Geoscience Reference
In-Depth Information
NO
2
(g)
Nitrogen
dioxide
NO(g)
Nitric
oxide
O(g)
Atomic
oxygen
decomposition by the reverse of Reaction 4.34 is about
twenty-five minutes. At 280 K, its lifetime increases to
thirteen hours. As such, PAN's mixing ratios decrease
with increasing temperature.
Conversely, when PAN rises vertically to above the
boundary layer, where temperatures are low, it persists
longer than at the surface and can be blown long dis-
tances by the wind. When it comes back to the surface,
it can dissociate again, releasing ozone-forming pollu-
tants far from urban areas.
Acetaldehyde also produces ozone. The peroxyacetyl
radical in Reaction 4.34, for example, converts NO(g)
to NO
2
(g) by
CH
3
C(
+
h
→
+·
<
420 nm
(4.29)
M
→
O(g)
Atomic
oxygen
·
+
O
2
(g)
Molecular
oxygen
O
3
(g)
Ozone
(4.30)
4.2.8. Ozone and Peroxyacetyl Nitrate
Production from Acetaldehyde
An
important
by-product
of
ethane
oxidation
is
acetaldehyde
[CH
3
CH(
O)(g)], produced from the
ethoxy radical formed in Reaction 4.28. The reaction
producing acetaldehyde is
C
2
H
5
O(g)
Ethoxy
radical
=
O) O
2
(g)
Peroxyacetyl
radical
NO(g)
Nitric
oxide
O) O(g)
Acetyloxy
radical
=
+
→
CH
3
C(
=
H O
2
(g)
Hydroperoxy
radical
(4.31)
+
O
2
(g)
Molecular
oxygen
→
CH
3
CH(
O)
Acetaldehyde
=
+
NO
2
(g)
Nitrogen
dioxide
+
(4.35)
This reaction is relatively instantaneous. Acetaldehyde
is a precursor to
peroxyacetyl nitrate
(PAN), a day-
time component of the background troposphere that
wasdiscovered during laboratory experiments of pho-
tochemical smog formation (Stephens et al., 1956). The
only source of PAN is chemical reaction in the presence
of sunlight. As such, its mixing ratio peaks during the
afternoon, at the same time that ozone mixing ratios
peak. Like formaldehyde, PAN is an eye irritant and
lachrymator, but it does not have other serious human
health effects. It does damage plants by discoloring their
leaves. Mixing ratios of PAN in clean air are typically
2to100 pptv. Those in rural air downwind of urban
sites are up to 1 ppbv. Polluted air mixing ratios are
generally not higher than 35 ppbv, with typical values
of 10 to 20 ppbv. PAN is not an important constituent
of air at night or in regions of heavy cloudiness. The
reaction pathway producing PAN is
NO
2
(g) forms O
3
(g) through Reactions 4.2 and 4.3. A
second mechanism of ozone formation from acetalde-
hyde is through photolysis:
CH
3
(g)
Methyl
radical
H CO(g)
Formyl
radical
CH
3
CHO(g)
Acetaldehyde
+
h
→
+
(4.36)
The methyl radical from this reaction forms ozone
through Reactions 4.17 to 4.20. The formyl radical
forms ozone through Reaction 4.24, followed by Reac-
tions 4.13 to 4.15.
4.3. Chemistry of Photochemical Smog
Photochemical smog
is a soup of gases and aerosol
particles. Some of the substances in smog are emitted,
whereas others form chemically or physically in the air
from precursor gases or particles. In this section, the
gas-phase components of smog are discussed. Aerosol
particles in smog are discussed in Chapter 5.
Photochemical smog differs from background air in
two ways. First, smog contains more high-molecular-
weight organics, particularly aromatic compounds, than
does background air. Because most high-molecular-
weight and complex organic compounds break down
quickly in urban air, they are unable to survive long
enough to disperse to the background troposphere. Sec-
ond, the mixing ratios of nitrogen oxides and organic
gases are higher in polluted air than in background air,
causing mixing ratios of ozone to be higher in urban air
than in background air.
Photochemical
O H(g)
Hydroxyl
radical
CH
3
C(
CH
3
CH(
O)(g)
Acetaldehyde
=
+
→
O)(g)
Acetyl
radical
=
+
H
2
O(g)
Water
vapor
(4.32)
M
→
CH
3
C(
O) O
2
(g)
Peroxyacetyl
radical
O)(g)
Acetyl
radical
=
+
O
2
(g)
Molecular
oxygen
CH
3
C(
=
(4.33)
M
O) O
2
(g)
Peroxyacetyl
radical
NO
2
(g)
Nitrogen
dioxide
CH
3
C(
=
+
CH
3
C(
O)O
2
NO
2
(g)
Peroxyacetyl nitrate
(PAN)
=
(4.34)
The last reaction in this sequence is reversible and
strongly temperature dependent. At 300 K and at sur-
face pressure, PAN's
e
-folding lifetime against thermal
smog
involves
reactions
among
nitrogen oxides
[NO
x
(g)
NO
2
(g)] and
reac-
tive organic gases
(ROGs, total organic gases minus
=
NO(g)
+
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