Atmospheric ozone is concentrated predominantly inthe stratosphere. Approximately 10% of the ozone-column density resides in the troposphere, and aneven smaller part in the mesosphere. Stratosphericozone is the major species that absorbs solar UV ra-diation from 220 to 320 nm, preventing the radiationfrom penetrating the lower atmosphere and reachingthe Earth’s surface. Hence the presence of the ozone
T. Ingold [email protected]!, C. Matzler, and N. Kampferre with the Institute of Applied Physics, University of Bern, Sid-erstrasse 5, 3012 Bern, Switzerland. A. Heimo is with the Swiss
eteorological Institute ~MeteoSwiss!, Les Invuardes, 1530 Pay-erne, Switzerland. C. Wehrli and R. Philipona are with thePhysikalische-Meteorologisches Observatorium Davos and WorldRadiation Center, Dorfgasse 33, 7260 Davos-Dorf, Switzerland.
Received 23 March 2000; revised manuscript received 11 Decem-ber 2000.
shield is crucial for any life on our planet. With ad-ditional absorption and emission features in the infra-red, ozone strongly influences the Earth’s radiationbalance; i.e., it is of high relevance to the dynamics ofthe troposphere and the stratosphere. Since the be-ginning of the 1970’s, there have been controversialdiscussions of the destruction of the stratosphericozone layer by anthropogenic activities.1–3 It wasproved that chemicals manufactured by humankinddestroy stratospheric ozone4,5; the most dramatic ef-fect of ozone depletion, over Antarctica, has led to theAntarctic ozone hole.6 There is also evidence for sig-nificant Arctic declines and a statistically significantdecrease of the ozone shield at mid-latitudes.4 For amore detailed description of the history of ozone de-pletion and the conceptual framework to explain thekey processes involved, with a focus on chemistry,see, for example, Ref. 7 and references therein.Thus it is important to monitor ozone abundancesglobally. Although satellite measurements permitthe measurement of ozone on a global basis, ground-
20 April 2001 y Vol. 40, No. 12 y APPLIED OPTICS 1989
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based continuous observations are most suitable forlong-term monitoring at special sites. Furthermore,they are valuable for use in assessing the calibrationstability of satellite measurements.
Network measurements of ozone-column densityare routinely acquired by two standard types of in-strument, the Dobson spectrophotometer8–10 andBrewer spectrophotometer.11,12 Both instrumentsare based on sunphotometry in the UV range from300 to 340 nm. The Dobson instrument was devel-oped by Dobson in the 1920’s.8 Since 1958 thesepectrometers have been deployed in a worldwideetwork. During the 1970’s the Brewer instru-ents were developed as alternatives to the Dobson
nstruments. In the 1980’s their usage increasedith the introduction of fully automated units. In-
truments of both types contribute to the longest con-inuous study of ozone-column density at Arosa,witzerland, which began in 1921.13 Attempts toeasure ozone-column density by use of a filter spec-
rophotometer and problems related to the use ofuch instruments were reported in the 1960’s byojkov14 and Vanier and Wardle15 and in the 1970’s
by Matthews et al.,16 Sutherland et al.,17 Basher andMatthews,18 and Basher.19 Even in the past decadethere have been a number of publications on thatsubject: Ground-based ozone-column density mea-surements, for example, were reported by Labow etal.,20 Dahlback,21 and Cordoba et al.22 A network ofUV multifilter rotating shadow-band radiometerswas realized within the U.S. Department of Agricul-ture UV-B radiation-monitoring network.23 Com-parisons of column ozone-density retrievals by asingle UV multifilter rotating shadow-band radiom-eter that uses total horizontal irradiances with thosefrom the Dobson and Brewer instruments are sum-marized by Slusser et al.24
Recently, the Physikalisch-Meteorologisches Obser-vatorium Davos and World Radiation Center at Davos~PMODyWRC Davos! developed a new sunphotometertype, the precision filter radiometer ~PFR!, for mea-surements of aerosol optical depth. Within the Euro-pean Community project CUVRA ~Characteristics ofthe UV Radiation Field in the Alps!, the PMODyWRCextended its PFR into the UV range, building a four-channel narrowband UV PFR for high-quality spectralUV measurements. Such fully automated filter in-struments are normally less expensive than the Dob-son and the Brewer spectrophotometers. StandardPFR’s are or will be operated within the Swiss Atmo-spheric Radiation Monitoring program25,26 ~CHARM!y the Swiss Meteorological Institute ~MeteoSwiss! atarious sites that are representative of high and lowltitudes, urban and rural environments, at the northnd south sides of the Alps. These operations arentended to determine the radiation climatology andts possible long-term trends.
Using the principle of sunphotometry in the samepectral regions as the Dobson or the Brewer instru-ent allows us to estimate the atmospheric ozone-
olumn density. Test measurements started at theichtklimatologisches Observatorium ~LKO! in
990 APPLIED OPTICS y Vol. 40, No. 12 y 20 April 2001
rosa @46.78°, 9.68°, 1850 m above sea level ~asl!# inid-July 1999, permitting parallel measurements to
e performed during the World Meteorologicalrganization–Global Atmosphere Watch ~WMOyAW! International Dobson Comparison ~17 July–10ugust 1999!. The fully automated design of the UVFR allows data to be obtained more frequently thanoes the semiautomated Dobson at the same site.ecause no operator attention is needed, the UV PFRas so far been easier to maintain than the Dobsonnd Brewer instruments.In this paper we describe the methodology withhich the ozone-column density can be determinedith the UV PFR from measurements obtained atrosa. Starting with an instrumentation section, theetrieval concept is outlined and the error analysisdiscussed in Appendix A! is summarized. Then weescribe the validation of UV PFR ozone-column den-ity during a two-month period in 1999 at Arosa byndependent data from collocated Dobson and Brewerpectrophotometers. The same data are furthermoreompared with measurements of the total ozone-apping spectrometer onboard the Earth Probe Totalzone Mapping Spectrometer27 ~EP-TOMS! and with
data from the Global Ozone Monitoring Experiment~GOME! instrument28 on the Second EuropeanRemote-Sensing Satellite ~ERS-2! platform. Fi-nally, in Section 6, we provide a summary and con-clusions.
2. Instrumentation
A. Precision Filter Radiometer
The PFR’s are designed with emphasis on instrumen-tal stability; thus the detectors are kept in a con-trolled environment and exposed to solar radiationonly during actual measurements. The PFR ishoused in a weatherproof tube of 88-mm diameterand 390-mm length with an entrance window of syn-thetic quartz and a weight of 3 kg. The tube is her-metically sealed and maintains an internalatmosphere of dry nitrogen over extended periods oftime. The temperature of the detector head is sta-bilized at 20 6 0.1 °C by an active, Peltier-type ther-mostatic controller over an ambient temperaturerange from 220 to 135 °C. An internal shutteropens for only a few seconds every 2 min to keepdose-related degradation of the filters to a minimum.Two apertures define a field of view of 2.5° and a slopeangle of 0.7°, and an optical position sensor accu-rately monitors the solar pointing within a 60.7°range. A CR10 data acquisition system ~Campbell
cientific, Leicestershire, UK! is an integral part ofthe PFR instrument for applications within the Swisscontribution to the WMOyGAW program. However,for the CHARM program a CR23 system from thesame manufacturer with 16-bit resolution is used,along with autoranging software. It performs si-multaneous measurements of the four photometricand several housekeeping signals while controllingthe shutter and measuring sequence. The standardcadence of 2 min results in 720 measurements per
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day. Optionally, the barometric pressure can bemeasured with relatively coarse accuracy by a solid-state sensor. Data are retrieved by means of aserial communication link to a personal computer,usually once per day, but internal memory can cap-ture data during a period of several days betweendownloads. In combination with a suitable solartracker, the CR23 system makes fully automatedoperation possible. The PFR instrument has fourindependent channels centered at the WMO-recommended aerosol wavelengths29,30 of 368, 412,500, and 862 nm equipped with dielectric interfer-ence filters manufactured by ion-beam-assisteddeposition. In the UV version used here ~hereafteralled the UV-PFR!, the wavelengths at 305, 311,18, and 332 nm correspond to those used in Dobsonpectrophotometers.
