Geology Reference
In-Depth Information
HO
2
þ
NO
-
OH
þ
NO
2
(2.19)
As both reactions (2.37) and (2.19) lead to the oxidation of NO to NO
2
the subsequent photolysis leads to the formation of ozone (see reactions
(2.2) and (2.20)). The individual reaction mechanism depends on the
identity of the organic compounds and the level of complexity of the
mechanism. Although OH is the main tropospheric oxidation initiator,
reaction with NO
3
,O
3
,O(
3
P) or photolysis may be an important loss
route for some NMHCs or the partially oxygenated products produced
as intermediates in the oxidation (see reaction (2.38)).
In summary, the rate of oxidation of VOCs and therefore by inference
the production of ozone is governed by the concentration of the catalytic
HO
x
radicals. There are a large variety of VOCs with a range of
reactivities; therefore this remains a complex area.
2.5.4 Urban Chemistry
In some respects, the story of atmospheric chemistry and particularly ozone
photochemistry begins with urban chemistry and photochemical smog. The
term smog arises from a combination of the words smoke and fog. In the
1940s, it became apparent that cities like Los Angeles (LA) were severely
afflicted with a noxious haze.
24
Though, at the time it was thought to be a
relatively local phenomenon, with the understanding of its chemistry came
the development of a photochemical theory for the whole of the tropo-
sphere. The LA smog is often termed photochemical smog and is quite
differentinorigintotheLondonsmogsofthe19thand20thcenturies,
which have their origins in abnormally high concentrations of smoke
particles and sulfur dioxide. The London smogs have been alleviated with
the effective application of legislation that has reduced the burning of coal
in the London area. The major features of photochemical smog are high
levels of oxidant concentration in particular ozone and peroxidic com-
pounds, produced by photochemical reactions. The principal effects of
smog are eye and bronchial irritation as well as plant and material damage.
The basic reaction scheme for the formation of photochemical smog is
VOC
þ
hn
-
VOC
þ
R
(2.40)
R
þ
NO
-
NO
2
(2.41)
NO
2
þ
hn(l
o
420 mm)
-
NO
þ
O
3
(2.2)
