Environmental Engineering Reference
In-Depth Information
In urban areas with smog, the concentration of ozone is higher than that predicted
above. This can be interpreted using the expression [O
3
]=
J
1
[
NO
2
]
/k
3
[NO]. Any
reaction that increases the transformation of NO to NO
2
can bring about increased
ozone concentrations.What types of materials are responsible for this effect? Table 6.12
summarizes the most common constituents of urban smog and their adverse effects.
TABLE 6.12
Common Constituents of Urban Smog that React with Ozone
Compound
Formula
Health Effects
Aldehydes
R-CHO
Eye irritation, odor
Hydrocarbons
R-CH
3
Eye irritation, odor
Alkyl nitrates
R-ONO
2
Peroxyacyl nitrates (PAN)
R-CO-OONa
Toxic to plants, eye irritant
Aerosols
(NH
4
)
2
SO
4
,NH
4
NO
3
, etc.
Visibility reduction
During daytime automobiles generate NO via combustion:
N
2
+
O
2
−→
2NO.
(6.195)
However, automobiles also generate hydrocarbons (HR) and CO via incomplete
combustion, both of which react with atmospheric OH radicals to produce peroxy
radicals:
OH
+
HR
+
O
2
−→
RO
2
+
H
2
O.
(6.196)
In general, only C
3
and higher hydrocarbons are reactive and are called “non-methane
hydrocarbons (NMHC).” The peroxy radicals are very efficient in converting NO to
NO
2
:
RO
2
+
NO
+
O
2
−→
RO
+
NO
2
+
HO
2
.
(6.197)
Thus, the ratio [NO
2
]/[NO] and O
3
concentration increase during the daytime.
NO
2
and HR also react producing aldehydes, ozone, and peroxy acetyl nitrate (PAN).
For example,
CH
3
-CH
3
+
OH
+
O
2
−→
CH
3
CH
2
O
2
+
H
2
O,
CH
3
CH
2
O
2
+
NO
+
O
2
−→
CH
3
CHO
+
HO
2
+
NO
2
,
CH
3
CO
•
+
OH
+
CH
3
CHO
−→
H
2
O,
(6.198)
CH
3
CO
•
+
CH
3
COOO
•
,
O
2
−→
CH
3
COOO
•
+
NO
2
−→
CH
3
COOONO
2
.
PAN, so formed, is an eye irritant. Thus to eliminate smog in an urban area, the first
line of defense is to reduce HR emissions and NO from automobiles and power plants.
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