Chemistry Assignment/ Civic Engagement

Page 973 21.7 Photochemical Smog

The word smog was coined to describe the combination of smoke and fog that shrouded London during the 1950s. The primary cause of this noxious cloud was sulfur dioxide. Today, however, photochemical smog, which is formed by the reactions of automobile exhaust in the presence of sunlight, is much more common.

Automobile exhaust consists mainly of NO, CO, and various unburned hydrocarbons. These gases are called primary pollutants because they set in motion a series of photochemical reactions that produce secondary pollutants. It is the secondary pollutants—chiefly NO2 and O3—that are responsible for the buildup of smog.

Nitric oxide is the product of the reaction between atmospheric nitrogen and oxygen at high temperatures inside an automobile engine:

Once released into the atmosphere, nitric oxide is oxidized to nitrogen dioxide:

Sunlight causes the photochemical decomposition of NO2 (at a wavelength shorter than 400 nm) into NO and O:

Atomic oxygen is a highly reactive species that can initiate a number of important reactions, one of which is the formation of ozone:

where M is some inert substance such as N2. Ozone attacks the C═C linkage in rubber:

where R represents groups of C and H atoms. In smog-ridden areas, this reaction can cause automobile tires to crack. Similar reactions are also damaging to lung tissues and other biological substances.

Ozone can be formed also by a series of very complex reactions involving unburned hydrocarbons, nitrogen oxides, and oxygen. One of the products of these reactions is peroxyacetyl nitrate (PAN):

PAN is a powerful lachrymator, or tear producer, and causes breathing difficulties.

Figure 21.25 shows typical variations with time of primary and secondary pollutants. Initially, the concentration of NO2 is quite low. As soon as solar radiation penetrates the atmosphere, though,

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more NO2 is formed from NO and O2. The concentration of ozone remains fairly constant at a low level in the early morning hours. As the concentration of unburned hydrocarbons and aldehydes increases in the air, the concentrations of NO2 and O3 also rise rapidly. The actual amounts depend on the location, traffic, and weather conditions, but their presence is always accompanied by haze (Figure 21.26). The oxidation of hydrocarbons produces various organic intermediates, such as alcohols and carboxylic acids, which are all less volatile than the hydrocarbons themselves. These substances eventually condense into small droplets of liquid. The dispersion of these droplets in air, called an aerosol, scatters sunlight and reduces visibility. This interaction also makes the air look hazy.

Figure 21.25 Figure 21.25 Typical variations with time in concentration of air pollutants on a smoggy day.

Figure 21.26 Figure 21.26 A smoggy day in a big city. Credit: © Kent Knudson/PhotoLink/Getty Images.

As the mechanism of photochemical smog formation has become better understood, major efforts have been made to reduce the buildup of primary pollutants. Most automobiles now are equipped with catalytic converters designed to oxidize CO and unburned hydrocarbons to CO2 and H2O and to reduce NO and NO2 to N2 and O2 [ Section 14.6]. More efficient automobile engines and better public transportation systems would also help to decrease air pollution in urban areas. A recent technological innovation to combat photochemical smog is to coat automobile radiators and air conditioner compressors with a platinum catalyst. So equipped, a running car can purify the air that flows under the hood by converting ozone and carbon monoxide to oxygen and carbon dioxide:

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In a city like Los Angeles, where the number of miles driven in one day equals nearly 300 million,

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this approach would significantly improve the air quality and reduce the “high-ozone level” warnings frequently issued to its residents.

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