B. Other Instruments Used During the LKOIntercomparison Period
1. Ground-Based InstrumentsAt the same location at LKO, Dobson and Brewerspectrophotometers are installed for routine mea-surements of ozone-column density.31 Since 1988MeteoSwiss has been responsible for the opera-tional performance of these measurements. Thereported accuracy of ozone-column density mea-surements of both types at Arosa is of the order of62%. In addition, a collocated standard PFR pro-vides spectral information in the visible and thenear IR.
Dobson instrument 101 ~Dobson101! measures di-rect or scattered sunlight; it is semiautomated anduses a double monochromator. Ozone-column den-sity is measured in time steps of a few minutes basedon wavelength-pair differences in the UV under di-rect visibility of the Sun. Here the double pair,~305.5y325.4 and 317.6y339.8 nm!, is used and is rec-ommended as the international standard based onthe world standard method.9 For a detailed descrip-tion of the instrument and the algorithm, see Stahe-lin et al.31 Note that we adopted a similar conceptor the UV-PFR instrument and its channels; see alsoection 3 below.Brewer instruments 40, 72, and 156 ~Brewer40,
Brewer72, and Brewer156! are similar in their prin-ciples to the Dobson instrument but have an im-proved design and are fully automated. Theymeasure the intensity of the incident solar light atfive wavelengths in the UV ~302.3, 306.3, 310.1,313.5, 316.8, and 320.1 nm! sequentially in 1.6 s,which permits the determination of ozone-columndensity. For a description of the algorithm and theinstrument, see Kerr et al.11,12
The standard PFR provides simultaneous mea-surements of the direct spectral irradiance at its fourstandard wavelengths ~368, 412, 500, and 862 nm!.Besides this information we used the aerosol opticaldepth ~AOD! derived at each wavelength, using themethod presented by Schmid et al.32
2. Satellite-Based InstrumentsThe total ozone-mapping spectrometer aboard theEP-TOMS27 provides measurements of ozone-columndensity by measuring the backscattered Earth radi-ance in six 1-nm bands in the UV-A and UV-B spec-tral regions, similarly to previous TOMS instrumentsaboard the Nimbus-7, Meteor-3, and ADEOSTOMS33–35 satellites. Regular measurements ofozone-column density by the EP-TOMS started on 25July 1996. The instrument uses a single monochro-mator and a scanning mirror to sample the backscat-tered solar UV radiation at 35 samples points locatedat 3° intervals along a line perpendicular to the or-bital plane. The measurements used for ozone re-trieval are made during the sunlit portions of theorbit. The dimensions of the instantaneous field ofview vary from 38 km 3 38 km at nadir to 70 km 3140 km at the extreme off-nadir during the high750-km orbit period after 12 December 1996.
The basic approach of the ozone algorithm is to usea radiative transfer model to calculate ratios of radi-ances to irradiance for different ozone-column densi-ties. Radiances were calculated at approximately0.05-nm intervals across each of the TOMS slits, basedon climatological temperature and ozone profiles, byuse of the appropriate ozone-absorption coefficient andtemperature dependence36 for each wavelength. Theozone-column density can be derived from comparisonswith TOMS measurements. For a detailed algorithmdescription, see Sec. 4 of Ref. 27. For EP-TOMSozone-column density, the absolute error is 63%, therandom error is 62%, and the drift in the first 1.5 yearsf operation was found to be less than 0.6%. Here wesed version 7 overpass data, which are available forrosa. These data are normally observed once everyay from 09:00 to 11:00 UT.The GOME is an across-track scanning optical
pectrometer onboard the ERS-2 that covers theavelength range from 250 to 790 nm. The GOME’s
ask is to sense the sunlight that is reflected or scat-ered in the Earth’s atmosphere and at the Earth’surface. The measured spectrum contains absorp-ion features, which can be used to derive quantita-ive information on the ozone-column density that isresent and on a number of other atmospheric spe-ies. The ozone retrieval is based on the differentialptical absorption spectroscopy method with theeasured reflectances from 325 to 335 nm. We used
zone-column densities generated with the off-lineOME Data Processor V. 2.7 ~Ref. 37! for Arosa.he GOME data processor uses flight model crossections38 with temperature parameterization simi-
lar to that suggested by Bass and Paur.36 Represen-tative temperatures are selected that correspond tothe maximum concentration in an appropriate ozoneclimatological profile.39,40
As the ERS-2 flies in a heliosynchronous polar or-bit, GOME measurements above Arosa are acquirednear 11:00 UT. A GOME ground pixel is ;320 kmn longitude and ;40 km in latitude. The accuracyf the GOME ozone vertical-column density measure-
20 April 2001 y Vol. 40, No. 12 y APPLIED OPTICS 1991
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Table 1. Manufacturer Specifications of Central Wavelength and
1
ment varies with the solar zenith angle ~SZA! and, inany cases, with the ozone column. At a SZA of70°, it is 2–5%, and at a larger SZA it is 10% oretter for mid-latitudes.
3. Methodology
A. UV-PFR Ozone Retrievals
The atmosphere’s ozone column density is deter-mined by the measurement of narrow ~1-nm! bands ofsolar UV radiation and the use of the Beer–Lambert–Bouguer law to describe the atmospheric attenuationof monochromatic radiation. In Fig. 1 the zenithtransmittance that is due to ozone absorption isshown for a MODTRAN 3.7 simulation41 in the UV spec-tral range from 300 to 340 nm. In addition, the filterresponses F~l! of the UV-PFR are superimposed,where the filter characteristics of the four UV-PFRchannels as given in Table 1 are used.
We use the column ozone-retrieval concept appliedfor the Dobson instruments.31 The UV-PFR outputvoltage at the central wavelength li of filter i asmeasured at the Earth’s surface is
V~li! 5 R22V0~li!exp@2m1a1~li!V 2 mRtR~li!
2 mata~li! 2 m2t2~li! 2 m3t3~li!#,
i 5 1, . . . , 4, (1)
where V0~li! is the extraterrestrial signal at the top ofthe atmosphere and R is the Earth–Sun distance inastronomical units at the time of observation; see alsoAppendix A.6 for details on its derivation. The argu-ment of the exponential function describes the totaloptical depth t of the atmosphere in the direction to-ward the Sun. The first term is due to O3 absorptionwith ozone-column density V, i.e., the number of mol-ecules per unit area above the observer’s location in thezenith direction. Temperature-weighted absorptioncoefficients a1 are derived from zenith calculations ofozone absorption in MODTRAN 3.7 simulations. MOD-TRAN describes ozone absorption by a temperature-ependent ~quadratic! continuum based on the mea-
Fig. 1. Zenith transmittance of ozone absorption in the UVthrough the MLS ~V 5 331.7 DU! computed with MODTRAN 3.7.Resolution is set to 5 cm21 ~'0.05 nm, depending on wavelength!.
uperimposed are the spectral responses of the four UV-PFR chan-els.
