Fossil Fuels
Fossil fuels are burned in most motor vehicles and power plants (Figure22.4). They fuel
manufacturing and other industries. Pure coal and petroleum can theoretically burn cleanly,
emitting only carbon dioxide and water, which are both greenhouse gases. But most of
the time, these fossil fuels do not completely burn, so these incomplete chemical reactions
produce pollutants. In addition, few fossil fuels are pure and so other pollutants are usually
released. These pollutants include carbon monoxide, nitrogen dioxide, sulfur dioxide and
hydrocarbons.
Figure 22.4: A power plant and its emissions before emission control equipment was added.
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In large car-dependent cities such as Los Angeles and Mexico City, 80% to 85% of air
pollution is from motor vehicles. Auto emissions are the most common source of ozone.
Carbon monoxide is toxic in enclosed spaces like tunnels. Nitrous oxides come from the
exhaust from a vehicle or a factory. Lead was once put in gasoline to improve engine knock,
but is now banned in the United States. Still, enormous quantities of lead are released into
the air every year from other sources.
A few pollutants come primarily from power plants or industrial plants. They pour out of
smokestacks that burn coal or oil. Sulfur dioxide (SO 2 ) is a major component of industrial
air pollution. It is released whenever coal and petroleum are burned. SO 2 mixes with H 2 O in
the air to produce sulfuric acid (H 2 SO 4 ). The heavy metal mercury is released when coal and
some types of wastes are burned. Mercury is emitted as a gas, but as it cools, it becomes a
droplet. Mercury droplets eventually fall to the ground. If they fall into sediments, bacteria
convert them to the most dangerous form of mercury: methyl mercury. Highly toxic, methyl
mercury is one of the metal’s organic forms.