Environmental Engineering Reference
In-Depth Information
10
1
10
1
UVC
UVB
UVA
10
0
10
0
DNA
damage
10
-1
10
-1
10
-2
10
-2
10
-3
10
-3
Ozone
depletion
10
-4
10
-4
present
Earth
early Earth
10
-5
10
-5
10
-6
10
-6
UVC
UVB
UVA
10
0
10
0
DNA
damage
extraterrestrial
10
-1
10
-1
10
-2
10
-2
10
-3
10
-3
10
-4
10
-4
10
-5
10
-5
10
-6
10
-6
180
200
220
240
260
280
300
320
340
Wavelength, nm
Figure 1.
Solar extraterrestrial and terrestrial UV irradiance spectra, action spectrum for DNA
damage and biological effectiveness spectra for extraterrestrial and terrestrial conditions.
spectrum by 8.3 %, with 0.5% UV-C (200-280 nm), 1.5% UV-B, and 6.3% UV-A
(315-400 nm), respectively
10
. Although the UV-C and UV-B regions make up only 2%
of the entire solar irradiance prior to attenuation by the atmosphere, they are mainly
responsible for the high lethality of extraterrestrial solar radiation to living organisms.
This was demonstrated in spores of
Bacillus subtilis
, that were killed effectively within
a few seconds by extraterrestrial sunlight (>190 nm)
11
, whereas at the Earth's surface
about thousand times longer exposure times were required to reach the same effect
(Figure 2). The reason for this high lethality of extraterrestrial solar UV radiation -
compared to conditions on Earth - lies in the absorption characteristics of the DNA
which is the decisive target for inactivation and mutation induction at the UV range. The
UV action spectrum for inactivation of
B. subtilis
spores spans over 7 orders of
magnitude and declines exponentially with increasing wavelength at wavelengths longer
than the peak wavelength around 260 nm
13,14
. Its spectral profile is similar to that
averaged for affecting DNA (Figure 1).
On its way through the atmosphere, solar radiation is modified by scattering and
absorption processes
15
. During the first 2.5 billion years of Earth's history, UV-radiation
of wavelengths >200 nm reached the surface of the Earth due to lack of an effective
ozone shield
16
. Following the rapid oxidation of the Earth's atmosphere about 2.1 billion
years ago, an UV-absorbing ozone layer was built up in the stratosphere. Today, the
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