992 APPLIED OPTICS y Vol. 40, No. 12 y 20 April 2001
urements of Bass and Paur,36 in the range 244–333nm. Measurements by Molina and Molina42 havebeen used up to 340 nm in combination with those ofYoshino et al.43 to expand the temperature-dependentrange of the absorption coefficients to 340 nm with afinal extension to 360 nm from the values of Caccianiet al.44 Calculated MODTRAN data were weighted bythe mid-latitude summer ~MLS! vertical temperatureand ozone profiles according to
a1~l! 52log@T1~l!#
VMLS, (2)
where T1 is the transmittance of ozone absorptionand VMLS 5 331.7 DU ~DU is a Dobson unit; 1 DUcorresponds to 2.687 3 1016 moleculesycm2!. Theoptical air mass m1 that relates the ozone mass of theactual path in the atmosphere to that in the zenithdirection is
m1 5r0 1 h
@~r0 1 h!2 2 ~r0 1 r!2 sin2 u#1y2 , (3)
where r0 is the mean radius of the Earth, r is theltitude of the station above sea level, h is the alti-ude of the ozone layer above sea level, and u is theZA. An appropriate value of h for Switzerland is
21 km, and r0 is set to 6371.229 km. The secondterm in Eq. ~1! is due to Rayleigh scattering ~tR! andis computed according to Bucholtz45 with actual at-mospheric pressure values at the sunphotometer site.For the air mass function mR we apply the formula ofKasten and Young.46 The third term encompassesMie scattering and absorption caused by aerosols ta.Absorption that is due to NO2 and SO2 is described byt2 and t3, respectively. For all three gases we ap-proximate the air mass ~ma, m2, and m3, respectively!by the water-vapor air mass given by Kasten.47
Applying Eq. ~1! to two different wavelengths, build-ing the difference of the output voltages, and rearrang-ing, we determine the ozone-column density by
V 5N 2 tRmR 2 t2m2 2 t3m3 2 tama
am1, (4)
where
N 5 ln~V0yV09! 2 ln~VyV9!, (5)
a 5 a1 2 a19, tR 5 tR 2 tR9, t2 5 t2 2 t29,
t3 5 t3 2 t39, ta 5 ta 2 ta9, (6)
Bandwidth for the Nominal UV-PFR Channels of the Instrument Installedat Arosa
Table 2. Error Contribution of Ozone Retrievals in the UVa for V 5 300 DU and SZA 5 25° for Cloud-Free Sky and Cirrus Clouds with a
M
in which a prime refers to the longer wavelength.The magnitude of the aerosol extinction in the UV isnot well known. Fortunately, aerosol extinction isonly weakly dependent on the wavelength in the 305–332-nm range. Thus the error introduced by ne-glecting ta is small. Combining two wavelengthpairs ~double difference! can minimize the aerosol’sinfluence on the ozone retrieval. The procedure is tobuild two differences from output voltages, using Eq.~1! at two wavelengths. Then one difference is sub-tracted from the other. Finally, by rearranging forV, we can write
V 5NA 2 NB
m1~aA 2 aB!2
mR~tR, A 2 tR,B!
m1~aA 2 aB!
2ma~ta, A 2 ta,B!
m1~aA 2 aB!, (7)
here A and B index the two different pairs and theterms that are due to NO2 and SO2 have been omit-ted. Neglecting the aerosol term introduces asmaller error than does a single-difference pair for anoptimal choice of wavelengths. For example, for twodifferent wavelength pairs with the same wavelengthdifference, this term will vanish if the AOD is as-sumed to be a linear function of wavelength, i.e.,ta,A 2 ta,B 5 0.
Different single-wavelength pairs can be used forthe single-pair retrieval of the UV-PFR instrument atArosa to retrieve the ozone-column density accordingto Eq. ~4!. For the four UV channels ~Table 1! it was
ossible to apply this retrieval to six different pairs,amely, 305y311, 305y318, 305y332, 311y318, 311y32, and 318y332. In addition, we also applied theouble-pair retrieval to ozone estimates, using Eq. ~7!or the two pairs 305y311 and 311y318. This com-ination comes closest to a negligible aerosol term in
Total, ¥ x 25.0–4.0 26.5–4.0 25.5–6.5Retrieval parameterf
DV0g 2.5 2.5 4.0
Da1h 6.0 4.0 3.0
Total, =¥ x2 6.5 4.5 5.0
aMeasured ozone-column density minus true ozone-column denbThe SO2 and aerosol terms are neglected. All data are givencFor the algorithm the model error is ,0.5 for all pairs; that fordAccording to the range of measured data.eInvariant temperature-weighted ozone-absorption coefficients afPressure ,0.5 for all pairs.gFor SZA 5 25° ~air mass, '1.10!.hFor V 5 300 DU.
q. ~7!. For both retrievals the NO2 term is calcu-lated according to the absorption coefficients as de-rived from MODTRAN for a climatological mean of 2 31015 moleculesycm2 for the NO2 column density. Noroutine measurements are available for columnaramounts of SO2. However, its contribution is smallfor clean atmospheric conditions, and we set t3 5 0 inEq. ~4!. We neglect the ozone column density in cal-culating the aerosol term.
B. Data Uncertainty
Uncertainties in the ozone values derived from UV-PFR measurements have several sources: errors inthe parameterization of atmospheric properties, lim-itations in the way in which the computation repre-sents the physical process in the atmosphere, errorsin the measurements of the incoming signals, anderrors in the values of input physical quantities ob-tained from laboratory measurements. Each ofthese sources of uncertainty can be manifested inrandom errors, an absolute error independent oftime, a time-dependent drift, or a systematic error.All major contributions are discussed in Appendix A.In Table 2 we summarize the errors in the UV-PFRretrievals for V 5 300 DU and SZA 5 25° for mea-surements obtained when the Sun is visible. Insuch cases cirrus clouds ~cloud optical depth, #0.17 at500 nm! could obscure the Sun, but optically thickerclouds are excluded.
The model errors are dominated by the ignoredaerosol extinction and the fixed temperature depen-dence of the ozone absorption coefficients. Both pa-rameters can considerably bias UV-PFR ozone-column densities. All other contributions are rathersmall with respect to these values. The range ~min-imum to maximum value of summation! is 9.0 DU~305y311! to 20.5 DU ~318y332! for the single pairsand only 3.5 DU for the double-difference pair. The
bson units, rounded to 0.5 DU.cirrus clouds is 20.5–0.5; that for the passband is ,1.0.
V 5 300 DU.
al Me
sity.in Do
the
1 for
20 April 2001 y Vol. 40, No. 12 y APPLIED OPTICS 1993
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conversion to relative values yields values of 3.0% for305y311, 3.5% for 305y318, 4.0% for 305y332, 5.2%for 311y318, 5.7% for 311y332, and 6.8% for 318y332and only 1.2% for the double pair. The effect that isdue to aerosols is found to be most dominant for the318y332 single-difference pair unless low turbidity ispresent. Thus we will probably overestimate ozonefor this pair.
