Solar spectral actinic flux

Table 4.2 tabulates the solar spectral irradiance, normalized to a solar constant of 1367 Wm , Solar spectral actinic flux at the surface (0km), 20, 30, 40, and 50km is shown in Figure 4.11. 

FIGURE 4.11 Solar spectral actinic flux (photons cm-2 s 1 nm"1) at various altitudes and at the Earth s surface (DeMore et al. 1994). 

The direct physical measurement of the spectral actinic flux F(X) is not easy, although attempts have been made (Shetter and Muller 1999 Hofzumahaus et al. 1999). Generally, irradiance E(k) (radiation flux per unit area, W nm ) is measured by radiometers, and experiments to compare solar spectral intensity in the field with radiative transfer models have been made in order to convert the spectral irradiance E(X) to F(X). In these analyses, downward actinic flux Fd (A) is obtained by upper-hemispherical integration of observed spectral radiance L(k,9,4>) (radiation flux per solid angle, W sr m nm ), and F (A) is expressed as the sum of the flux of direct radiation Fq (X) and downward diffusive... 

In the spectral region near 120 nm, the solar hydrogen emission line at Lyman a represents an important source of ionization and dissociation. During quiet periods, it contains more energy than the rest of the spectrum at shorter wavelengths. The total flux of this line as well as its shape vary with solar activity. The actinic flux varies between a minimum of about 3.0 x 1011 photons cm 2s 1 for quiet solar activity and a maximum of about 6.0 x 1011 during high solar activity. 

For wavelengths below about lOOOnm (1 pm), the predominant absorbing species in the atmosphere are 02 and 03 (Figure 4.9). The spectral solar actinic flux at various altitudes, as shown in Figure 4.11, can, therefore, be computed fairly accurately by considering only the absorption by 02 and 03. The absorption cross sections of 02 and 03 are shown in Figures 4.12 and 4.13. All the absorption shown in Figure 4.12 and 4.13 leads to dissociation. 

Kazandzis, S., Bais, A.F., Balis, D., Zerefos, C., Blumthaler, M. Retrieval of downwelling UV actinic flux density spectra from spectral measurements of global and direct solar UV irradi-ance. J. Geophys. Res. 105, 4857-4864 (2000)... 

The objectives of the study presented in this paper were to observe and characterize phototransients produced by laser excitation of natural waters and humic substance (HS) solutions. The photosystems were studied on two scales. On the laboratory scale pulsed laser flash photolysis was used to study the time resolved and spectral behavior of the photochemical transients. Studies to Identify and quantify the transients Included adding energy and electron acceptors and model compounds to the solution and varying parameters such as pH, metal, and oxygen content. On the field scale laboratory data taken at solar actinic wavelengths Is extrapolated using published solar photon fluxes to predict environmental effects of the phototransients studied In this work. This paper thus contains an overview of many experiments performed over four years (16). 

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