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urban air than in the background troposphere. Mid-
morning sources of OH(g) in urban air are aldehyde
photolysis and oxidation. The major afternoon source of
OH(g) in urban air is ozone photolysis. In sum, the three
major reaction mechanisms that produce the hydroxyl
radical in urban air are
Table 4.2.
Percentage emission of several gases by
source category in Los Angeles Basin, 2005
Source
category CO(g) ROG NO
x
(g) SO
x
(g) NH
3
(g)
Stationary
11
67
17
38
74
Mobile
89
33
83
62
26
Early morning source
HONO(g)
Nitrous
acid
TOTAL
100
100
100
100
100
OH(g)
Hydroxyl
radical
NO(g)
Nitric
oxide
+
h
→
+
<
400 nm
Source
: U.S. EPA (2011b).
(4.42)
Midmorning source
HCHO(g)
Formal
-
dehyde
H CO(g)
Formyl
radical
H(g)
Atomic
hydrogen
field, or industrial facility, over which many fixed pol-
lutant sources aside from point source smokestacks
exist. Together, point and area sources are
stationary
sources
.Table 4.2 shows that carbon monoxide and
nitrogen oxides originate predominantly from mobile
sources. The thermal combustion reaction in automo-
biles that produces nitric oxide at a high tempera-
ture is
+
h
→
+
<
334 nm
(4.43)
M
→
H(g)
Atomic
O
2
(g)
Molecular
oxygen
H O
2
(g)
Hydroperoxy
radical
hydrogen
+
(4.44)
H CO(g)
Formyl
radical
H O
2
(g)
Hydroperoxy
radical
+
O
2
(g)
Molecular
oxygen
→
CO(g)
Carbon
Monoxide
+
(4.45)
NO(g)
Nitric
oxide
H O
2
(g)
Hydroperoxy
radical
NO
2
(g)
Nitrogen
dioxide
OH(g)
Hydroxyl
radical
+
→
+
(4.46)
High temperature
−−−−−−−→
2 NO(g)
Nitric
oxide
N
2
(g)
Molecular
nitrogen
+
O
2
(g)
Molecular
oxygen
(4.41)
Afternoon source
O
3
(g)
Table 4.2 shows that mobile sources emit less ROG
and ammonia than do stationary sources. In the 1980s,
the distribution of ROGs between stationary and mobile
sources was roughly equal. The decrease in mobile
source ROGs since then is due primarily to the catalytic
converter, a control device required in all new U.S.
automobiles since the 1970s (Section 8.1.7). Because
Los Angeles has few coal-burning sources, most of its
SO
x
(g) emissions are from mobile sources.
O(
1
D
)(g)
Excited
atomic
oxygen
Ozone
+
h
→
O
2
(g)
Molecular
oxygen
+·
<
310 nm
(4.47)
O(
1
D
)(g)
2 OH(g)
Hydroxyl
radical
·
+
H
2
O(g)
Water
vapor
→
(4.48)
Excited
atomic
oxygen
ROGs emitted in urban air include alkanes, alkenes,
alkynes, aldehydes, ketones, alcohols, aromatics, and
hemiterpenes. Table 4.3 shows lifetimes of these ROGs
against breakdown by six processes. Photolysis breaks
down aldehydes and ketones; OH(g) breaks down all
eight groups during the day; HO
2
(g) breaks down alde-
hydes during both day and night; O(g) breaks down
alkenes and terpenes during the day; NO
3
(g) breaks
down alkanes, alkenes, aldehydes, aromatics, and ter-
penes during the night; and O
3
(g) breaks down alkenes
and terpenes during both day and night.
ROG chemical breakdown produces radicals that lead
to ozone formation. The ozone-forming potential of an
ROG is a function not only of the ROG's emission rate,
butalso of its reactivity. The
reactivity
of a gas is the
ozone-forming potential of the gas for given emission
rates of the gas and NO
x
(g). For example, 1 kg of emit-
ted formaldehyde can form two orders of magnitude
more ozone than 1 kg of emitted ethane under typical
urban conditions (Russell et al., 1995). Knowing both
4.3.2. Reactive Organic Gas Breakdown
Processes
Once ROGs are emitted, they are broken down chem-
ically into free radicals. Six major processes break
down ROGs - photolysis and reaction with OH(g),
HO
2
(g), O(g), NO
3
(g), and O
3
(g). OH(g) and O(g) are
present during the day only because they are short lived
and require photolysis for their production. NO
3
(g) is
present during the night only because it photolyzes
quickly during the day. O
3
(g) and HO
2
(g) may be
present during both day and night.
OH(g) is produced in urban air by some of the same
reactions that produce it in the background troposphere.
An early morning source of OH(g) in urban air is pho-
tolysis of
nitrous acid
[HONO(g)]. HONO(g) may be
emitted by automobiles; thus, it is more abundant in
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