Extraterrestrial Solar Spectrum

Akimoto, Atmospheric Reaction Chemistry, Springer Atmospheric Sciences, DOI 10.1007/978-4-431-55870-5 3 

Wavelength (nm) Radiation intensity (photons cm s 5 nm ) Wavelength (nm) Radiation intensity (photons cm s 5 nm ) Wavelength (nm) Radiation intensity (photons cm s 5 nm ) 

100-300 km above the ground, but the absorption and photolysis of N2 will not be discussed in this book since it focuses on the troposphere and the stratosphere. 

Absorption spectrum, cross sections and energy diagram of molecular O2, the second major component of the earth s atmosphere, are shown in Fig. 3.4, Table 3.2, and Fig. 3.5, respectively. As shown in Fig. 3.4, O2 has a strcmg crmtinuum called the Schumann-Runge Continuum (S-R Continuum) in the 130-175 nm region, which corresponds to the allowed transition in Fig. 3.5. This 

The photolysis of O2 at the Schumann-Runge system is an important chemical process to form the ozone layer in the stratosphere. The chemistry of the stratosphere will be discussed in Chap. 8. The absorption spectmm of O3 formed there plays an important role to determine the vertical distribution of the solar spectrum at different altimde and also the shorter wavelength edge in the troposphere. 

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Radiometric quantities are important to describe and measure UV and VUV radiation. They are usually subdivided into energetic, spectral and photonic terms. Energetic terms (Tab. 3-9) are based on the energy of the radiation and they refer to all relevant wavelengths. Eor each of these terms a spectral derivative can be defined (Bolton, 1999) which is correlated with a specific wavelength X. Eor example, the extraterrestrial solar spectrum incident on the upper atmosphere is represented by the mean spectral irradiance Eq in W m nm over a defined wavelength interval AX in nm (CIE, 1989). Further, each of the spectral units can easily be transferred to photon-based units, which themselves are related to radio-metric units (cf Braun et al., 1991). 

Switzerland, yielded the spectrum reproduced in Fig. 5.40. The numerical values upon which this diagram is based can be found in M. Iqbal [5.34]. The maximum of EfAn lies in the visible light region at A pa 0.45 /tm. 99 % of the irradiance falls in the wavelength band A < 3.8 fim. Fig. 5.40 also shows the spectral irradiance EXa of the radiation emitted by a black sun at Ts = 5777 K. The areas under the two curves (up to A —> oo) are equal — they each yield the solar constant E0 —, but the spectrum of the extraterrestrial solar radiation deviates significantly at some points, in particular at A < 0.6//.in, from the spectrum of radiation from a black body. 

FIGURE 2 Extraterrestrial solar irradiance, measured by a spectrometer onboard an Earth-orbiting satellite. The UV spectrum (119 < 1 < 420 nm) was measured by the SOLSTICE instrument on the UARS satellite (modified from a diagram provided by G. J. Rottmann, private comm. 1995). The vertical lines divide the various spectral subranges defined in Table I. The smooth curves are calculated blackbody spectra for a number of emission temperatures. 

Fig. 5.1 Solar spectrum comparison between extraterrestrial and ground level fluxes. The data used are taken from Ref. [4]...

There are two critical outcomes to the patterns of upper atmosphere ozone levels, which are vital to life on Earth. The first is the vertical layering of the atmosphere as already described. The second is the fact that, after O2, the O3 filters out virtually all the remaining UV-B (280-315 nm) from the solar spectrum at the Earth s surface. The DNA damage inflicted by extraterrestrial levels of UV would severely restrict the survival of life forms at the surface it is O2 combined with O3 which provides the essential UV filter. 

WMO Wehrli Standard Extraterrestrial Solar Irradiance Spectrum. World Meteorological Organization, Geneva (1985)... 

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