Significant uncertainties regarding retrieval pa-rameters arise from the channel calibration and theozone-absorption coefficients. Considering that thefractional uncertainty that is due to the ozone-absorption coefficient is only an estimate ~AppendixA.6!, the two parameter uncertainties contribute inalmost equal parts to the uncertainty of the singlepairs. The error is 6.5 DU ~2.2%! for 305y311, 4.5DU ~1.5%! for 305y318, 5.0 DU ~1.7%! for 305y332,7.0 DU ~2.3%! for 311y318, 6.5 DU ~2.2%! for 311y332, 12.0 DU ~4.0%! for 318y332, and 14.5 DU ~4.8%!or the double pair. For an increasing air mass.25° SZA! the contribution of the uncertainties of V0
decreases ~Appendix A.6!. Additional errors in val-es of ozone-column density could be introduced bylter alterations. During the 2-month period weound no indications of such an effect, as we discuss inppendix A.7.
4. Experiment
We derived ozone-column density for the single- anddouble-pair retrievals, respectively, in the periodfrom mid-July 1999 to mid-September 1999. Dataare obtained for solar zenith angles from 26° to 77.5°~air mass range from 1.11 to 4.62!. The upper limits selected to maintain a sufficient signal-to-noise ra-io at 305 nm determined by the strong absorptionAppendix A.6!. Reliable measurements for thether channels can be extended to somewhat lowerun elevations.Data selection was performed with the following
rocedure: We first rejected data with an air massf .4.62 and total zenith optical depth of .3 at 305
nm. For the remaining data we built the ratio X ofV for the single-difference pair 305y311 and thedouble-difference pair,
X 5V305–311
Vdp, (8)
nd its standard deviation sX. This ratio willemerge as a good indicator of data quality. Thevalue of X is normally near 1 unless the data areffected, for example, by clouds. For such cases theensitivity is different in V for these pairs. Thus we
started an iteration to remove outliers ~7.0%! beyond2.95 3 sX, where sX was recalculated after each it-eration. We finally obtained 7081 measurements ~2-
in intervals!. This data set comprised alleasurements when direct sunlight was the only
omponent ~2907 cloud-free data! and most measure-ments when direct sunlight was scattered by opti-cally thin clouds ~cloudy data with cloud zenithptical depth at 500 nm up to ;2.5!.
994 APPLIED OPTICS y Vol. 40, No. 12 y 20 April 2001
To separate the cloud-free periods from the cloud-ontaminated data set, we looked at the irradianceeasurements of the collocated standard PFR. In
eneral, the data obtained in the visible are more sen-itive to cloud-scattered sunlight than measurementsn the UV. If a cloud obscured the Sun, the observedecrease in the signal allowed such contaminated datao be excluded. UV-PFR ozone estimates were ob-ained on 52 of 60 days in the two-month period ~87%!.owever, the cloud-free data were collected on only 24ays ~40%!. Ozone-column density determined from
single-difference pair 305y311 ranged from 279.0 to39.2 DU, with a mean value of 303.2 DU and a stan-ard deviation of 13.6 DU.
5. Results and Data Analysis
A. Comparison with Dobson and Brewer Instruments
Data on the Dobson and Brewer instruments at LKOwere used to validate the UV-PFR ozone-column den-sity estimates of the single-difference pairs and thedouble-difference pair. UV-PFR ozone values werederived on almost the same days as the Brewer andDobson data. They provided additional data on 1 to 2days within the 60-day period, depending on instru-ment. Here we considered time-resolved values of V.We have summarized the results of the comparisonwith the ground-based instruments in Table 3. Allthe UV-PFR data are averaged over 20-min intervalscentered at times of measurements made with theDobson and Brewer instruments. In addition wehave listed the results for comparisons of Brewer40minus Brewer72, Brewer40 minus Brewer156, andBrewer40 minus Dobson101 data obtained in the sameperiod. Intervals of the Brewer40 data are set to 10min centered at Brewer72, Brewer156, or Dobson101time. The best agreement for UV-PFR data is foundfor the single pair 305y311 followed by 305y318, thedouble pair ~305y311 and 311y318!, 305y332, 311y318,311y332, and the 318y332 combination. Normally,these results are similar to comparisons of Breweralong Dobson or Brewer data. With respect to thebest UV-PFR single pair, the findings for comparisonsamong Brewer instruments are slightly better,whereas the Dobson with the Brewer instrumentsyield somewhat worse results.
Figure 2 shows a scatterplot of UV-PFR 305y311and Brewer72 data. The data pairs that show thelargest deviation were obtained when the Sun wascovered by clouds. For the reduced cloud-free dataset ~n 5 518! the mean difference decreases by only0.7 DU ~0.2%! and the standard deviation ~std differ-ence! by 0.6 DU ~0.2%!, compared with the results forthe whole data set. Regarding the data selection,this minor loss in retrieval performance can be at-tributed mostly to clouds in the measurements for thetwo-month period. Similar findings are obtainedwhen the same pair is compared with those from allother Dobson and Brewer instruments.
Figure 3 shows the mean and the standard deviation~stdev! of V differences for both the cloud-free and thewhole data set. The results for the different Dobson
Table 3. Total Ozone Statistics of Measurements Performed at Arosa
and Brewer instruments are averaged for each UV-PFR retrieval. The difference in standard deviations~0.6–1.9 DU! between the two situations is quite small.
The statistics of the comparisons presented aboveshow that the UV-PFR single pair 305y311 yields thebest overall performance in retrieving V. The mean
and the standard deviation of the differences arewithin 1% of the Dobson and Brewer data. The 305y318 pair yields similar data for the standard devia-tion of the differences but slightly underestimatesozone-column density. Although the double pairyields only the third-best overall agreement, the rel-atively small influence of aerosols, atmospheric tem-perature, and clouds makes this pair a valuablecandidate for use in retrieving ozone as well andfurthermore to provide the X values for quality con-trol ~Section 4!. The MLS atmosphere and ozoneprofile used for weighting the absorption coefficientsdescribes in a satisfactory way the state of the atmo-sphere in these two months ~see Appendix A.2 forexpected deviations in other seasons!.
Fig. 2. Scatterplot of ozone-column density retrievals of the UV-PFR 305y311 and the Brewer72, as measured in Arosa ~1850 m asl!from mid-July 1999 to mid-September 1999. UV-PFR data wereaveraged over 20-min intervals.
Fig. 3. Mean ~top! and standard deviation ~bottom! of ozone-column density difference, UV-PFR minus Dobson or Brewer. Re-sults for each UV-PFR pair are averaged and are given for all dataand for a reduced data set when no clouds obscured the Sun.
~1850 m asl! of UV-PFR, Dobson, and Brewer Data from Mid-July 1999to Mid-September 1999a
aData of instrument A are averaged over 20-min intervals cen-tered at the time of measurement of instrument B. All relativevalues are given with respect to instrument B.
bStandard deviation.c10-min intervals instead of 20-min intervals.
