλ The radaton source producng E (λ) can ether be a standard lamp or the Sun. In the former case, E (λ) s known from the standard s certfcate; n the latter case, E(λ) s typcally measured * by a spectroradometer deployed next to the flter nstrument to be calbrated. In theory, C does not depend on the lght source beng measured, whch can be the Sun or any artfcal lght source. * * In practce, C s subject to uncertanty f R (λ) s not accurately known. The uncertanty of C as a functon of varous characterstcs of R (λ) are dscussed n Annex C. The radatve quantty beng measured by flter nstruments calbrated wth ths approach s response-weghted rradance E wth V E = C * Ths quantty s dfferent for every nstrument. However, standardzed data products such as erythemal rradance can be calculated from E wth hgh accuracy. Ths s dscussed n Secton CHARACTERIZATION OF MULTI-CHANNEL FILTER INSTRUMENTS Proper characterzatons of angular and spectral response of mult-channel flter nstruments are crucal for obtanng accurate measurements. Informaton from the characterzaton s used to convert raw sgnals of the nstruments to physcal quanttes such as spectral rradance. For approprate qualty control and assurance of flter nstruments, characterzaton of the spectral response and angular response should be undertaken at regular ntervals. Characterzng nstruments requres well-desgned systems. It s suggested that qualfed laboratores carry out spectral response and cosne response characterzatons. In addton, the stablty and calbraton of any nstrument needs to be montored over tme. The followng gves a general descrpton of typcal characterzaton systems and procedures. 5.1 Characterzaton of spectral response functons The spectral senstvty of each channel should be measured wth a dynamc range and wavelength range large enough to detect small flter leakages outsde the man flter bandpass. Generc response functons should not be used because t has been shown that the spectral transmsson of flters may vary sgnfcantly, even for flters of the same batch [Bernhard et.al., 2005]. The centre (nomnal) wavelength of each channel should be calculated from the measurement response functons, e.g. (1) Determne centrod wavelength of the response functon, (2) Multply the response functon wth a reference spectrum and determne ts centrod wavelength. A system for characterzng the spectral response of mult-channel radometers requres a spectral lght source. Ths can be provded ether by a tunable laser or by an optcally dspersng nstrument, such as a monochromator. The followng descrpton wll concentrate on the latter. Typcally, the output from a hgh-ntensty lght source such as a xenon arc lamp s maged onto the 12

19 entrance slt of a monochromator. The monochromator scans across the desred wavelength range (e.g., nm) n wavelength ncrements small enough to resolve the spectral response. The output of the monochromator s maged onto one of two separate detecton systems, the multchannel flter radometer (MCFR) under test and a reference detector wth known spectral response. A measurement of the MCFR and the reference detector output sgnals are taken at each wavelength step. The spectral response R ( λ ) of the MCFR s calculated from measurements of the MCFR and the reference detector: ( V R ( λ ) = DUT,, L ( V R, L ( λ ) V ( λ ) V DUT R,0,,0( λ )) S ( λ )) R ( λ ) Here VDUT,, L( λ ) s the lght sgnal of channel of the MCFR, VDUT,,0 ( λ ) s the correspondng dark sgnal, VR, L( λ ) and VR, 0 ( λ ) are the lght and dark sgnal of the reference detector, respectvely, and S R ( λ ) s the spectral response of the reference detector. Instrument specfc spectral response functons may be avalable from the manufacturer, but can change wth tme. The reference detector SRF should deally be confrmed by a standards laboratory, and must be checked perodcally to assure stablty. Measurng R ( λ ) accurately n absolute terms s dffcult and t s therefore conventonal to normalze R ( λ ) to one at ts maxmum value. The monochromator s bandwdth should deally be more than an order of magntude smaller than the bandwdth of the MCFR. Its wavelength accuracy should be better than 0.03 nm for flters centred at 305 nm to acheve solar rradance errors less than 2% (see Annex C.1), n partcular f the MCFR s calbrated aganst a lamp (Secton 4.1.3) Ths crteron s less strct when measurng spectral response at longer wavelengths. The stray-lght rejecton of the monochromator should be suffcent to ensure spectral purty at each measurement step. Ths typcally requres the use of a double-monochromator. The lght output from the monochromator should be suffcent to gve a dynamc range of at least three orders of magntude n the measured spectral response of the MCFR. The spectral response measurement system should have an optmum balance between acceptable stray-lght rejecton, band-pass sze, wavelength step sze and adequate sgnal throughput to obtan the spectral response curve of the MCFR wth the desred dynamc range. It s often not possble to set the monochromator to a bandwdth that s one order of magntude smaller than that of the MCFR s channels, and stll have suffcent sgnal over the desred dynamc range. It may therefore be necessary to deconvolve the spectral response functon wth the monochromator s slt functon. A sutable technque has been suggested by Bernhard et al. [2005]. As an alternatve, the