data on total and spectral solar irradiance

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  • Data on total and spectral solar irradiance

    Ann T. Mecherikunnel, James A. Gatlin, and Joseph C. Richmond

    This paper presents a brief survey of the data available on solar constant and extraterrestrial solar spectralirradiance. The spectral distribution of solar radiation at ground surface, computed from extraterrestrialsolar spectral irradiance for several air mass values and for four levels of atmospheric pollution, is also pre-sented. The total irradiance at ground level is obtained by integration of the area under the spectral irra-diance curves. It is significant that, as air mass increases or as turbidity increases, the amount of energy inthe infrared relative to the total increases and that the energy in the UV and visible decreases.

    1. IntroductionQuantitative data on solar irradiance at ground lo-

    cations are needed in the study of atmospheric optics,pollution, solar energy for energy conversion, climato-logy, and remote sensing. Total and spectral solar ir-radiance at a particular site at ground level depends onseveral parameters such as altitude, latitude, longitude,earth-sun distance, cloudiness, time of day, and atmo-spheric attenuation due to ozone, water vapor, carbondioxide, and dust particles. Solar irradiance data atground level necessary for several applications can beobtained from the extraterrestrial solar spectrum bycomputing the fractional loss due to various atmo-spheric constituents. Accurate data on extraterrestrialsolar irradiance and atmospheric attenuation factorsare needed for this.

    This paper will present a brief account of the currentstatus of solar constant and air mass zero solar spectralirradiance values. Computed values of solar spectralirradiance at ground level for air mass values, 1, 1.5, 2,3, 4, 7, and 10 for four sets of af, Angstr6m turbidityparameters, 20 mm of precipitable water vapor, and3.4-mm ozone from NASA/ASTM Standard solarspectral irradiance are also presented in graphicalform.

    11. Solar Constant and ExtraterrestrialSolar Spectral Irradiance

    Solar radiation is usually described in terms of solarspectral irradiance and the solar constant. The solarconstant is the amount of total radiant energy (usually

    Joseph Richmond is with U.S. National Bureau of Standards,Washington, D.C. 20234; the other authors are with NASA GoddardSpace Flight Center, Greenbelt, Maryland 20771.

    Received 19 August 1982.

    expressed in W * m-2) received from the sun per unittime, per unit area exposed normal to the sun's rays, atthe mean earth-sun distance in the absence of theearth's atmosphere. Air mass zero solar spectral irra-diance is the distribution of this power (surface) densityas a function of wavelength.

    Earlier estimates of the solar constant and the solarspectral irradiance were obtained from ground-basedmeasurements. The most extensive investigations ofsolar constant and the spectral distribution of solarradiant flux were those made by the Smithsonian In-stitution from high altitude ground stations between1902 and 1950. The Moon' and Johnson 2 values of thesolar constant and the spectral energy distributionwidely used before 1970 were derived from the Smith-sonian data, with additive corrections for the UV andIR regions of the solar spectrum attenuated by the at-mosphere. Measurements made from the ground arelimited in accuracy due to the strong and highly variableabsorption and scattering characteristics of the atmo-sphere. In recent years several attempts have beenmade to minimize the errors due to the atmosphere bymaking measurements from high altitude platformssuch as aircraft, balloons, satellites, and rockets. Ex-perimental results obtained independently by severalgroups of investigators from high altitude flights duringthe 1960s resulted in the solar constant and air masszero solar spectral irradiance data 3 4 widely used in thedesign criteria for NASA space vehicles 5 and as an en-gineering standard of the American Society of TestingMaterials (ASTM).6 The NASA/ASTM6 standardsolar constant value 1353 i 21 W m-2 is an average ofthe results derived from nine series of measurementsusing different types of total irradiance detector, dataacquisition, and analysis technique. Two major sourcesof uncertainty are associated with the standard values.(1) Most of the solar irradiance determination requiredcorrections for the residual atmosphere and water vaporabove the aircraft or balloon and the transmittance of