20 April 2001 y Vol. 40, No. 12 y APPLIED OPTICS 1995
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Table 4. Total Ozone Statistics of Measurements from UV-PFR Data
1
B. Comparison with the EP-TOMS
Similar agreements were found for V. 7 EP-TOMSoverpass data. The statistics of the comparison aresummarized in Table 4. We averaged all the UV-PFR data over 1-h intervals centered at the measure-ment time of the EP-TOMS. We found slightlylarger average shifts than for the previous compari-sons. The EP-TOMS overestimates V with respectto the data of the UV-PFR single pairs that have lowaerosol sensitivity. This result was also found whenthe EP-TOMS was compared with Brewer and Dob-son data measured at the LKO station. All thesevalues are consistent with the reported overestima-tion of approximately 1.0% with respect to a networkcomposed of 30 mid-northern-latitude stations withDobson and Brewer ozone-measuring instruments.27
Moreover, all results are within the uncertainties ofboth the EP-TOMS and the UV-PFR instruments.
C. Comparison with the GOME
GOME overpass data for Arosa were compared withUV-PFR data. To account for the overpass timenear 11:00 UT, we averaged the PFR-UV data over a1-h interval centered at 10:00 UT. The statistics ofthe comparison are summarized in Table 4. In gen-eral, the GOME measured less ozone than all theother instruments. All the standard deviations ofthe differences are within the reported uncertainty ofthe GOME of 2–5%. The increased values comparedwith the results of using all other instruments re-
Obtained at Arosa ~1850 m asl! and Satellite Overpass Data for Arosaa
aUV-PFR data are averaged over a 1-h interval centered at themeasurement time of the EP-TOMS or at 10:00 UT for GOMEdata. All relative values are given with respect to instrument B.
bStandard deviation.
996 APPLIED OPTICS y Vol. 40, No. 12 y 20 April 2001
flects the natural ozone variability in the largerGOME field of view.
D. UV-PFR Ozone Estimates
Figure 4 shows the variation of daily means of ozone-column densities as determined by single-pair re-trieval with the 305y311 pair and double-pairretrieval as well as Dobson101, EP-TOMS, andGOME data for coincident days. With respect to thestandard deviation of the mean differences, theagreement in daily averaged V was found to be as
uch as 1.5 DU worse than for the time-resolvedata. This increase can be explained mostly by theifferent numbers of measurements that are requiredhen one is building daily averaged values. There
s only one value per day for the EP-TOMS and theOME, respectively, whereas multiple data were used
or the ground-based instruments. Thus the acquisi-ion times of all these measurements differ as well.
An example of a diurnal ozone variation is shown inig. 5 for 18 July 1999 for the single pairs 305y311nd 318y332 and the double-difference pair for whichhe UV-PFR data are not averaged. Cloud-free dataere obtained until 11:00 UT, as shown by nonmo-
ecular optical depth data at 500 nm. Afterward,louds partly obscured the Sun and contributed toonmolecular optical depth as well as to AOD. Inddition, Dobson101 and Brewer72 data are super-mposed and show a similar variation. As predictedy the MODTRAN simulations, there is an enhanced
overestimation for ozone-column density retrieved forthe 318y332 pair, as no aerosol term is included.This offset increases after 11:00 UT because of thepresence of clouds. Simulations by MODTRAN for op-ically thicker clouds lead to the reverse effect. Thisehavior, shown in Fig. 5, is often observed for opti-ally thin clouds. There seems to be an AOD in-rease as well, possibly as a result of aerosol–cloud
Fig. 4. Variation of daily means of ozone-column density as mea-sured at Arosa ~1850 m asl! with the Dobson101 spectrometer andhe UV-PFR with the single-difference pair 305y311 and theouble-difference pair. In addition, satellite-based EP-TOMS andOME overpass data are plotted for the selected dates.
s2aamUory
A
interaction. But how far this is the case is difficultto determine unless additional information on cloudand aerosol properties is available. We estimatedthe influence of aerosols by introducing the correctionaccording to ta, using AOD measurements in the vis-ible along the Ångstrom turbidity law48 at the samelocation. For the UV-PFR data in Fig. 5 we foundthese corrections in the range from 21.0 to 21.0 DU,depending on the selected wavelength pair at the timebefore 11:00 UT. These findings are consistent withthe data of the simulations. In the afternoon therange was 22.0 to 216 DU. The double pair yieldsthe expected low aerosol dependence because the twopairs at 305y311 and 311y318 represent an almostoptimal choice for such a correction. However, thenoise in retrieved ozone increases by a factor of ap-proximately 2–3 when estimates of the involved sin-gle pairs are considered separately. Similar findingsregarding the aerosol term and the noise were ob-tained for all other days. Because there are moreparameters in the double-pair expression given by Eq.~7!, a small disturbance in atmospheric turbidity leadsto increased scatter. However, this scatter is stronglyreduced when 20-min averages are computed.
6. Summary and Conclusions
The UV-PFR instrument operated within the SwissAtmospheric Radiation Monitoring program CHARMcan provide ozone-column densities obtained frommeasurements of direct and forward-scattered sun-
Fig. 5. Time series of ozone-column density ~top! and nonmolecu-lar zenith optical depth ~NMOD! at 500 nm ~bottom! measured at
rosa ~1850 m asl! for 18 July 1999. The ozone data were derivedfrom the UV-PFR instrument ~305y311, 318y332, and double pair!,the Brewer72 spectrometer, and the Dobson101 spectrometer.Until 11:00 UT clear sky ~no clouds! was observed; i.e., NMODequals the aerosol optical depth.
light. Data were obtained for SZA’s up to 77.5°. Atlarger angles the 305-nm voltage readings fall belowthe instrument detection limit. Following the Dob-son scheme, we retrieved ozone-column densities forsix single-difference pairs and a double-differencepair ~305y311 and 311y332!. The most significantretrieval uncertainties are found to be uncertaintiesof the channel calibration constants and the ozone-absorption coefficients. Most single pairs show sub-stantially smaller calibration uncertainties than thedouble pair. Model errors arise from neglectingaerosol extinction and using ozone-absorption coeffi-cients weighted with the built-in MODTRAN MLS tem-perature and ozone profiles. Both can bias the ozoneestimates that depend on a single pair, whereas thedouble pair is quite insensitive to such effects. How-ever, for the two-month period from mid-July to mid-September there is almost no error that is due toinvariant temperature-weighted absorption coeffi-cients. In a clean Alpine environment, both NO2and SO2 are expected to interfere extremely littlewith the retrieved ozone.
As a result of this study we suggest using the 305y311 single-difference pair and the double-differencepair for ozone determination with the UV-PFR. The305y311 pair shows only minor scatter in measureddata and shows the least expected error for single-difference pairs. The double-difference pair yieldsthe lowest retrieval uncertainty when the contribu-tions of the parameter uncertainties are neglected.No significant change in mean difference and stan-dard deviation was found in the ozone-column den-sity comparisons presented here for cloud-free andcloud-contaminated measurements ~cloud opticaldepth at 500 nm up to 2.5!. Only the scatter in theingle measurements that has increased by a factor of–3 compared with the scatter in the best single pairsffects the retrieval performance. We can eliminatemajor part of the scatter by averaging consecutiveeasurements as indicated by the 20-min averagedV-PFR in Table 3. Furthermore, the combination
f both retrievals by the ratio X yields informationelevant to data quality control. If the atmosphereields unexpectedly large SO2 and NO2 amounts or
aerosol loads, information on other single pairs mightbe helpful in deriving a better estimate.
