Page 207 - Fundamentals of Air Pollution

III. Gas-Phase Chemical Reaction Pathways 173

state, the change in concentration with time no longer occurs, so that
d[conc]/dt is equal to zero.

From Eq. (12-13), it is possible to obtain an expression for the relationship
of NO, NO 2, and O 3:

Equation (12-17) is called the photostationary state expression for ozone. Upon
examination, one sees that the concentration of ozone is dependent on the
ratio NO 2/NO for any value of k v The maximum value of ki is dependent
on the latitude, time of year, and time of day. In the United States, the
1
range of ki is from 0 to 0.55 min" . Table 12-5 illustrates the importance of
the NO 2/NO ratio with respect to how much ozone is required for the
photostationary state to exist. The conclusion to be drawn from this table
is that most of the NO must be converted to NO 2 before O 3 will build up
in the atmosphere. This is also seen in the diurnal ambient air patterns
shown in Fig. 12-2 and the smog chamber simulations shown in Fig. 12-
3. It is apparent that without hydrocarbons, the NO is not converted to
NO 2 efficiently enough to permit the buildup of O 3 to levels observed in
urban areas.
The cycle represented by Eqs. (12-9), (12-10), and (12-11) is illustrated by
the upper loop (a) in Fig. 12-4. In this figure, the photolysis of NO 2 by a
photon forms an NO and an O 3 molecule. If no other chemical reaction is
occurring, these two species react to form NO 2, which can start the cycle
over again. In order for the O 3 concentration to build up, oxidizers other
than O 3 must participate in the oxidation of NO to form NO 2. This will

TABLE 12-5

[O,l Predicted from Phntostationarv State Annrnximation as a Function of Initial fNO,l 5
[NO ]°(ppm) [N0 2] fmal (ppm) [O 3]finai (Ppm) [NOJ/[NO]
2
0.1 0.064 0.036 1.78
0.2 0.145 0.055 2.64
0.3 0.231 0.069 3.35
0.4 0.319 0.081 3.94
0.5 0.408 0.092 4.43
1
1
1
" frj = 0.5 min" ; £3 = 24.2 ppm^ mirT .