Environmental Engineering Reference
In-Depth Information
Terrestrial solar spectra
As a result of the diurnal rotation of the Earth the solar elevation angle and thus
the length of the path through the atmosphere changes in the course of the day. This
gives rise to a variation of the spectral composition and the intensity of terrestrial solar
radiation. Fig. 4 shows the spectral irradiance in the UV range of the solar spectrum on
ground at different times of the day respectively at different solar elevation angles,
measured at Neuherberg near Munich (11.6° E, 48.22° N, 495 m asl.), Germany, on 17
April 1996. The short wave edge (the 'ozone edge') is shifted towards shorter
wavelengths with increasing solar elevation angle. At low solar elevation angles, UV-B
radiation is more attenuated as compared with longer wavebands because of the more
pronounced scattering and absorption with decreasing wavelength. From Fig. 4 we
conclude that the measurement of solar spectral UV irradiances has to cover at least 6
orders of magnitude within a small wavelength range of 30 nm. This implies very
stringent demands on the technical equipment used for such measurements as will be
discussed in section 3.2.
50°
10 0
40°
30°
20°
10°
10 -1
10 -2
10 -3
10 -4
10 -5
10 -6
300
320
340
360
380
400
W avelength,
λ
(nm )
Figure 4. Typical UV spectra for different solar elevation angles measured on a clear sky day 2 .
Further prominent factors influencing the transfer of UV radiation through the
atmosphere are the total ozone column, aerosols, clouds and the ground albedo. As
mentioned above, ozone strongly absorbs below 340 nm. In good approximation, the
UV-B irradiance shows an exponential dependence on the total ozone column as it is
expected from Lambert-Beer's rule. The ground albedo which is the ratio between the
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