core part of the response functons may be scanned wth a small bandwdth and the wngs wth a large bandwdth to have suffcent sgnal. Measurements wth the two bandwdth settngs can then be sttched together. Ths technque has been successfully used by Johnsen et al. [2008b]. Spectral response functons should deally be characterzed once per year. Ths s often not possble n practce. At a mnmum, nstruments should be retested f comparsons wth other nstruments ndcate potental changes n the detector s spectral response. The followng publcatons provde descrptons of systems for the characterzaton of spectral response functons: Bernhard et al. [2005]; Bolsée et al. [2000]; d Sarra et al. [2002]; Hülsen and Gröbner [2007]; Johnsen et al. [2008b]; and Lantz et al. [2005]. These papers should be consulted f a user of mult-channel nstruments chooses to perform such characterzatons hmself. As these measurements are rather demandng, t s advsed these characterzatons are performed by establshed laboratores such as the WMO/GAW regonal calbraton centers. 13

20 5.2 Angular response As wth any other nstrument measurng solar UV rradance the angular response of a mult-channel flter nstrument should be characterzed for zenth angles between -90 and 90, and several azmuth angles n ncrements suffcent to obtan any structure that may be present (e.g., 1 zenth angle ncrement, 45 azmuth angle ncrement). The system for measurng the angular response typcally conssts of a lght source, a computer-controlled rotary table, and algnment fxtures. The MCFR s mounted to the rotary table such that ts axs of rotaton touches the reference plane of the MCFR s dffuser. For performng the angular response measurement, the rotary table s turned from -90 to 90 whle the sgnals of the MCFR detectors are recorded. The measurement s repeated after turnng the nstrument n ts holder to a dfferent azmuth angle. Cosne and azmuthal errors can be calculated from these measurements usng the defntons gven n the Glossary. The lght source may ether be a hgh-ntensty ncandescent lamp, such as a 1000-W FEL lamp, or a dscharge lamp, such as a Xenon lamp. If a convex mrror or a lens s used to collmate the lamp s output, t has to be ensured that the beam s homogeneous and overflls the dffuser of the MCFR. The measurement system should have optcally flat black surroundngs to lmt scattered lght. A large baffle should be nstalled between lamp and MCFR such that scattered off-axs radaton from the lamp cannot reach the MCFR. For accurate measurements t s crtcal that the MCFR s algned correctly. Ths can be acheved by means of an algnment laser mounted behnd the lght source and drected toward the rotary stage. Frst, lamp and MCFR are algned such that the laser goes through the centre of the lamp and the centre of MCFR s dffuser. Second, a mrror s placed n front of the MCFR such that t s parallel to the MCFR s reference surface used for levellng the nstrument n the feld. The algnment of the MCFR s then adjusted such that the laser beam s back-reflected to the laser. Wth ths method the 0 poston of the rotary stage can also be determned accurately. If the measurements ndcate that the nstrument has a sgnfcant azmuthal error, t s mportant that the orentaton of the nstrument can be marked and transferred to the feld such that azmuthal asymmetres n solar measurements can be nterpreted and corrected. The followng publcatons provde descrptons of systems for the angular response characterzatons: Harrson et al. [1994a]; Hülsen and Gröbner [2007]; Johnsen et al. [2008b]. 5.3 Stablty tests Several methods can be used to check the temporal stablty of the calbraton of mult-flter nstruments. These nclude: Comparson wth solar measurements performed by well-mantaned spectroradometers. Comparson wth other mult-flter nstruments. Va the Langley Method of analyss and extrapolaton to extraterrestral solar rradance. A combnaton of the methods lsted above Comparson wth spectroradometers For the mplementaton of ths method, the mult-flter radometer to be tested and the spectroradometer measure sde by sde for a perod of deally one week or longer. The measurements of the spectroradometer are weghted wth the response functons of the mult-flter radometer as descrbed n Secton 4.2, and compare wth the net sgnal of the mult-flter radometer. The rato of the two measurements should deally not depend on tme. If the drft s beyond an acceptable lmt, the mult-flter radometer should be recalbrated. The method s routnely appled to measurements performed by the NSF UV Montorng Network [Bernhard et al., 2008]. 14