Assuming an ozone-column density of 300 DU, wefind that the uncertainty in retrieved ozone of the305y311 single pair ranges from 25 to 4 DU for modelerrors and is 6.5 DU for errors that are due to re-trieval parameter uncertainties. The agreement ofUV-PFR ozone-column density with the Dobson andBrewer data is within 3 DU ~1%! for both the meandifference and the standard deviation of the differ-ence. The uncertainty in the UV-PFR ozone-columndensity values of the double pair ranges from 20.5 to3.0 DU for model errors and is 14.5 DU for retrievalparameter uncertainties. For the double pair weslightly overestimate ozone-column density by asmuch as 4.8 DU ~1.6%!, depending on Dobson orBrewer data with a standard deviation of the differ-ences below 3.8 DU ~1.3%! between the V data sets.
20 April 2001 y Vol. 40, No. 12 y APPLIED OPTICS 1997
atea~pamwt
1
Comparisons with satellite-based EP-TOMS orGOME measurements yield a somewhat increasedstandard deviation of the differences for both UV-PFR pairs. The most probable reason for these dif-ferences is the different air volumes observed by thesatellite-based instruments, whereas all ground-based instruments at Arosa estimate ozone-columndensity from an almost identical volume in the samedirection. For the GOME there are mean differ-ences of 3.8 DU ~1.3%! for the 305y311 pair and 7.9DU ~2.6%! for the double pair. For the EP-TOMS wefound a mean difference of 24.1 DU ~21.3%! for the305y311 pair but a negligible offset of 0.2 DU ~0.0%!for the double pair. All comparisons indicate thatthe UV-PFR data are consistent with the UV-PFRuncertainties and also with the expected accuraciesin V of the other instruments.
Within the instrument trial phase the automatedUV-PFR proved to be a satisfactory alternative forthe Brewer and semiautomated Dobson spectropho-tometers at the LKO. It permitted measurements ofozone-column densities with similar precision and ac-curacy at an even higher temporal resolution. Theadvantages of the UV-PFR are its design, optimizedfor minimal maintenance with no operator attention,and the normally lower purchase costs that resultfrom the use of filters. However, we need to extendthe research presented here to check the long-termstability of the instrument. Because of the atmo-spheric conditions at the LKO altitude of 1890 m asl,this stability can be controlled with Langley plots.The use of independent measurements of Dobson andBrewer instruments at the same location can provideexcellent validation of the data. If the long-termobservations yield results similar to those for the two-month period in 1999, the UV-PFR will be a suitableinstrument for wider implementation in a networkfor routine, unattended measurements of ozone-column density.
Appendix A: Error Analysis
To clarify the relationship between the individualuncertainties and how they arise, we have organizedthe following error analysis in order of position in theretrieval process where the error arises.
1. Error in the Parameterization
The retrieval robustness was studied by use of opticaldepth calculations of MODTRAN simulations, where datafrom SRS 400 radiosondes49 collected on October 1998through September 1999 were evaluated. These datawere obtained from the Swiss Meteorological station atPayerne, Switzerland ~46.82 °N, 6.95 °E, 490 m asl!.Besides temperature, pressure, and water vapor,ozone profiles up to approximately 30–35 km weremeasured every 2 or 3 days at 12:00 UT. For theprofile data above, up to 100 km, we added fixed MLSvalues. The aerosol profile was set to the built-in ru-ral extinction with a visibility of 23 km in the tropo-sphere and to background conditions above in thestratosphere. We kept gaseous absorption of NO2and SO2 fixed by applying the built-in density profiles
998 APPLIED OPTICS y Vol. 40, No. 12 y 20 April 2001
that describe a clean-air environment. The path wasselected from the observer altitude at Arosa ~1850 masl! to space along a number of SZA’s in the range from0° to 80°. Altogether, we computed 134 differentcloud-free cases in MODTRAN that were representative ofa 1-yr period. Considering temperature-weightedozone-absorption coefficients and including the termsof SO2, NO2, and the aerosols, these simulationsyielded an ozone-column density retrieval precision forEqs. ~4! and ~7! within 0.5 DU for the selected SZArange that is valid for all pairs.
2. Atmospheric Temperature
Absorption coefficients of the various gaseous attenu-ators in the UV range are temperature dependent.For the ozone retrievals presented here, only the effectfor ozone is found to be relevant. Increasing temper-ature leads to a stronger ozone absorption in the UV.50
However, within the algorithms presented, by usingEq. ~2! we weighted all ozone-absorption coefficients bythe MLS vertical temperature and ozone profile asgiven by the MODTRAN ozone-transmittance simulationalong a vertical path to space.
Using the 1-y set of radiosondes as evaluated inMODTRAN, we also calculated a representative set oftemperature-weighted ozone-absorption coefficientswith Eq. ~2! from the simulated ozone transmittancesT1~l! and the ozone column density VRS as integratedalong the vertical path starting at the observer altitudeat Arosa ~1850 m asl!. We found the variation in a1 tobe 2.2% at 305 nm, 2.6% at 311 nm, 1.6% at 318 nm,and 12.1% at 332 nm for mean values of 1.648 3 10219,7.774 3 10220, 3.236 3 10220, and 1.690 3 10221 cm2,respectively. Our invariant temperature-weightedabsorption coefficients ~MLS, 0 m asl! are nearer themaximum values than are the simulations; i.e., theycan be regarded as an upper limit for all simulatedvalues. Selecting an observer altitude of 1850 m aslthat is appropriate for Arosa instead of 0 m asl for theMLS profile yielded coefficients that were smaller byless than 0.25%. We obtained the lower limit by re-placing the MLS atmosphere with the mid-latitudewinter atmosphere in the MODTRAN simulation. Werepeated all MODTRAN simulations with the MLSweighted absorption coefficients and compared theozone estimates with the first simulations. Depend-ing on the retrieval and the selected pair, there will bean annual sinusoidal variation of the ozone value from23.5% ~underestimation! to 10.5% ~overestimation!,s shown in Fig. 6. The underestimation is found forhe single-pair retrieval, whereas the double pair over-stimates ozone-column density. We found closestgreement in summer ~winter! for the single pairsdouble pair!. In the worst case, i.e., for the 311–318air in wintertime, this variation can lead to a system-tic underestimation of the ozone-column density by asuch as 10 DU for V ' 300 DU. Using mid-latitudeinter data instead of MLS data would yield almost
he same variation, with a shift in the y axis by ;3%,i.e., an agreement in winter for the single pairs. Forlong-term retrievals it is suggested that monthly val-
wA
ccuNdfpAawa
a
i
R
t
b
icp
No
afi
ues of Eq. ~2! be taken from climatological mean pro-files.
3. Atmospheric Optical Depths
The absorbers NO2, SO2, and aerosols contribute tothe column ozone in Eq. ~4! according to linear ex-pressions of absorber amounts Xi:
DVi 5 2Xi
~ai 2 ai9!mi
~a1 2 a19!m1< 2Xi
~ai 2 ai9!
