Urban Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 89

methane) in the presence of sunlight. The most recog-
nized gas-phase by-product of smog reactions is ozone
because ozone has harmful health effects (Section 3.6.5)
and is an indicator of the presence of other pollutants.
On a typical day, ozone forms following the emission
of NO(g) and ROGs. Emitted pollutants are called pri-
mary pollutants . Pollutants, such as ozone, that form
chemically or physically in the air are called secondary
pollutants .
Primary pollutant ROGs are broken down by chem-
ical reaction into peroxy radicals , denoted by RO 2 (g).
Peroxy radicals and NO(g) form secondary pollutant
ozone by the following sequence:
NO(g)
Nitric
oxide

The plot also shows that, for low ROGs, increases in
NO x (g) above 0.05 ppmv decrease ozone. For high
ROGs, increases in NO x (g) always increase ozone.
The plot is useful for regulatory control of ozone. If
ROG mixing ratios are high (e.g., 2 ppmC) and NO x (g)
mixing ratios are moderate (e.g., 0.06 ppmv), the plot
indicates that the most effective way to reduce ozone
is to reduce NO x (g). Reducing ROGs under these con-
ditions has little effect on ozone. If ROG mixing ratios
are low (e.g., 0.7 ppmC) and NO x (g) mixing ratios are
high (e.g., 0.2 ppmv), the most effective way to reduce
ozone is to reduce ROGs. Reducing NO x (g) under these
conditions actually increases ozone. In many polluted
urban areas, the ROG:NO x (g) ratio is lower than 8:1,
indicating that limiting ROG emissions should be the
most effective method of controlling ozone. However,
because ozone mixing ratios depend not only on chem-
istry, but also on meteorology, deposition, and gas-to-
particle conversion, such a conclusion is not always
clear cut.
Figure 4.13 shows the evolution of NO(g), NO 2 (g),
and O 3 (g) during one day at two locations - central
Los Angeles and San Bernardino - in the Los Angeles
Basin. In the basin, a daily sea breeze transfers primary
pollutants [NO(g) and ROGs], emitted on the west side
of the basin (i.e., central Los Angeles) to the east side
of the basin (i.e., San Bernardino), where they arrive as
secondary pollutants [O 3 (g) and PAN]. Whereas NO(g)
mixing ratios peak on the west side of Los Angeles, as
shown in Figure 4.13a, O 3 (g) mixing ratios peak on the
east side, as shown in Figure 4.13b. Thus, the west side
of the basin is a source region , and the east side is a
receptor region of photochemical smog.
Asignificant difference between ozone production in
urban air and the background troposphere is that peroxy
radicals convert NO(g) to NO 2 (g) in urban air but less
so in clean air. As a result, the NO 2 (g):NO(g) ratio is
much higher in urban air than clean air. Reaction 4.4
suggests that an increase in the NO 2 (g):NO(g) ratio
increases ozone levels above the pure photostationary-
state ozone level.

R O 2 (g)
Organic
peroxy
radical

NO 2 (g)
Nitrogen
dioxide

R O(g)
Organic
oxy
radical

+ → + (4.37)

NO(g)
Nitric
oxide

NO 2 (g)
Nitrogen
dioxide

+ O 3 (g)

Ozone →

+ O 2 (g)

Molecular
oxygen

(4.38)

NO 2 (g)
Nitrogen
dioxide

NO(g)
Nitric
oxide

O(g)
Atomic
oxygen

+ h → +· < 420 nm

(4.39)

M
→

O(g)
Atomic
oxygen

· + O 2 (g)

Molecular
oxygen

O 3 (g)
Ozone

(4.40)

Figure 4.12 shows ozone mixing ratios resulting from
different initial mixtures of NO x (g) and ROGs. This
plot is called an ozone isopleth .The figure shows that,
for low mixing ratios of NO x (g), ozone mixing ratios
are relatively insensitive to the quantity of ROGs. For
high NO x (g), an increase in ROGs increases ozone.

0.25

0.4

0.2

0.15

0.1

0.05

4.3.1. Emissions of Photochemical
Smog Precursors
Major gases emitted in urban air include nitrogen
oxides, ROGs, carbon monoxide [CO(g)], and sulfur
oxides [SO x (g)

0

0 0.5 1 1.5 2

ROG (ppmC)

Figure 4.12. Peak ozone mixing ratios resulting from
different initial mixing ratios of NO x (g) and reactive
organic gases (ROGs). The ROG:NO x (g) ratio along
the line through zero is 8:1. Adapted from
Finlayson-Pitts and Pitts (1999).

SO 3 (g)]. Of these, NO x (g)
and ROGs are the main precursors of photochemi-
cal smog. Sources of CO(g), SO x (g), and NO x (g) are
primarily incomplete combustion. Sources of ROGs

= SO 2 (g) +