Geology Reference
In-Depth Information
Table 4 Percentage of NMHC classes measured in the morning at various
locations
Urban Los Angeles
Urban Boston
Rural Alabama
Alkanes
42
36
9
43
a
Alkenes
7
10
Aromatics
19
30
2
46
b
Other
33
24
Source: From ref. 31.
a
Large contribution from biogenic alkenes.
b
Mainly oxygen containing.
NO
þ
O
3
-
NO
2
þ
O
2
(2.24)
R
þ
R
-
R
(2.42)
R
þ
NO
2
-
NO
y
(2.43)
There is a large range of available VOCs in the urban atmosphere,
driven by the range of anthropogenic and biogenic sources.
5,21,31
Table 4
illustrates the different loadings of the major classes of NMHC in urban
and rural locations. These NMHC loadings must be coupled to meas-
ures of reactivity and the degradation mechanisms of the NMHC to give
a representative picture of urban photochemistry. The oxidation of the
VOCs drives, via the formation of peroxy radicals, the oxidation of NO
to NO
2
, where under the sunlit conditions the NO
2
can be dissociated to
form ozone (reaction (2.2)). Pre-existing ozone can also drive the NO to
NO
2
conversion (reaction (2.24)).
The basic chemistry responsible for urban photochemistry is essen-
tially the same as that takes place in the unpolluted atmosphere (see
Section 2.5). It is the range and concentrations of NMHC fuels and the
concentrations of NO
x
coupled to the addition of some photochemical
accelerants that can lead to the excesses of urban chemistry. For
example, in the Los Angeles basin it is estimated that 3333 ton day
1
of organic compounds are emitted as well as 890 ton day
1
of NO
x
.In
addition, to the reactions forming OH in the background troposphere,
i.e. via the reaction of O(
1
D) with H
2
O, viz
O
3
þ
hn(l
o
340 nm)
O
2
þ
O(
1
D)
-
(2.7)
O(
1
D)
þ
H
2
O
-
2OH
(2.8)
