Environmental Engineering Reference
In-Depth Information
drops
a
r
0
2
dN
i
dt
D
N
i
τ
4
π
Dr
0
N
i
N
dr
D
,
where
a
0.2 s. Equalizing this rate to the rate of ion
formation under the action of cosmic rays for optimal altitudes
h
D
1
μ
m as earlier and
τ
D
D
11
15 km,
30 cm
3
s
1
wherethisrateis
Q
D
[161], we obtain for the equilibrium number
density of molecular ions
r
0
a
2
6cm
3
N
i
D
.
In particular, for the optimal drop size
r
0
D
20
μ
mthisformulagives
N
i
10
3
cm
3
. Therefore, ignoring the influence of the processes of attachment
of molecular ions to water drops does not change the character of the charging
processes under consideration.
We also note that in the above estimations we assumed the atmosphere to be
uniform, that is, these parameters do not depend on the altitude. But this is not cor-
rect because the charge separation leads to a nonuniform distribution of charged
particles, including molecular ions, whose number density may depend on the al-
titude, and this dependence may be different for positive and negative molecular
ions. Nevertheless, this complexity does not violate the general character of charge
separation in the Earth's atmosphere.
We have the character of charge separation is determined by molecular ions in
the atmosphere, which in turn are connected to corresponding chemical processes
and may be different depending on certain atmospheric conditions [164]. In the
first stage of these processes, ionization of air under the action of cosmic rays leads
to the formation of O
or O
2
ions, and these negative ions in turn partake in chem-
ical processes which result in the formation more stable negative ions. In the same
manner, positive ions, which start from simple O
C
,O
2
,N
C
,andN
2
ions, are
transformed into other ions depending on some admixtures to air molecules in the
atmosphere. Subsequently, more stable positive and negative ions attach to aerosol
particles which are charged negatively if the diffusion coefficient or the mobility
of negative ions is higher than that for positive ions, and the aerosol particles are
charged positively for the inverse relation between ion mobilities. Therefore, small
additions to atmospheric air may change the sign of the charge of falling aerosol
particles, which is evidence of the complexity of charge separation in the Earth's
atmosphere.
Above we considered charging of water drops in atmospheric air as a result of
attachment of molecular ions to the drop surface, and this process proceeds in
clouds where the water concentration is relatively high [165-169]. But there are
other mechanisms of charge transfer to aerosol particles in the Earth's atmosphere.
The characteristic feature of the condensation process for water is that it proceeds
at low temperatures close to the meting point, and then ice particles may be formed
simultaneously with liquid drops. These conditions are typical for clouds which
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