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Ground-Based Spectral Measurements of Solar Radiation (II) - Global and Diffuse Sky Radiation

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Ground-Based Spectral Measurements of Solar Radiation (II) - Global and Diffuse Sky Radiation Ground-based Spectral Measurements of Solar Radiation (II) - Global and Diffuse Sky Radiation - by Keizo Murai, Masaharu Kobayashi, Ryozo Goto and Toyotaro Yamauchi Meteorological Research Institute, Tokyo (Received December 2, 1978) Abstract A newly designed spectro-pyranometer was used for the measurement of the global (direct+diffuse) and the diffuse sky radiation reaching the ground. By the subtraction of the diffuse component from the global radiation, we got the direct radiation compo- nent which leads to the spectral distribution of the optical thickness (extinction coeffi- cient) of the turbid atmosphere. The measurement of the diffuse sky radiation reveals the scattering effect of aerosols and that of the global radiation allows the estimation of total attenuation caused by scattering and absorption of aerosols. The effects of the aerosols are represented by the deviation of the real atmosphere measured from the Rayleigh atmosphere. By the combination of the measured values with those obtained by theoretical calculation for the model atmosphere, we estimated the amount of absorp- tion by the aerosols. Very strong absorption in the ultraviolet region was recognized. absorption alone. To clarify the character- 1. Introduction istics of absorption by the aerosols, it is In recent studies on the solar radiation, necessary to make some additional measure- many authors have investigated the absorp- ments of the solar radiation. And theoretical tion by the aerosol particles in the atmos- calculations are needed for comparison with phere (e. g. Drummond and Robinson, 1974 ; the measurements, assuming various values Robinson, 1962 and 1966 ; Liou and Sasamori, of the parameters contained in the transfer 1975). In them, the optical property of the equation in the turbid atmosphere. aerosol, especially the imaginary part of the For the purpose of studying the aerosol refractive index of the particle, is the es- absorption under various atmospheric condi- sential parameter to determine the amount tions, we made the spectral measurements of absorption by the aerosols. of the global and the diffuse sky radiation_ In a previous paper (Murai et al., 1977), at the ground surface, as well as of the we described the analysis of the measure- direct solar radiation. We further analysed ments of the direct solar radiation and the the data of the diffuse sky radiation with year-to-year variation of the extinction co- the aid of the theoretical calculation for the efficient of aerosols in Tokyo. The extinction model atmosphere. In this report, we de- coefficient thus obtained represents the total scribe the effects of the aerosols on the attenuation due to the scattering and the global and the diffuse sky radiation and absorption of the aerosols, but does not give estimate the absorption of solar radiation in us any information on the scattering or the the atmosphere. 2. Instruments and Measurement ation component. The direct componemt thus obtained for various optical air masses The spectro-pyranometer was used to was applied to the long method to derive measure the spectral distributions of global the spectral conversion factor corresponding and sky radiation reaching the ground. As to the extraterrestrial value of solar radi- the details of the instruments were explained ation. According to these conversion factors, in the previous paper (loc. cit.), our descrip- all measured values are represented by the tion of them will be necessarily brief. The fractions relative to the solar radiation in- instrument is composed of a double mono- cident on the unit horizontal surface at the chromator, an integrating sphere, a disk for top of the atmosphere. shielding the direct beam, an amplifier and a data recording system. A diffraction grat- ing and a quartz prizm are combined for 3. Results of Measurement building up the double monochromator, so The spectral distributions of the extinc- that the dispersion of wavelengths is nearly tion coefficient were calculated from the constant throughout the wavelength region direct components which were derived by for the measurement. The integrating sphere the subtraction described above. An example providing the receiving surface of incident of the relative values of the global, G(2), fluxes is attached to the entrance slit of the and the diffuse sky radiation, D(2), measured monochromator. The scanning of wave- is shown in Fig. 1. In the figure, GR(2) and lengths from 0.35 to 2.00 pm is performed by DR(2) represent the values of the global and a pulse motor with which we can fix the the diffuse sky radiation in the Rayleigh wavelength interval for the data sampling. atmosphere, respectively. An example of the The time needed for the scanning with wave- spectral extinction coefficient determined length interval 250A is about 10 