~a1 2 a19!, (A1)
ith index i ~52, 3, a! for the different absorbers.ssuming that m1 5 mi for all i, a condition that is
fulfilled with negligible error in the expected SZArange up to the selected upper limit of 77.5°, the airmass dependence cancels out of Eq. ~A1!. Owing totheir absorption properties of NO2, SO2, and the aero-sols in the UV ~Ref. 50! and the UV-PFR channelharacteristics, Eq. ~A1! leads to a positive ~negative!ontribution in V for NO2 and aerosols ~SO2!; i.e., wenderestimate ~overestimate! V when we neglectO2 and aerosols ~SO2!. In contrast to the single-ifference retrieval, the reverse situation is obtainedor the double-difference pair, where the same ex-ression is valid if the nonprimed a are indexed with
and those with a prime with B. Under normaltmospheric conditions, incorporating exact amountsill have only a small effect on V unless there will belarge increase of these values in the atmosphere.
. NO2
NO2 column density is measured by the GOME.37 Inthe period from July 1999 to September 1999 the over-pass GPD2.7 Level-2 product for Arosa ranged from3 3 1015 to 6 3 1015 moleculesycm2, with an overalluncertainty between 10% and 20%. Considering ourNO2 estimates of 2 3 15 moleculesycm22, the changen the retrieved ozone-column density is below 61 DU
for all pairs. For larger values of NO2 the error when
Fig. 6. Relative variation of the ozone-column density aboveArosa from temperature and ozone profile weighted absorptioncoefficients for MODTRAN simulations from radiosonde data. Neg-tive data yield an underestimation of ozone-column density if thexed MLS absorption coefficients are used.
we neglect this contribution increases. However,NO2 measurements over Arosa since 1995 have rarelyexceeded 6 3 1015 moleculesycm2. The upper limitwas found to be '10 3 1015 moleculesycm2. The re-ported range is in good agreement with ground-basedNO2 observations from 1991 to 1993 over the Jung-fraujoch ~46.55 °N, 7.98 °E, 3580 m asl! by Van
oozendael et al.51 They report values between 1 31015 and 4 3 1015 moleculesycm2, with a measurementprecision of ;11%. As for the GOME measurements,he larger values were found during summer.
. SO2
Most of the global SO2 emissions are the result ofhuman activity, i.e., combustion of fossil fuels ~mainlycoal and oil! and smelting of ores. Some SO2 comesfrom volcanoes, either in eruptions or in continuouseffusive activity. Both anthropogenic and volcanicSO2 can be detected by the ERS-2 GOME instrument.Eisinger and Burrows52 report an estimated detec-tion limit of 0.4 DU ~1.08 3 1016 moleculesycm2! forcloud-free scenes at low solar zenith angles. Theyfound peak values for anthropogenic SO2 of 2.6 DU~6.99 3 1016 moleculesycm2! over the Balkans on 8February 1998. However, most data gathered overEurope yielded values below the detection limit. Inaddition, data for two volcanic eruptions are pre-sented with columns of as much as 6 DU ~1.612 31017 moleculesycm2! and 33 DU ~8.867 3 1017 mole-culesycm2!, respectively. To study the influence ofSO2 absorption, we set the absorber amount to aclean-air condition of 0.2 DU ~5.37 3 1016 moleculesycm2!. This value is two ~three! times the proposedvalue in MODTRAN 3.7 when the MLS atmosphere isselected for an observer altitude at 0 ~1850! m asl.We believe that this is an appropriate value for alocation situated in a clean Alpine environment, suchas Arosa. Depending on the wavelength pair, V var-es by as much as 0.4 DU for values near 300 DU withonsiderably smaller influence on the single-differenceairs 305y311, 305y318, and 305y332.
c. AerosolsInvestigations of aerosols in the estimation of ozone-column density were conducted with simulations inMODTRAN. Figure 7 shows the aerosol sensitivity inboth retrievals for 0° SZA when three different caseswere considered for boundary layer aerosol extinction,i.e., the tropospheric case with a visibility of 50 km andtwo rural cases with visibilities of 23 and 5 km, respec-tively. Note that we found almost the same resultsfor lower Sun elevations, as expected from Eq. ~A1!.
eglecting the aerosol term leads to an overestimationf V for all single-difference pairs and an underesti-
mation for the double-difference pair. However, theeffect for the double pair is small. This is not surpris-ing, as the wavelengths ~305y311 and 311y318! wereselected to minimize the aerosol influence. In gen-eral, the effect in retrieved ozone increases for largerturbidities, as the wavelength dependence is more pro-nounced in the optical depth difference. Owing to theseasonal variation of AOD at Arosa, with larger values
20 April 2001 y Vol. 40, No. 12 y APPLIED OPTICS 1999
h
tps
ci
2
during summer, we expect a lower overestimation inwinter. The simulated AOD’s at 500 nm are 0.137 forthe tropospheric case, 0.266 for rural 23 km, and 0.651for rural 5 km; i.e., the effect in V is almost propor-tional to an AOD at 500 nm. Comparing these datawith AOD measurements at 500 nm, we expect tur-bidities below the value of the rural case with a visi-bility of 23 km; see the AOD distribution in Fig. 8 forthe two months when we estimated ozone-column den-sity. In more than 80% of the time the AOD is evenbelow the tropospheric case. Thus the effect is nor-mally in the range of several Dobson units, with anoverestimation of as much as 15 DU for the worst case,the single pair 318y332. An example of this effect isshown in Fig. 5.
4. Clouds
Degradation in retrieved ozone-column density is ex-pected when clouds obscure the Sun. In such casesthe UV-PFR measures the directly transmitted sun-
Fig. 7. MODTRAN simulation of the aerosol term in the ozone re-rievals for the single-difference pair and the double-differenceair, from different built-in aerosol profiles along a vertical path topace ~0° SZA!. Neglecting the aerosol term leads to an overesti-
mation ~underestimation! for the single pairs ~double pair!.
Fig. 8. Distribution of aerosol optical depth measured at 500 nmfor Arosa during the period from mid-July to mid-September 1999.The AOD values of the MODTRAN simulations for the troposphericase ~3! and the rural case with visibility of 23 km ~1! are super-mposed. stdev, Standard deviation.
000 APPLIED OPTICS y Vol. 40, No. 12 y 20 April 2001
light and the forward-scattered scattered light. Forwavelength pairs that yield low ozone absorption atboth channels we expect the largest effect. The con-taminated data cause an additional optical depth tobe exhibited, resulting in a similar term for both re-trievals, as described in Eq. ~A1!. We used MODTRAN
simulations to study the sensitivity of cirrus clouds tocolumn-ozone density. For the three built-in cirruscloud models we found a maximum change in theretrieved V of only 60.5 DU for all pairs, normallyresulting in a slight underestimation when this termis omitted. For other types we applied no simula-tions and tried to study the influence of clouds fromthe measurements itself. By comparing the reducedcloud-free and full data set with Brewer or Dobsonresults at the LKO we can estimate such cloud deg-radations; see also Figs. 3 and 5.
5. Finite Filter Passbands
The retrievals are based on the assumption that theBeer–Lambert–Bouguer law is applicable with neg-ligible error. However, the effective spectral re-sponse function F9~l! of a single UV-PFR channel isthe product of the Sun’s extraterrestrial irradianceS~l! and the instrument’s response, F~l!. With in-creasing air mass, the general trend is a progressiveshift in the balance of F9~l! toward a longer effectivewavelength li. This is the result of increased ab-sorption and scattering of atmospheric constituentsat shorter wavelengths, mainly as a result of ozoneabsorption and Rayleigh scattering. Additionally,there are small-scale changes in the impression of theozone-absorption band structure on F~l!. Bothlarge-scale and small-scale changes lead to an en-hancement of the less attenuated components of afilter’s spectral response function. Thus an increasein the passband’s effective central wavelength is ob-served. The effect is greatest at the shortest wave-length, where the gradient is largest. For a detaileddiscussion of such effects regarding Dobson spectro-photometers and filter instruments the reader is re-ferred to Vanier and Wardle15 and Basher.19
We studied these effects with MODTRAN simulationsin which both the monochromatic filter passbands atli and the finite filters F~l! as given in Fig. 1 wereconsidered. We found an agreement in V estimatesto better than 1 DU for a SZA in the range from 0° to77.5° for both cases.
