Page 202 - Fundamentals of Air Pollution

168 12. Atmospheric Chemistry

TABLE 12-1
Values of exp(-£ a/J?T) as a Function of the
Activation Energy for T = 298K"
exp(-£./RT)
£ a
1.0 0.67
3.0 0.30
10.0 0.0177
30.0 5.51 x 10 6
100.0 2.95 x 10" 18
300.0 2.56 x MT 53
1
!
• In SI units, R = 8.31434 kj~ mol .
III. GAS-PHASE CHEMICAL REACTION PATHWAYS

The complexity of the atmospheric chemical reactions occurring in major
metropolitan areas can be staggering. Urban atmospheres are characterized
as complex mixtures of hydrocarbons and oxides of sulfur and nitrogen.
Table 12-3 show the hydrocarbons identified in the urban air of St. Peters-
burg, Florida (5). The interactions among this large number of compounds
can be understood by studying simpler systems. Figure 12-2 shows the
diurnal patterns of NO, NO 2, and O 3 for St. Louis, Missouri (6). These
diurnal patterns are interrelated. The concentration profiles of Fig. 12-2 are
the result of a combination of atmospheric chemical and meteorological
processes. To uncouple this combination of factors, laboratory (smog cham-
ber) studies such as those of the propene-NO x system (Fig. 12-3) have been
undertaken (7). These profiles show chemical transformations separated
from meteorological processes.
Similar chemical steps occur in the ambient air and in laboratory smog
chamber simulations. Initially, hydrocarbons and nitric oxide are oxidized

TABLE 12-2
Activation Energies for Atmospheric Reactions
Reaction E a (kj/mol)
N 2 + O 2 -» N 2O + O 538
CO + O 2 -* CO 2 + O 251
SO 2 + NO 2 -» SO 3 + NO 106
O + H 2S -» OH + HS 6.3
O + NO 2 -» NO + O 2 <1
HO 2 + NO -» NO 2 + OH <1

Source: Campbell, I. M., "Energy and the Atmo-
sphere," pp. 212-213. Wiley, New York, 1977.