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ethanyl radical then reacts to produce NO
2
(g). The
sequence is
OH
OH
OH
+ NO(g)
H
H
H
H
H
H
H
H
O
M
M
CC
(4.49)
CC
CC
O
CC
O
+ O
2
(g)
H
H
+ OH(g)
H
H
H
H
H
H
NO
2
(g)
Ethene
Ethanyl radical
Ethanolperoxy
radical
Ethanoloxy
radical
The
excited Criegee biradical
isomerizes, and its
product, excited formic acid, thermally decomposes by
NO
2
(g) produces ozone by Reactions 4.2 and 4.3. The
ethanoloxy radical
,aby-product of ethene oxidation,
60% CO(g) + H
2
O(g)
Carbon
monoxide
Water
vapor
H
H
O
*
M
M
21% CO
2
(g) + H
2
(g)
(4.52)
CO
CO
Carbon
dioxide
Molecular
hydrogen
H
H
O
*
+
O
2
(
g)
Excited formic
acid
Excited Criegee
biradical
CO(g) + OH(g) + HO
2
(g)
19%
Carbon
monoxide
Hydroxyl
radical
Hydroperoxy
radical
goes on to produce formaldehyde and
glycol aldehyde
[HOCH
2
CHO(g)], both of which contribute to further
ozone formation. The formaldehyde ozone process is
described in Section 4.2.6.
In sum, ozone attack on ethene produces HCHO(g),
HO
2
(g), CO(g), and NO
2
(g). These gases not only
reform the original ozone lost, but also produce new
ozone.
4.3.4.2. Alkene Reaction with Ozone
When ethene reacts with ozone, the ozone substitutes
into ethene's double bond to form an unstable
ethene
molozonide
.The molozonide decomposes to products
that are also unstable. The reaction sequence of ethene
with ozone is
4.3.5. Ozone Production from Aromatics
To luene
[C
6
H
5
CH
3
(g)] originates from the combus-
tion of gasoline, diesel, biofuels, and outdoor biomass;
petroleum refining; detergent production; and off-
gassing from paints, solvents, and building materials.
H
H
O
+
CO
CO
37%
H
H
H
H
M
O
(4.50)
Formaldehyde
Criegee biradical
+ O
3
(g)
CC
OO
H
H
H
H
O
*
H
2
CCH
2
+
CO
CO
63%
H
H
Ethene
Ethene molozonide
Formaldehyde
Excited Criegee
biradical
Formaldehyde produces ozone as in Section 4.2.6. The
Criegee biradical
from Reaction 4.50 forms NO
2
(g) by
H
2
CO O(g)
Criegee
biradical
After methane, it is the second most abundantly emit-
ted organic gas in Los Angeles air and the fourth most
important gas in terms of abundance and chemical reac-
tivity (Table 4.4). Mixing ratios of toluene in polluted
NO(g)
Nitric
oxide
NO
2
(g)
Nitrogen
dioxide
+
→
HCHO(g)
Formal
-
dehyde
+
(4.51)
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