Environmental Engineering Reference
In-Depth Information
photodissociation, we have [O 2 ]
D
N /4, where the concentration of molecular oxy-
gen is [O 2 ]
10 11 cm 3 , and a typical transport time is
τ
L / w
10 4 s. This
dr
gives the inequality
dis , so the photodissociation process does not disrupt
the barometric distribution of molecular oxygen at altitudes where photoabsorption
of solar radiation takes place.
To estimate the number density of atomic oxygen, we can compare the flux w [O]
of oxygen atoms and the rate of their formation, which is of the order of the flux
of solar photons causing photodissociation. This flux is I
τ
τ
dr
10 12 cm 2 s 1 at
3
optimal altitudes. The number density of atomic oxygen is
I
w
[O]
0.1 N
(6.99)
at altitudes where it is formed, and (6.98) has been used for the atom drift velocity.
From this it follows that the concentration of atomic oxygen (approximately 10%)
does not vary with altitude until the mechanisms for loss of atomic oxygen become
weak. Decay of atomic oxygen is determined by the three body process for recom-
bination of atomic oxygen 2O
M, where M is N 2 ,O,orO 2 .The
maximum number density of atomic oxygen [O] max is observed at altitudes where
a typical time for the three body association process is equal to the time for atomic
transport to lower layers, that is,
C
M
!
O 2
C
w [O] max
L
K [O] 2 max N .
10 33 cm 6 /s, we obtain
Because the value of the three body rate constant is K
10 13 cm 3 . Part of the atomic oxygen penetrating to lower layers in the
atmosphere is transformed into ozone by processes 40 and 41 in Table 6.6. Thus,
the photodissociation process in the upper atmosphere leads to a high ozone con-
centration (up to 10 4 ) at altitudes from 40 to 80 km.
[O] max
6.4.5
Ions in the Upper Atmosphere
Ions in the upper atmosphere such as N 2 ,O 2 ,O C ,andN C are formed by pho-
toionization of the corresponding neutral species (see Figure 6.22) The spectrum
of solar radiation in the VUV range that is responsible for ionization of atmo-
spheric atomic particles is created by the solar corona. Hence, the intensity of
this radiation can vary over a wide range. The average flux of photons with the
wavelength below 100 nm is about 2.4
10 10 cm 2 s 1 . This part of the spec-
trum causes Lyman-series transitions of atomic hydrogen, including the Rydberg
spectrum and the continuum. The contribution in the spectral range from 84 to
103 nm averages about 1.3
10 10 cm 2 s 1 . Among the more notable process-
es are the CIII transition at 99.1 nm (9
10 8 cm 2 s 1 ), the CIII transition at
10 9 cm 2 s 1 ), the OV transition at 63 nm (1.3
10 9 cm 2 s 1 ), the
97.7 nm (4.4
10 9 cm 2 s 1 ), and the Lyman HeII transition at
HeI transition at 58.4 nm (1.3
30.4 nm (7.7
10 9 cm 2 s 1 ). The maximum rate constant for the generation of
 
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