Environmental Engineering Reference
In-Depth Information
place i.e. in air molecules, water drops, ice crystals, and aerosol particles.
Rayleigh and Mie diffusion are differentiated. Rayleigh diffusion is diffusion at
particles with a radius significantly smaller than the wavelength of the incident
light (e.g. air molecules). The Mie diffusion takes place at particles with a ra-
dius within the wavelength of the incident light and larger (e.g. aerosol parti-
cles). The larger the particles at which the sunlight is diffused, the more they
diffuse into a forward direction. Mie diffusion then turns into diffraction.
Absorption. Absorption is the conversion of solar radiation into other energy
forms. In general, solar radiation is converted into heat during this process.
Such absorption can take place in aerosol, cloud and precipitation particles.
Additionally, a selective absorption is possible; here selected spectral and
wave-length ranges of solar radiation are absorbed by some gases existing
within the atmosphere. This is especially the case for ozone (O 3 ) and water va-
pour (H 2 O). Ozone, for example, almost completely absorbs the spectral range
between 0.22 and 0.31 µm. Carbon dioxide (CO 2 ), in comparison, only mini-
mally absorbs solar radiation.
This weakening is described by the so-called transmission factor τ G (Equation
(2.3)); it covers all weakening effects affecting solar global radiation incident on
the outer atmospheric layer passing through the atmosphere. G g is the global ra-
diation and E SC the solar constant.
GE
=
τ
(2.3)
g
SC
G
The transmission factor consists of the Rayleigh diffusion τ RD , the Mie diffu-
sion τ MD and the absorption within gases τ GA as well as the absorption within parti-
cles τ PA (Equation (2.4)).
τ =
τ
τ
τ
τ
(2.4)
G
RD
MD
GA
PA
Spectral range. Due to the weakening of the radiation within the atmosphere of
the earth, the energy distribution spectrum of sunlight is changed. Fig. 2.8 shows
the spectrum of the solar radiation before and after passing through the earth's
atmosphere.
Due to the described radiation weakening processes within the atmosphere of
the earth, the energy distribution of solar radiation reaching the earth shows the
following characteristics.
The energy maximum is within the visible spectral range between 0.5 and
0.6 µm (green to yellow light).
With a decreasing wave length (i.e. in the ultraviolet spectrum) the radiated
power decreases rapidly.
With an increasing spectral range (i.e. in the infrared spectrum) the radiation
decreases more slowly.
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