SOLAR RADIATION AND ITS MEASUREMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 25

At the top of the Earth's surface, the total irradiance is 1367 W·m 2 (solar
constant) of which 111 W·m 2 or 8 % contribute to the 200 - 400 nm range. The
extraterrestrial spectrum was measured by the 'Solar Ultraviolet Spectral Irradiance
Monitor' (SUSIM) aboard the Spacelab 2 mission and can be downloaded from the
internet site http://www.solar.nrl.navy.mil/susim_atlas_data.html.

Scattering and absorption in the atmosphere

Solar radiation entering the Earth's atmosphere is subject to scattering and
absorption processes which greatly influence the spectral composition and spatial
distribution of the radiation field on the Earth's surface 4,5 . Scattering is a result of the
interaction of small particles with the radiation. It is determined by the wavelength of
the radiation and the size of the particle. In Rayleigh scattering the particles are much
smaller than the wavelength, e.g. by air or trace gas molecules. The spatial distribution
of scattered radiation is the same in the forward and backward direction. The probability
for Rayleigh scattering exhibits a 1/O 4 dependence which results in the blue sky over the
day and the red sky at sunrise or sunset. In contrast, Mie scattering, where the particles'
size is in the order of the wavelength, e.g. for aerosols, the scattered radiation is
strongly peaked to the forward direction.
The main absorbers influencing the UV range of the spectrum at the Earth's
surface are oxygen and nitrogen (both atomic and diatomic) and ozone. Between 200
and 300 nm stratospheric ozone is the main absorber of UV radiation, the absorption
spectrum of ozone leads to the steep edge of the terrestrial solar spectrum around
290 nm. The amount of UV radiation in the UV-B waveband is strongly dependent on
the total ozone column thickness.
As a result of scattering of radiation in the atmosphere, a diffuse radiation field is
formed. Besides scattering, absorption is responsible for the attenuation of the Sun's
beam which is called direct component of the radiation field. The sum of direct and
diffuse radiation defines the global radiation (compare Fig. 3).

Sun (extraterrestrial solar radiation)

Atmosphere

Diffuse radiation Direct radiation

Terrestrial global solar radiation

Figure 3. Schematic presentation of atmospheric multiple scattering events and absorption processes.