Environmental Engineering Reference
In-Depth Information
process that is the inverse of process 5 gives the main contribution to glow in the
night sky, and processes 7-10 determine auroral radiation.
As an introduction to the study of atmospheric plasmas, we consider general
properties of the Earth's atmosphere. The heat balance of the Earth is represent-
ed in Table 6.7. The source of the Earth's heat balance is solar radiation. Solar
radiation can be approximated by radiation of a blackbody with an effective tem-
perature of 5800 K, corresponding to a radiation flux of 6.4 kW/cm
2
from its sur-
face, mostly in the visible region of the spectrum. But in contrast to a blackbody,
solar radiation contains a remarkable flux in UV and vacuum UV (VUV) spec-
tra that is responsible for ionization processes in the Earth's atmosphere. The
radiative flux is 0.137 W/cm
2
at the distance of the Earth from the Sun, which
10
14
kW of solar power reaching the Earth's atmosphere. The
same amount of power then emanates from the Earth in the form of emission
from the Earth's surface and atmosphere as infrared radiation, and reflection by
the Earth's surface and atmosphere. The surface of the Earth receives and returns
2.5
translates to 1.74
10
14
kW of power, whereas the Earth including its atmosphere receives and
returns 2.69
10
14
kW of power. If the Earth is regarded as an ideal blackbody,
the emitted infrared radiation corresponds to an effective surface temperature of
291 K. Absorption of solar UV radiation by molecules and atoms of the Earth's
atmosphere determines ionization and chemical processes in upper atmosphere
layers. The optical density of the Earth's atmosphere for this radiation is high, and
therefore radiation of short wavelengths is absorbed at high altitudes. Figure 6.20
shows the altitudes at which solar radiation of a given wavelength is absorbed [134].
The Earth's atmosphere at sea level consists mostly of molecular nitrogen
(78%), molecular oxygen (21%), and argon (about 1%); the total number densi-
tyofmoleculesandatomsis2.7
10
19
cm
3
(at a pressure of 1 atm). The number
density of molecules decreases with increasing altitude
h
as given by the barometric
Figure 6.20
The boundary altitudes for absorption of solar UV radiation of a given wave-
length [134].
