122 ATMOSPHERIC CHEMISTRY
URBAN PHOTOCHEMICAL OXIDANTSThe photochemical-oxidant problems exist in a number of
urban areas, but the Los Angeles area is the classic example
of such problems. Even more severe air-pollution problems
are occurring in Mexico City. The most commonly studied
oxidant is ozone (O 3 ), for which an air-quality standard exists.
Ozone is formed from the interaction of organic compounds,
nitrogen oxides, and sunlight. Since sunlight is an important
factor in photochemical pollution, ozone is more commonly
a summertime problem. Most of the ozone formed in the
troposphere (the lowest 10 to 15 km of the atmosphere) is
formed by the following reactions:NO 2 hν ( 430 nm) → NO O(^3 P) (1)
O(^3 P) O 2 M → O 3 M (2)Nitrogen dioxide (NO 2 ) is photolyzed, producing nitric
oxide (NO) and a ground-state oxygen atom (designated as
O(^3 P)). This oxygen atom will then react almost exclusively
with molecular oxygen to form ozone. The M in reaction (2)
simply indicates that the role of this reaction depends on the
pressure of the reaction system. NO can also react rapidly
with ozone, reforming NO 2 :NO O 3 → NO 2 O 2 (3)These three reactions allow one to derive the photostationary
state or Leighton relationship[O 3 ] [NO]/[NO 2 ] = k 1 / k 3 or [O 3 ] = k 1 [NO 2 ]/ k 3 [NO]This relationship shows that the O 3 concentration depends
on the product of the photolysis rate constant for NO 2 ( k 1 )
times the concentration of NO 2 divided by the product of the
rate constant for the NO reaction with O 3 ( k 3 ) times the NO
concentration. This photolysis rate constant ( k 1 ) will depend
on the solar zenith angle, and hence will vary during the day,
peaking at solar noon. This relationship shows that the con-
centration of ozone can only rise for a fixed photolysis rate
as the [NO 2 ]/[NO] concentration ratio increases. Deviations
from this photostationary state relationship exist, because as
we will see shortly, peroxy radicals can also react with NO to
make NO 2.
Large concentrations of O 3 and NO cannot coexist, due to
reaction (3). Figure 2 shows the diurnal variation of NO, NO 2 ,
and oxidant measured in Pasadena, California. Several fea-
tures are commonly observed in plots of this type. Beginning
in the early morning, NO, which is emitted by motor vehi-
cles, rises, peaking at about the time of maximum automobile
traffic. NO 2 begins rising toward a maximum value as the NO
disappears. Then the O 3 begins growing, reaching its maxi-
mum value after the NO has disappeared and after the NO 2
has reached its maximum value. The time of the O 3 maximum
varies depending on where one is monitoring relative to the
urban center. Near the urban center, O 3 will peak near noon,
while further downwind of the urban center, it may peak in
the late afternoon or even early evening.^Hydrocarbon PhotooxidationThe chemistry of O 3 formation described thus far is overly sim-
plistic. How is NO, the primary pollutant, converted to NO 2 ,
which can be photolyzed? A clue to answering this question
comes from smog-chamber studies. A smog chamber is a rela-
tively large photochemical-reaction vessel, in which one can
simulate the chemistry occurring in the urban environment.
Figure 3 shows a plot of the experimentally observed loss rate
for propene (a low-molecular-weight, reactive hydrocarbon
commonly found in the atmosphere) in a reaction system ini-
tially containing propene, NO, and a small amount of NO 2.
The observed propene-loss rate in this typical chamber run
was considerably larger than that calculated due to the known
reactions of propene with oxygen atoms and ozone. Hence,
there must be another important hydrocarbon-loss process.
Hydroxyl radicals (OH) react rapidly with organics.
Radicals, or free radicals, are reactive intermediates, such as
an atom or a fragment of a molecule with an unpaired elec-
tron. Let’s look at a specific sequence of reactions involving
propene.
The hydroxyl radical reacts rapidly with propene:OH CH 3 CH=CH 2 → CH 3 CHCH 2 OH (4a)
OH CH 3 CH=CH 2 → CH 3 CHOHCH 2 (4b)These reactions form radicals with an unpaired electron
on the central carbon in (4a) and on the terminal carbon
in (4b). These alkyl types of radicals react with O 2 to form
alkylperoxy types of radicals.CH 3 CHCH 2 OH O 2 → CH 3 CH(O 2 )CH 2 OH (5a)
CH 3 CHOHCH 2 O 2 → CH 3 CHOHCH 2 (O 2 ) (5b)FIGURE 2 Diurnal variation of NO, NO 2 , and total oxidant in
Pasadena, California, on July 25, 1973. From Finlayson-Pitts and
Pitts (2000). With permission.0 500 1000 1500 2000 25000.000.040.080.120.160.200.240.280.320.360.400.440.48Time (hours)Concentration (ppm)OxidantNONO2C001_008_r03.indd 122C001_008_r03.indd 122 11/18/2005 10:15:19 AM11/18/2005 10:15:19 AM