6. Error in Retrieved Ozone Caused by ParameterUncertainties
Measurement error is determined by m1, V, V9, V0,V09, a1, a19, tR, and tR9. For the double-differencepair @Eq. ~7!# the parameters for two difference pairs
ave to be considered. Errors of m1, tR, tR9, V, andV9 are neglected in the description here.
Because of the simultaneity of recordings to withina few seconds and because we limited our analysis to a,77.5° SZA, the value of air mass m1 when the pro-posed air mass expression is used is fairly accurate.An incorrect pressure value can lead to a differentvalue of Rayleigh scattering optical depth tR. Pres-
nfwt
ftcrl
sP
sure is measured at the UV-PFR facility every 10 minwith 0.1-hPa resolution and a precision of a few hec-topascals. Thus the change is less than 0.5 DU for allpairs. A possible model error is canceled by formingdifferences. Furthermore, the signals V and V9 weremeasured simultaneously within a few seconds to highaccuracy by a precision 16-bit data acquisition system.A critical signal-to-noise ratio can be obtained for theshortest UV-PFR wavelength at 305 nm near the up-per air mass limit of 4.62 as a result of strong atmo-spheric extinction. Ratios as low as 20 were observedfor some cloud-contaminated data, leading to uncer-tainties of a few Dobson units in V. However, with anincreasing Sun elevation of a few degrees the error isnormally reduced to less than 0.5 DU. In addition,such data points are often rejected by the data selec-tion procedure ~Section 4! and by the cloud opticaldepth limit of 2.5 at 500 nm.
Neglecting these uncertainties, we find that theabsolute error of V for the single-difference pair ~ j 51! and the double-difference pair ~ j 5 2! for the re-maining input parameters is composed of the frac-tional errors pj and qj that are due to errors of V0 anda1, respectively:
DV 5 Îpj2 1 qj
2. (A2)
a. Fractional Error That is Due to Uncertainty ofthe Calibration ConstantCalibration of filter radiometers is crucial for accu-rate measurements of optical depth and irradiance.Schmid et al.32 discuss the methodology of lamp andLangley calibrations of an 18-channel sunphotometerwith filters that cover the spectral range from 300 to1025 nm, whereas Slusser et al.53 explore the Langleycalibration method for UV multifilter rotatingshadow-band instruments. Here, the radiometriccalibration of V0~li! is based on the method of refinedLangley calibration54 and the general method of For-gan.55 First we performed a refined Langley calibra-tion of the 412-nm channel of the collocated standardPFR. Then the calibration of the UV-PFR channelswas related to the calibrated data of the 412-nmchannel within the general method. All the datawere reduced to an air mass range from 2 to 5. Thenthe modified Thompson t technique56 was applied forautomatic elimination of outliers in each data set.This procedure set an upper air mass limit of between2.6 and 3.4 for the 12 morning Langley calibrations.By rejecting four periods we obtained eight differentV0 values for each UV channel. From the average ofthese values found on different days the standarddeviation DV0 was determined to be 1.44% for 305
m, 1.05% for 311 nm, 0.86% for 318 nm, and 1.54%or 332 nm. However, each set of V0 for all channelsas found to be correlated. Thus we had to apply
he law of propagation for correlated variables57 toobtain the absolute fractional uncertainties, asshown in Fig. 9. The values are independent of Vand proportional to m1
21. The largest uncertaintiesare found for the single pair 318y332 with values upto 12.5 DU. The other single pairs and the double-
difference pair yield considerably smaller values, be-low 6 DU, depending on the pair.
b. Fractional Error That is Due to Uncertainty ofOzone-Absorption CoefficientsFor the UV ozone-absorption coefficients there aredifferent data sets; see, for example, Malicet et al.58
They report agreement to within 3%. These param-eters are certainly correlated, too; to what degree thisis the case is difficult to determine. Thus we tried toestimate the uncertainty by applying the law of prop-agation with a reduced uncertainty of 1% for a1 asobtained from MODTRAN. For a typical ozone-columndensity of V 5 300 DU we obtained uncertainties of6.2 DU for 305y311, 3.8 DU for 305y318, 3.0 DU for305y332, 5.6 DU for 311y318, 3.1 DU for 311y332, 3.2DU for 318y332, and 14.0 DU for the double pair. Incontrast with the data obtained for the calibrationconstants these values are air mass independent.These values are an estimate and could differ byseveral Dobson units according to the difficulties en-countered in modeling the uncertainty of the ozone-absorption coefficients.
7. Filter Stability Check
So far we have not addressed interference filter deg-radation or alteration of their shape over time. Suchchanges could lead to significant errors for ozone re-trievals, resulting in a drift in ozone-column density.For example, a 0.2-nm shift in the central filter wave-length for the 305-nm channel decreases the ozone-column density estimates by 10–20 DU for the singlepair containing the 305-nm channel and by '40 DUor the double pair. Therefore, knowledge of the fil-er response is necessary if one wishes to derive ac-urate ozone-column densities. In our case the filteresponses are characterized by central wavelengthsi and their passbands Dli as given by the manufac-
Fig. 9. Uncertainty p of V that is due to errors in V0 for theingle-difference and the double-difference retrieval of the UV-FR versus air mass.
20 April 2001 y Vol. 40, No. 12 y APPLIED OPTICS 2001
a~
BtC
11. J. B. Kerr, C. T. McElroy, and R. A. Olafson, “Measurement of
2
turer and summarized in Table 1. Normally, theeffects described here are also manifested in a filtercalibration change in V0; see, for example Schmid et
l.32 Because of the altitude of the LKO facility1850 m asl!, V0 calibrations performed with Langley
plots are adequate for verifying the filter stability; seealso Bigelow and Slusser.59 However, for the two-month period we found no indication of filter degra-dation, as was also true for laboratory recalibrationmeasurements of the instrument at PMODyWRC af-ter three months.
The MODTRAN program was made available by theU.S. Air Force Research Laboratory at Hanscom AirForce Base, Massachusetts. We thank the NASAyGoddard Space Flight Center TOMS Ozone ProcessingTeam for providing Earth Probe TOMS overpass dataand B. Schmid for helpful comments. ERS-2 GOMElevel-2 data were processed at the DeutschesFernerkundungsdatenzentrum (DFD) on behalf of theEuropean Space Agency. Overpass data were pro-vided by the Belgian Institute for Space Aeronomy aspart of the Environment AO1-158 project. This re-search was supported by the Swiss National ScienceFoundation under grant 2000-049426.96, by theSwiss Meteorological Institute—MeteoSwiss as partof the CHARM project, and by the Bundesamt fur
ildung und Wissenschaft as part of the characteris-ics of the UV radiation field in the Alps, Europeanommunity contract ENV-497-0575.
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