Environmental Engineering Reference
In-Depth Information
The only precursor that can initiate ozone formation in the troposphere is nitrogen dioxide,
NO
2
. As we shall see later, other gases, both man-made and natural ones, can abet ozone formation,
but the initiation of the process is solely due to NO
2
. Nitrogen dioxide is a brown gas, formed by
oxidation of nitric oxide, NO. The sum of NO and NO
2
is termed nitric oxides, NO
x
. Nitric oxides
are formed primarily in the combustion of fossil fuels. A part of the NO
x
is formed on account
of the inherent nitrogen content of fossil fuels, especially coal and petroleum. This is called “
fuel
NO
x
.” A greater part is formed during combustion. At the high flame temperatures some of the air
O
2
and N
2
combine to form NO and NO
2
. This is called “
thermal NO
x
.”
Nitrogen dioxide gas can photo-dissociate in sunlight, at wavelengths shorter than 420 nm.
The resulting atomic oxygen combines with molecular oxygen to form ozone:
NO
2
+
(
ν)
λ
≤
420 nm
→
+
h
NO
O
(9.10)
O
+
O
2
+
M
→
O
3
+
M
(9.11)
where M is an inert molecule which is necessary to bring about the combining of atomic and
molecular oxygen. Since the formed O
3
can be destroyed by the same NO which is formed in
(9.10):
O
3
+
NO
→
O
2
+
NO
2
(9.12)
this cycle of reactions cannot explain the large concentrations of ozone that are observed in various
regions of the world. It was assumed in the late 1950s and early 1960s by scientists at the California
Institute of Technology that instead of reaction (9.12), NO is reoxidized to NO
2
by some atmospheric
oxidant other than O
3
, thereby starting reaction (9.10) all over again, allowing the build-up of O
3
from a relatively small concentration of NO
2
. That “mystery” oxidant turned out later to be a peroxy
radical, RO
2
. This radical is formed in the following sequence of reactions:
RH
+
OH
→
R
+
H
2
O
(9.13)
R
+
O
2
→
RO
2
(9.14)
RO
2
+
NO
→
NO
2
+
RO
(9.15)
RH designates a hydrocarbon molecule, OH is a hydroxyl radical, RO is a an alcoxy radical, and
RO
2
is a peroxy radical. The naturally occurring peroxy radical, HO
2
, can also oxidize NO to NO
2
.
The hydroxyl radical appears to be omnipresent in the atmosphere. It is formed by reaction of water
vapor with an excited oxygen atom O(
1
D). The latter is formed in the photodissociation of O
3
by
sunlight of wavelength less than 319 nm.
Indeed, the peroxy radicals were subsequently found in the lower troposphere and in laboratory
experiments, called smog chambers. Now the picture of the formation process of ozone and the
other photo-oxidants becomes much more complicated. While NO
2
is the spark that starts the flame,
other “fuels” are necessary to sustain the flame and raise its temperature, metaphorically speaking.
The other fuels, or precursors as they are called, are hydrocarbon molecules and other organic
compounds, collectively known as volatile organic compounds (VOC). Some of the VOC are of
anthropogenic origin, such as products of incomplete combustion, evaporation of fuels and solvents,
emissions from refineries, and other chemical manufacturing. Others are of biogenic origin, such as