21 5.3.2 Comparson wth a reference mult-flter radometer As n the prevous secton, the mult-flter radometer to be tested and the reference radometer measure sde by sde for a suffcently long perod. Measurements of the two nstruments are compared as descrbed n Secton Both nstruments should deally have very smlar spectral response functons. If ths s not the case, the tme seres analyss should be restrcted to a subset of measurements performed at smlar SZA and total ozone column Calbratons wth standards of spectral rradance The mult-flter radometer to be tested s regularly (e.g., annually) placed n front of a Standard of Spectral Irradance and the net-sgnal s measured. Assumng that the lamp s stable, the radometer should measure the same net-sgnal at every event. If dfferent lamps are used, measurements of the radometer should be converted to spectral rradance and compared wth the values provded n the lamp s certfcates. Addtonal correctons smlar to those descrbed n Secton may be necessary f the colour temperature of the varous lamps s not dentcal. Calbraton standards, ether reference standards or workng standards, should be recalbrated (or replaced) after 20 hours of use, unless otherwse stated n the lamp s certfcate. A recalbraton of reference standards should be performed by standard laboratores (see also Webb et.al., 1998) Repeated spectral response measurements Repeated spectral response measurements can help to determne reasons for changes n nstrument senstvty uncovered by one of the three methods descrbed above. For example, these measurements may detect changes n the response functons centre wavelengths, bandwdth, peak response, and wngs [Bgelow and Slusser, 2000]. Fgure 2 presents a tme seres of centrod wavelengths and bandwdths determned from measurements of an nstrument that s used by the USDA UV-B Montorng and Research Programme. Fgure 2 - Results of repeated spectral response measurements of a nstrument used by the USDA UV-B Montorng and Research Programme. Left panel: devaton of centrod wavelength from ntal measurement. Rght panel: Bandwdth expressed as fwhm Langley Method The Langley method allows determnaton of the solar spectrum outsde the Earth s atmosphere from measurements at varous armasses [Slusser et al., 2000]. Applcaton of the technque s possble only for nstruments that are equpped wth a shadowband. If the radometer s stable, repeated estmates of the extraterrestral spectrum should deally be dentcal. The technque s mostly suted to detect long-term changes of an nstrument s stablty as the Sun s a 15

22 very stable lght source. The method s not affected by changes n reference nstruments or lamps that mght affect the methods descrbed n Sectons However, the Langley method gves robust results only f the atmosphere s stable durng the Langley analyss, for example ths requres that the sky s cloud-free and atmospherc ozone and aerosol concentraton are constant over the tme requred for a Langley analyss (typcally a few hours are needed). Apart from hgh alttude stes, these condtons are rarely met. More nformaton on the Langley method s provded n Secton 6.6. Bgelow and Slusser [2000] have compared the methods descrbed n Sectons 5.3.3, 5.3.4, and and dscussed ther advantages and dsadvantages. More nformaton on long-term stablty of mult-channel nstruments can also be found n Johnsen et al. [2002], Janson and Slusser [2003], and Janson et al. [2004]. 5.4 Vsble and nfrared leakage test The senstvty to vsble and nfrared radaton can be tested wth cut-off flters that transmt vsble and nfrared radaton but block UV radaton (e.g., GG 400 produced by Schott). The measurement should be performed outdoors wth the Sun as the lght source. The flter should be placed on top of the radometer s dffuser. It s mportant that there s a good lght-tght seal between flter and radometer to prevent unfltered radaton from reachng the dffuser. Wth the flter n place the sgnal should be less than 1% compared to the sgnal wthout the flter for SZA smaller than 70. If the nstrument s calbrated wth a Standard of Spectral Irradance (Secton 4.1.3), then the lght leakage should also be tested usng the same calbraton lamp. Ths s mportant as ncandescent lamps have a larger contrbuton from the nfrared than the Sun [Lantz et al., 2005]. 5.5 Cosne error correcton The effect of the cosne error on solar data should be corrected. Correcton methods must take nto consderaton: (1) the devaton of the drectonal response of the radometer from the deal cosne response and (2) the dstrbuton of the radaton feld,.e., the dstrbuton of radance, when measurng solar radaton. Because the radaton feld s generally not known n detal, approxmatons have to be made. The most common approxmatons and smplfcatons are: The global spectral rradance s defned as the sum of drect horzontal spectral rradance and dffuse spectral rradance. For clear-sky condtons, the proporton of both can be ether measured drectly or calculated by a model. For overcast condtons, the drect spectral rradance s set to zero. For partly cloudy condtons, the accuracy of cosne error correcton methods s generally lmted. The drectonal dstrbuton of sky radance s regarded as sotropc. Ths assumpton has proved to be approxmately vald n the UV-B regon (Blumthaler et al., 1996). Methods of cosne error correctons should provde estmates of ther uncertanty. Descrpton of mplementatons and valdatons of cosne error correcton algorthms can be found n (Bernhard and Seckmeyer, 1997), (Seckmeyer and Bernhard, 1993), (Gröbner et al., 1996), (McKenze et al., 1992), (Fester et al., 1997), (Bas et al., 1998) and (Cordero et.al.,2008). 6. APPLICATIONS Several data products can be derved from mult-channel flter nstruments ncludng bologcally effectve rradance (such as erythemal rradance), reconstructed hgh-resoluton solar spectra, total column ozone, and aerosol and cloud optcal depth. The followng secton gves an overvew of methods for calculatng these data products. 6.1 Bologcally effectve rradance The followng methods are sutable for dervng values of bologcally weghted rradance (such as erythemally weghted rradance) from measurements of mult-channel flter nstruments. 16