Total and spectral solar irradiance measured at ground surface

Ann Mecherikunnel and Charles H. Duncan

This paper presents the results of a series of total and spectral solar irradiance measurements made atground surface (Table Mountain Facility, Calif., altitude 2.18 km). The spectral irradiance data are pre-sented for the 0.3-3.0-/im spectral region for air mass 1.5.

1. Introduction

Measurements were made to determine the total andspectral irradiance at ground surface due to the directirradiance from the sun on 26, 27 Feb. 1972. The solarirradiance data were needed in the calibration of themulti-spectral scanner (MSS). The impetus for thispaper at this time is the request from several scientistshere and abroad for solar spectral irradiance valuesmeasured at ground surface, especially for the 1-3.0-Irmspectral region. Very little experimental data areavailable in the literature for the spectral region. Totalirradiance measurements were made by Charles H.Duncan, and the spectral irradiance measurements weremade by the late M. P. Thekaekara.

II. Instrumentation

Direct total irradiance was measured with anAngstr6m pyrheliometer mounted on a heliostat. Solarspectral irradiance measurements were made with aLeiss quartz double-prism monochromator and a Per-kin-Elmer LiF prism monochromator. The three in-struments, Angstr6m pyroheliometer and Perkin-Elmerand Leiss monochromators, are described in literatureas they are the same instruments used for solar irra-diance measurements onboard NASA 711 in 1967.1-3Table I gives a listing of the energy detector and thespectral region pertinent to each instrument.

III. Measurement

Total irradiance measurements were made with theAngstr6m pyrheliometer mounted on the heliostat totrack the sun. The principle of the measurement is thatof balancing the temperature of two metal strips, oneof which is exposed to the radiation and the other

Both authors were with NASA Goddard Space Flight Center,Greenbelt, Maryland 20771, when this work was done; C. H. Duncanhas now retired.

Received 30 July 1981.

heated by an electric current. The calibration is withreference to the International Pyrheliometric Scale (IPS56).

Solar spectral irradiance measurements were madewith the Leiss double prism and the Perkin-Elmermonochromators. The calibration of the monochro-mators was based on NBS quartz-iodine lamps.4 Mostof the spectral scans were for the 0.3 -1.2-ym spectralregion. But two spectral scans were made on both daysfor the 0.3-3.0-Mum spectral region with the Perkin-Elmer monochromator. The spectral irradiance wasmeasured at 0 .01-ym intervals except at the absorptionbands, where closer intervals adequate for showing thewidth and depth of the bands have been chosen. Theamount of precipitable water vapor during the obser-vation time was -2 mm, very low compared to 19 mm,which is the average at sea level for midlatitudes. Theair-mass values were computed from the known valuesof ephemeris transit, solar declinaton, latitude, andlongitude of the place. The sun-earth distance for thetwo days of measurement was very close to average: 26Feb.: 0.989 a.u.; 27 Feb.: 0.990 a.u.

IV. Results and Discussion

The output of the pyrheliometer when multipled byits known calibration factor gives the total irradiancedue to the sun. The total irradiance observed remainedfairly constant, -1055 W m-2 throughout the obser-vation of 27 Feb. 1972. Table II gives the total irra-diance measured as a function of time. This irradiancealso includes a small portion of the circumsolar sky ir-radiance.

The direct solar spectral irradiance on Table Moun-tain measured for the 0.3-3.0-Am spectral region for airmass 1.5, time 10:34 h is presented in Table III and ingraphic form in Fig. 1. It is based on the data of 27 Feb.1972 measured by the Perkin-Elmer and the Leiss. Theirradiance is that of the whole solar disk, excluding thecircumsolar sky.

The air mass, also known as the optical air mass, de-scribed above refers to the altitude and the location and

554 APPLIED OPTICS / Vol. 21, No. 3 / 1 February 1982

N~~~~w 1600 \ ~~~~~~~~~~Sol. Sectral Irdae

1250 Tabte Moutin . C.l F.b 27. 1972Ta.m 10:34 H,,Ai, Mass 1.5

1000

= 50

0 .2 0.5 1.0 1.5 2.0 2.5 3.0

WAVELENGTH (MICROMETER)

Fig. 1. Solar spectral irradiance for the 0.3-3.0-/tm spectral region, measured at ground surface, Table Mountain, Calif. [altitude 2.18 km(7200 ft)] on 27 Feb. 1972. Time 10:34 h. Solar zenith angle 48.20; air mass 1.5. Precipitable water vapor 2 mm.

Table I. Energy Detector and Spectral Region Data.