    1354 APPLIED OPTICS / Vol. 22, No. 9 / 1 May 1983

  • the optical window materials. (2) The radiation de-tectors used in these measurements were calibrated inthree different radiometric scales: the InternationalPyrheliometer Scale 1956 (IPS 56), the Absolute Elec-trical Units Scale, and the Thermodynamic KelvinTemperature Scale.7 The difference between the threescales is not known accurately. Only three of the ninetotal irradiance measurements were performed withelectrically calibrated absolute radiometers. The otherdeterminations were made with pyrheliometers or ac-tinometers radiometrically calibrated by comparisonwith a reference instrument using IPS 56 scale. Thestability of the reference instruments for the IPS 56 wasquestioned at the International Pyrheliometric Com-parisons, and a series of solar irradiance determinationswith several absolute cavity radiometers resulted in anew pyrheliometric scale which is -2.2% higher than theIPS 56.8,9 Since the different experimental results usedin the derivation of the NASA/ASTM standard valuecould not use the same primary standard for the cali-bration of the irradiance detectors, a reduction to acommon reference is needed to enable direct compari-son of the values obtained by different experimenters.Frohlich reviewed the various solar constant determi-nations and reduced the values to a common referencescale (the Solar Constant Reference Scale, SCRS), andthe most probable value of the solar constant is pro-posed to be 1373 W * m 2 .8 A similar reduction of theindividual measurements used in the derivation of theNASA/ASTM standard solar constant to a commonreference scale based on electrical power equivalencewas reported by Forgan.10 According to him the solarconstant value is 1375 W . m- 2 compared with 1353W.m- 2 .

    The uncertainty of solar constant measurements hasimproved significantly of the order of 0.5% in recentyears, because of the development of self-calibratingpyrheliometers and the availability of space measure-ments. Several long-term solar constant monitoringprograms have been started in recent years to measurethe solar constant and its variability with self-cali-brating pyrheliometers from space to study the effectof solar irradiance variations on weather and climate ofthe earth. The results from Rocket Experiments of1976,11 1978,9,12 1980,9,13 Nimbus-7 Earth RadiationBudget Experiment (ERB),14 and the Active CavityRadiometer Irradiance Monitor (ACRIM)15 on theSolar Maximum Mission (SMM) satellite are summa-rized in Table I.Table 1. Average Value of Solar Constant Measured from Rocket Flights,

    Nimbus-7 ERB and the SMM ACRIM Experiments

    Solar Estimatedconstant error

    Platform/sensor Year W . m- 2 (%)Rocket experiment" June 1976 1367 +0.5Rocket experiment/ACR 9 Nov. 1978 1367.6 +0.5Rocket experiment/ACR 9 May 1980 1367.8 +0.5Nimbus-7/ERB,H-F1 6 Nov. 1978 to 1372.7 40.4

    July 1981SMM/ACRIM 9 Feb. to 1367.7 +0.2

    July 1980

    In many applications of solar irradiance values, bothtotal and spectral, a question of major concern is thevariability of these values. The solar constant is de-fined for the average sun-earth distance. As the earthmoves in its elliptical orbit around the sun, the totalsolar energy received varies by t3.5%. There are alsosmall and undetermined variations due to cyclic andsporadic changes in the sun itself. Temporary de-creases of 0.1-0.2% in solar total irradiance lasting fora few days and associated with the passage of sunspotgroups have been reported by Willson et al. 9 1 5 andHickey et al. 14 16 But the analysis of the solar constantdeterminations from 1965 to 1980 by Frohlich andBrusa revealed no indication of any significant changein the solar constant value.17

    Ill. Air Mass Zero Solar Spectral IrradianceSolar constant determination from space with active

    cavity radiometers has significantly reduced the un-certainty in solar constant values of the order of t0.5%.But no definitive measurements of solar spectral irra-diance have been made from space. Measurements ofextraterrestrial solar spectral irradiance in selectedspectral bands in the UV, visible, and near-IR regionsof the solar spectrum have been made by filtered solarchannels of both Nimbus-6181 9 and 720 ERB experi-ments. Preliminary analysis of the data from bothERB instruments shows that the filters have undergonesignificant degradation in space.21 22 According toPierce and Allen23 the best presently available data ofsolar spectral irradiance in the interval from 0.3 to 3.0um are those given by Thekaekara et al.,3,5 Arvesen et

    al. ,24 and Labs and Neckel.25 The spectral irradiancedata reported by Thekaekara et al. and Arvesen et al.are based on NASA Convair 990 aircraft (11.6-12.5-kmaltitude) measurements between 1967 and 1969. Labsand Neckel data for the spectral region from 0.33 to 1.25,m were derived from absolute intensity measurementsat the center of the solar disk made in the early 1960sat Jungfraujoch Scientific Station (3.6-km altitude) inSwitzerland. Beyond 1.25 m, Labs and Neckel usedthe values reported by Pierce and Allen.23 26 Neckeland Labs recently published a revised version of thesolar spectral irradiance data given in Ref. 25 using moreaccurate values for center to limb variation of the solardisk in the conversion of center disk intensities intomean disk intensities.27

    Figure 1 shows a comparison of solar spectral irra-diance values for the 0.3-1.2-,um solar spectr


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