Photochemical Smog
Any city can have photochemical smog, but it is most common in arid locations. A rise in
the number of vehicles in cities worldwide has increased photochemical smog. This smog
forms when car exhaust is exposed to sunlight. Nitrogen oxides are created in car combustion
chambers. If there is sunshine, the NO 2 splits and releases an oxygen atom (O). The oxygen
ion then combines with an oxygen molecule (O 2 ) to form ozone (O 3 ). This reaction can also
go in reverse: Nitric oxide (NO) removes an oxygen atom from ozone to make it O 2. The
direction the reaction proceeds depends on how much NO 2 and NO there is. If NO 2 is three
times more abundant than NO, ozone will be produced. If nitrous oxide levels are high,
ozone will not be created.
Ozoneis an acrid-smelling, whitish gas. Warm, dry cities surrounded by mountains, such
as Los Angeles, Phoenix, and Denver, are especially prone to photochemical smog (Figure
22.3). Photochemical smog peaks at midday on the hottest days of summer. Other com-
pounds in addition to ozone are found in photochemical smog. Ozone is also a greenhouse
gas.
Figure 22.3: Counties with such high ozone levels that they do not attain federal air quality
standards. ( 5 )
Causes of Air Pollution
Most air pollutants come from burning fossil fuels or plant material. Some are the result
of evaporation from human-made materials. Nearly half (49%) of air pollution comes from
transportation, 28% from factories and power plants, and the remaining pollution from a
variety of other sources.