min. and from the direct components is shown in Fig. during the scanning the light quantity enter-' 2. In the figure, the upper curve represents ing the monochromator is reduced by the the total extinction of the vertical column neutral density filters to get a suitable value of the atmosphere and the lower one the of photocurrent for the data recording. The extinction due to the aerosols. The hatched shade disk is for interrupting the direct area represents the ozone absorption in the beam radiation, and the global and the dif- Chappuis band. To get the spectral values fuse sky radiation were alternately measured of ozone absorption we used Vigroux's ab- with and without the disk respectively. sorption coefficients and the total amount The measurement was carried out in a of ozone obtained by routine observation at cloudless sky to clarify the effects of the Tateno (36°03'N; 140°08'E). To remove the aerosol particles on the solar radiation. The effect of water vapor absorption in the near data used for analysis were obtained from infrared region, we used the smooth curves measurements on six days during the period which pass through four measured values from January to March in 1978 in Tokyo. at 0.8, 1.05, 1.25 and 1.55 pm instead of the In these measurements, the sky was clear original measured curves. The absorption throughout the day and the time sequences at the above four wavelengths is negligibly of the global and the diffuse sky radiation small. were obtained by the continuous automatic According to the procedures described operation of the spectro-pyranometer. Thus above, we got the spectral distributions of we easily got the values for each component the aerosol extinction coefficient for all cases at the time corresponding to the optical air as shown in Fig. 3. For the sake of com- mass desired for the analysis. By the sub- parison, the distribution curve proportional traction of the diffuse component from the to 2. written in the figure. On average, global radiation we got the direct solar radi- the distributions in the shorter wavelength Fig., 1. Spectral distributions of global and diffuse sky radiation. C(A) and D(A) are measured distributions represented by the unit relative to the energy incident on the unit horizontal surface at the top of the atmosphere. G R(2) and D R(2) represent distribu- tions for the Rayleigh atmosphere. Fig. 2. Spectral distribution of the extinction Fig. 3. Spectral distributions of the aerosol coefficient calculated from the direct extinction coefficient obtained in the radiation component. r (A) represents the total extinction coefficient, rm(2) period of the measurement. the aerosol extinction coefficient. Hat- ched area shows the ozone absorption component. region are steeper and in the longer, flatter than 2-'-distribution. The size distributions corresponding to these extinction curves are calculated by using the inversion technique (Yamamoto and Tanaka, 1969) and are shown in Fig. 4. The absolute energy distributions of the global and the diffuse radiation meas- ured are represented in Fig. 5. The absolute values were obtained from the relative values represented in Fig. 1 as an example, multi- plied by the absolute values of solar radi- ation incident on the unit horizontal surface at the top of the atmosphere. We used the Table of the absolute energy of the extra- terrestrial solar radiation published by NASA (Thekaekara, 1971). In the figure, we can see that the spec- Fig. 5. Spectral distributions of the absolute tral distribution of the global radiation on energy of global and diffuse sky radi- Feb. 20, the most turbid case in our meas- ation. /0 represents the extrater- urements, is quite different from that on restrial irradiance, GR and DR the Jan. 25, the clearest case. For comparison, Rayleigh atmosphere, and G and D the distribution for the Rayleigh atmosphere the measured. is shown in the figure. The deficit of the measured values to those of the Rayleigh atmosphere generally increases with the in- crease of the extinction coefficient, and a part of it appears as the excess of the measured diffuse radiation reaching the ground over the Rayleigh case. The rest of the deficit is partly scattered out to the space and partly absorbed by the aerosols. The excess of diffuse radiation in the visible and the near infrared regions gener- ally increases with the increase of aerosol extinction, but in the region shorter than about 0.4 pm the excess becomes zero or nega- tive, the negative amount increasing with the increase of the extinction (See the following section). To compare the measurements with the theoretical calculations, the spectral dis- tributions of diffuse radiation for the model atmosphere are shown in Fig. 6 (Yamamoto, Tanaka and Ohta, 1974), in which the ex- tinction coefficient of aerosols, VM(2), is re- presented by Fig. 4. Size distributions of aerosol particles corresponding to the spectral distri- the refratictive index ii=-1.50-0.10i and butions of extinction coefficient re- =1.50-0.03i, and the surface albedo A,=0.15.
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