Geoscience Reference
In-Depth Information
that cause eyes to swell, redden, and tear are
lachryma-
tors
.The methoxy radical from Reaction 4.18 produces
formaldehyde by
CH
3
O(g)
Methoxy
radical
M
→
CH
3
(g)
Methyl
radical
CH
3
O
2
(g)
Methylperoxy
radical
+
O
2
(g)
Molecular
oxygen
(4.17)
NO(g)
Nitric
oxide
CH
3
O
2
(g)
Methylperoxy
radical
NO
2
(g)
Nitrogen
dioxide
CH
3
O(g)
Methoxy
radical
+
→
+
(4.18)
H O
2
(g)
Hydroperoxy
radical
+
O
2
(g)
Molecular
oxygen
→
HCHO(g)
Formal
dehyde
+
(4.21)
NO
2
(g)
Nitrogen
dioxide
NO(g)
Nitric
oxide
O(g)
Atomic
oxygen
+
h
→
+·
<
420 nm
(4.19)
The
e-
folding lifetime of CH
3
O(g) against destruc-
tion by O
2
(g) is
0.0001 seconds. Once formaldehyde
forms, it produces ozone precursors by
∼
M
→
O(g)
Atomic
oxygen
·
+
O
2
(g)
Molecular
oxygen
O
3
(g)
Ozone
(4.20)
H CO(g)
Formyl
radical
H(g)
Atomic
hydrogen
+
<
334 nm
HCHO(g)
Formal
-
dehyde
+
h
→
In the first reaction, OH(g)
abstracts
(removes) a
hydrogen atom from methane, producing the
methyl
radical
[CH
3
(g)] and water. In the stratosphere, this
reaction is an important source of water vapor. As with
Reaction 4.12, Reaction 4.17 is fast. The remainder of
the sequence is similar to the remainder of the carbon
monoxide sequence, except that the
methylperoxy rad-
ical
[CH
3
O
2
(g)] converts NO(g) to NO
2
(g), whereas in
the carbon monoxide sequence, the
hydroperoxy rad-
ical
[HO
2
(g)] performs the conversion.
CO(g)
Carbon
monoxide
+
H
2
(g)
Molecular
hydrogen
<
370 nm
(4.22)
OH(g)
Hydroxyl
radical
H CO(g)
Formyl
radical
HCHO(g)
Formal
-
dehyde
+
→
+
H
2
O(g)
Water
vapor
(4.23)
H CO(g)
Formal
radical
H O
2
(g)
Hydroperoxy
radical
+
O
2
(g)
Molecular
oxygen
→
monoxide
+
CO(g)
Carbon
(4.24)
M
→
H(g)
Atomic
hydrogen
H O
2
(g)
Hydroperoxy
radical
+
O
2
(g)
Molecular
oxygen
(4.25)
4.2.6. Ozone Production from Formaldehyde
An important by-product of the methane oxidation path-
way is
formaldehyde
[HCHO(g)]. Formaldehyde is a
colorless gas with a strong odor at mixing ratios higher
than 0.05 to 1.0 ppmv. It is the most abundant alde-
hyde in the air and moderately soluble in water. Aside
from gas-phase chemical reaction, the most important
source of formaldehyde is off-gassing from plywood,
resins, adhesives, carpeting, particleboard, fiberboard,
and other building materials (Hines et al., 1993). Mix-
ing ratios of formaldehyde in urban air are generally
less than 0.1 ppmv (Maroni et al., 1995). Indoor mix-
ing ratios range from 0.07 to 1.9 ppmv and typically
exceed outdoor mixing ratios (Anderson et al., 1975;
Jones, 1999).
Because formaldehyde is moderately soluble in
water, it dissolves readily in the upper respiratory tract.
Below mixing ratios of 0.05 ppmv, formaldehyde causes
no known health problems. Above this level, it causes
the following effects: 0.05 to 1.5 ppmv, neurophysio-
logic effects; 0.01 to 2.0 ppmv, eye irritation; 0.1 to 25
ppmv, irritation of the upper airway; 5 to 30 ppmv, irrita-
tion of the lower airway and pulmonary problems; 50 to
100 ppmv, pulmonary edema, inflammation, and pneu-
monia; and greater than 100 ppmv, coma or death (Hines
et al., 1993; Jones, 1999). Formaldehyde is but one of
many eye irritants in photochemical smog. Compounds
CO(g) forms ozone through Reactions 4.11 to 4.15, and
HO
2
(g) forms ozone through Reactions 4.13 to 4.15.
4.2.7. Ozone Production from Ethane
The most concentrated nonmethane hydrocarbons in
the background troposphere are
ethane
[C
2
H
6
(g)] and
propane
[C
3
H
8
(g)]. Background tropospheric mixing
ratios are 0 to 2.5 ppbv for ethane and 0 to 1.0 ppbv
for propane. These hydrocarbons originate substantially
from anthropogenic sources and have relatively long
lifetimes against photochemical destruction, so they
persist after leaving urban areas. The
e
-folding lifetime
of ethane against chemical destruction is about twenty-
three to ninety-three days, and that of propane is five
to twenty-one days. The primary oxidant of ethane and
propane is OH(g). In both reactions, OH(g) initiates the
breakdown. The sequence of reactions, with respect to
ethane, is
OH(g)
Hydroxyl
radical
C
2
H
5
(g)
Ethyl
radical
C
2
H
6
(g)
Ethane
+
→
+
H
2
O(g)
Water
vapor
(4.26)
M
→
C
2
H
5
(g)
Ethyl
radical
C
2
H
5
O
2
(g)
Ethylperoxy
radical
+
O
2
(g)
Molecular
oxygen
(4.27)
NO(g)
Nitric
oxide
C
2
H
5
O
2
(g)
Ethylperoxy
radical
NO
2
(g)
Nitrogen
dioxide
C
2
H
5
O(g)
Ethoxy
radical
+
→
+
(4.28)
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