Solar Irradiance at the Earth

The choice of the light source—form, emitted wavelengths, radiant power— depends inter alia on whether Ti02 is unsupported or supported, and on the type and shape of the supporting material. For example, a Ti02-coated flexible material can be wrapped around a cylindrical lamp placed inside the reactor. A plate covered by Ti02 can be installed perpendicularly to the beam of a lamp located outside the reactor. Obviously, the choice of the lamp, especially of the emission characteristics, also depends on the objective. For instance, in view of solar photocatalytic applications, lamps mimicking solar irradiation at the Earth s level or a solar box can be used in the laboratory. 

As can be seen from Fig. 15.9 and from Tables 15.3 and 15.4, the solar irradiance at the surface of the earth shows a sharp decrease in the uv-B region (uv-B 280-320 nm) with virtually no intensity below 290 nm. Hence, only compounds absorbing light above 290 nm undergo direct photolysis. Owing to the sharp decrease in light... 

Solar radiation is a form of thermal radiation having a particular wavelength distribution. Its intensity is strongly dependent on atmospheric conditions, time of year, and the angle of incidence for the sun s rays on the surface of the earth. At the outer limit of the atmosphere the total solar irradiation when the earth is at its mean distance from the sun is 1395 W/m2. This number is called the solar constant and is subject to modification upon collection of more precise experimental data. 

Figure 4-5. Wavelength distributions of the sun s photons incident on the earth s atmosphere and its surface. The curve for the solar irradiation on the atmosphere is an idealized one based on Planck s radiation distribution formula (Eq. 4.3a). The spectral distribution and the amount of solar irradiation reaching the earth s surface depend on clouds, other atmospheric conditions, altitude, and the sun s angle in the sky. The pattern indicatedby the lower curve is appropriate at sea level on a clear day with the sun overhead.

The Sun delivers a spectral irradiance at the Earth s surface at AM 1.0 (air mass), without concentrator, of 1.16 W m nm at 2 = 700 nm [2]. The solid angle represented by the Sun seen from the Earth is Q = 6.8 x 10 steradian. From Eq. (10), one calculates in this case Tr = 5500 K, and from Eq. (12) with T = 298 K one obtains rj = 0.946. If the solar spectrum were that of a black body, all wavelengths would lead to the same values of Jr and Figure 1 shows that this condition is fulfilled only if the receiver is outside the atmosphere. At the Earth s surface, absorption by atmospheric oxygen, ozone, water, and carbon dioxide makes the structured solar irradiance spectrum deviate significantly from the ideal black-body spectrum and requires rR(A) to be calculated for each wavelength. 

Direct photoreaction (eq 4) is important only for halocarbons (e.g., aromatic compounds) that significantly absorb radiation at wavelengths >295 nm, the cutoff for solar spectral irradiance at the earth s surface. Because saturated chlorinated and fluorinated organic compounds, including methylchloroform and chlorofluorocarbons, absorb solar radiation very weakly, their direct photoreaction is very slow in the sea and in fresh waters. As discussed in a later section, photoreactions of these compounds may be accelerated by sorption and indirect photoreactions in natural waters. Saturated and olefinic polv-brominated and iodinated organic compounds have long absorption tails that extend beyond 295 nm. Direct photoreaction of such compounds in aquatic environments may be significant. 

Figure 9.1 Solar irradiance at the surface of the Earth. The dark lines denote the bandgap energies of Si, GaAs, CdS and SrXi03, respectively at T = 300 K. The portions of the spectrum to the left of each line represent photons that are not substantially absorbed by the semiconductor.

It is well known that the spectral distribution and irradiance of the solar radiation at the Earth s surface depend on the location and is subjected to seasonal and diurnal variations. Therefore, a reference spectrum is needed as a basis for comparison with the spectral energy distribution of artificial light sources. Data from CIE No. 15 1971 (colorimetry official recommendations of the International Commission on Illumination) that recommend a standard illuminant D65 with a scheduled color temperature of approximately 6500 K have been used as a basis over the years. 

The solar luminosity (total radiant power emitted) is 3.861(T W, of which 1373 W/m reaches the top of the earth s atmosphere. To a zeroth approximation the sun can be considered a black body with an effective temperature of 5780 K, which implies a peak in the radiation at around 0.520 pm (5200 A). The actual solar spectral emission is more complex, especially at ultraviolet and shorter wavelengths. The graph below, which was taken from Reference 1, summarizes the solar irradiance at the top of the atmosphere in the range 0.3 to 10 pm. 

If extrapolated to the solar spectral irradiance at the Earth s surface under near-UV irradiation, the uptake coefficient (at 50 ppbv of NO2) becomes y = (8.8 0.5) X 10 . Such data can be used to estimate the HONO source flux from these urban surfaces as 130 pptv h just by assuming that only 1% of a street-canyon surface with 10 m street width and 20 m building height is covered by... 

When the attenuation of solar radiation penetrating to the earth s atmosphere is caused by the absorption of O2 and O3, the solar irradiance at the altitude zq and wavelength X on the surface perpendicular to the sun is expressed according to the Beer-Lambert law. 

Eo represents a solar constant defined as the irradiance value corresponding to the mean of distance between earth and sun, without including terrestrial atmosphere. This solar constant influences irradiance from solar spectra at the earth surface and its total value is 1120 W m when the sun reaches zenith, according to... 

The solar radiation incident on the atmosphere and the Earth s surface represents the largest external energy contribution. The optical irradiation at the upper boundary layers of the atmosphere is 4,435 kJ/cm2/year, of which around 1,108 kJ/cm2/year reaches the Earth s surface. 

Figures 7.4(a), (b) and (c) give the spectral characteristics of three sources (a) the sun at the earth s surface, (b) a JP-4 pool fire and (c) blackbody sources at typical room fire conditions (800-1100 K). The solar irradiance is mostly contained in about 0.3-2.4 fim while fire conditions span about 1-10 pm.

Franklin LA, Forster RM (1997) The changing irradiance environment consequences for marine macrophyte physiology, productivity and ecology. Eur J Phycol 32 207-232 Frederick JE, Snell HE, Haywood EK (1989) Solar ultraviolet radiation at the earth s surface. Photochem Photobiol 50 443 450... 

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