Wave-length

Instrument rangeInstrument Energy detector type (Am)

Angstrbm Resistance strip Total 0.3-4.0pyrheliometer

Perkin-Elmer 1 P 28 tube LiF Prism 0.3-0.7mono- thermocouple 0.7-4.0chromator

Leiss mono- EM 1 9558QA Quartz double 0.3-0.7chromator photomultiplier prism

tubePbS 0.7-1.6

not to the sea level. It is the relative air mass mr andis equal to the secant of solar zenith angle (seez).5 Ina plane-stratified atmosphere

secz = mr, (1)

in which refraction may be neglected, secz is equal tothe ratio of the optical path lengths in the oblique andvertical directions.5 The absolute air mass m may beestimated from the relative air mass mr from Eq. (2),where p is the pressure at the experiment site,

(P)- - ,r( " (2)

and po is the normal sea-level pressure. 5

The first column in Table III gives the wavelength inmicrometers. The second column gives the spectralirradiance, and the third column gives the integratedirradiance 0 to X. The total irradiance due to the solardisk is 1027 W m- 2, which is lower than the value mea-sured by the Angstrom pyrheliometer, because the py-rheliometer readings include irradiance from a smallportion of circumsolar sky of a half-cone angle of -3°.The spectral irradiance values (Ex) in Table III and Fig.1 are for the energy received per unit area exposednormally to the sun's rays (solar zenith angle Z = 48.2°).The spectral irradiance received on the ground is con-fined to the 0.3-3.0-,gm spectral region. Essentially, allthe solar radiation incident below 0.3 Am in the UV re-gion is absorbed by high-altitude atmospheric ozone.At wavelengths >3.0 gm in the IR range almost all the

Table II. Total Solar Irradiance Measured, With Angstrom Pyrheliometer,27 Feb. 1972 at Table Mountain, Calif.

Time

10:1510:2010:2510:3010:3510:4010:4510:5010:5511:00

11:0511:1011:1511:2011:2511:3011:3511:4011:4511:5011:55

12:0012:1512:2012:2512:3012:3512:4012:4512:5012:5513:0013:15

Total irradiance (W m-2)

1054105510551055105510551055i05610571056

10501054105310541055105510581061106110601058

106310561068105910561055105610501056105410431043

energy is absorbed by atmospheric water vapor andcarbon dioxide. The energy distribution in the 0.3-3.0-,m spectral region received at the ground surfaceis very sensitive to changes in the atmosphere. Thenormal incident direct solar spectral irradiance Ex canbe related to the atmospheric transmittance T by thefollowing equation:

Ex = Eox T, (3)

where EoN is the extraterrestrial solar spectral irra-diance.

1 February 1982 / Vol. 21, No. 3 / APPLIED OPTICS 555

Table ll. Solar Spectral Irradlance Measured at Table Mountain, Calif., 27 Feb. 1972: Time 10:34 hours, Air Mass 1.5, Solar Zenith Angle 48.2°= Wavelength In micrometers

EX = Solar spectral Irradlance averaged over small bandwidth centered at X, In W m- 2jum-

Eo-x = Integrated solar Irradlance In the o-X wavelength range, in W m- 2

Spectral irradianceWavelength Ex

A, (uM) (W M-2 m-1

0.30.310.320.330.340.350.360.370.380.39

0.400.410.420.430.440.450.460.470.480.49

0.500.510.520.530.540.550.560.570.580.59

0.600.640.670.680.700.710.7250.730.750.757

0.770.800.8020.8050.820.8360.840.850.860.87

546

152300375438461566557594

803103810971068124113861473152015661545

1530149014651450143914231376138514011385

1373133612901279124212251140118011201120

10951037985

1020995960880936910885

Integrated irradiance 0-XEo-x

(W m-2)

00.261.243.506.9

10.915.420.626.231.9

38.948.158.869.681.294.3

108.6123.6139.0154.5

170185200214229243257271285299

312367406419444456474480503511

525557559562577593597606615624

Spectral irradianceWavelength Ex

X, (m) (W m- 2 ym-')

0.880.890.8950.910.920.930.940.9550.971.0

1.051.11.121.1391.151.1651.1811.191.2251.27

1.291.3051.331.351.3751.381.441.4871.521.54

1.581.711.81.881.922.12.22.32.42.5

2.63.0

850800780770735695624725760713

637540450330370499483474447405

377341383230110100183238275265

24819314730188776656231

100

Integrated irradiance 0-XEo-W

(W M-2)

633641645656664671678688699721

755784794801805812820824840859

867872881888892892901911919925

935964979986987996

1004101210181023

10251027

References1. M. P. Thekaekara, R. Krueger, and C. H. Duncan, Appl. Opt. 8,

1713 (1969).2. M. P. Thekaekara, "The Solar Constant and the Solar Spectrum

Measured From a Research Aircraft," NASA TR R-351 (1970).3. M. P. Thekaekara, Sol. Energy 14, 109 (1973).4. R. Stair, W. E. Schneider, and J. K. Jackson, Appl. Opt. 2, 1151

(1963).5. N. Robinson, Solar Radiation (Elsevier, New York, 1966), Chap.

3, pp. 48-52.

556 APPLIED OPTICS / Vol. 21, No. 3 / 1 February 1982