Environmental Engineering Reference
In-Depth Information
transferred in this way is 3.2
1.2 on average [154]. In addition, there are 100
negative flashes per second over the entire globe on average [152].
Thus, although it may seem strange at first glance, the charging of the Earth oc-
curs as a result of lightning in the atmosphere. Charge transfer processes in clouds
that are connected with charging and precipitation of small drops or aerosols lead
to a redistribution of the charges therein. The lower part of a cloud is usually neg-
atively charged and the upper part is usually positively charged. The electric poten-
tial of a cloud can reach hundreds of millions of Volt, causing breakdown of the
air. The discharge of a cloud to the Earth's surface in the form of a lightning stroke
is accompanied by the transfer of electric charge to the Earth's surface. About 10%
of thunderstorms transfer positive charge to the Earth's surface, and 90% transfer
negative charge. The net result is that the Earth acquires a negative charge.
We can estimate the timescales for the electric processes of the Earth. Positive
and negative ions recombine under atmospheric conditions in three body colli-
sions, with an effective ion recombination coefficient of
˙
10
6
cm
3
/s for
recombination according to processes 17-19 in Table 6.6. We note that electrons are
practically absent in the atmosphere near the Earth's surface although electron for-
mation is the first stage of ionization processes in the lower Earth atmosphere. But
according to processes 20 and 21 in Table 6.6, conversion of electrons into negative
ions O
2
near the Earth's surface proceeds during a time of approximately 10
8
s,
which is short compared with the lifetime of charged particles in the atmosphere.
Hence,weassumethenegativechargeintheloweratmospheretobeconnected
partially to the negative molecular ions. One can estimate from this a time for re-
combination of molecular ions as about
α
D
2
N
i
)
1
10
3
cm
3
.
τ
D
(
α
D
0.5 h for
N
i
D
This contrasts with the much shorter characteristic time of
q
/
I
6min for dis-
charging of the Earth, where
q
is the Earth's charge and
I
is the average atmospheric
current.
One can estimate a typical distance an ion travels during its lifetime
D
τ
,which
is
s
,where
K
is the ion mobility and
E
is the electric field strength near
the Earth. Table 6.10 contains the measured mobilities of some ions in nitrogen.
These ions may be formed in a humid atmosphere and are characterized by high
electron binding energies. Indeed, the electron affinity is 2.3 eV for the NO
2
,3.7eV
for NO
3
,and2.8eVforCO
3
. For comparison, the electron affinity for O
2
is 0.43 eV
and for the oxygen atom is 1.5 eV. These stable negative ions can form complex
ions in a humid atmosphere. Using the data in Table 6.10, we take the ion mobility
K
D
KE
τ
2cm
2
/(V s), which gives for the distance traveled by an ion during its lifetime
as
s
50m. This distance is short compared with the size of the atmosphere,
which is evidence for continuous formation of charged particles in the atmosphere.
One can estimate the average number density of molecular ions taking into ac-
count, as follows from measurements, the average current density over land of
2.4
10
16
A/cm
2
[149]. Let us assume
the mobility of negative and positive molecular ions to be close to 2 cm
2
/(V s) as
follows from Table 6.10. Taking the number densities
N
i
of negative and posi-
tive molecular ions to be identical, on the basis of the electric field strength
E
10
16
A/cm
2
and over the ocean of 3.7
D
1.3 V/cm we obtain that the indicated current density over land is provided by a ion
