Environmental Engineering Reference
In-Depth Information
A major contributor to air pollution in Southern California was the
exhaust from cars and trucks, a fact vehemently denied by the automobile
industry until it was irrefutably proven by Arie Haagen-Smit at the Cali-
fornia Institute of Technology in the early 1950s. Today, cars and light
trucks account for about 60 percent of smog-creating emissions in the
region, so any successful effort to reduce air pollution has to take full
account of the emissions produced by the region's large vehicle population.
Motor vehicle emissions are converted to smog through a series of
chemical reactions that occur in the presence of sunlight.
1
Uncontrolled
vehicles produce the constituents of smog in a number of ways: through the
venting of vaporized gasoline, the emission of gases from the engine's
crankcase, and most important through the combustion process that con-
verts gasoline into the power that propels them. When a charge of air and
vaporized fuel is compressed and then ignited in an engine's combustion
chamber, not all of the fuel is completely combusted; some unburned
hydrocarbons are emitted. At the same time, high temperatures and pres-
sures within the combustion chamber convert atmospheric nitrogen into
various oxides of nitrogen (NOx for short). The exhaust gases are then
released into the atmosphere, where the ultraviolet portion of sunlight
breaks down NO
2
, one of the oxides of nitrogen, into NO. The liberated
oxygen atoms then combine with atmospheric oxygen (O
2
) to produce one
of the major constituents of photochemical smog: ozone (O
3
), a major irri-
tant to the respiratory system.At the same time, other oxides of nitrogen are
converted into a variety of compounds, notably the peroxyacyl nitrates that
contribute to the eye-burning effects of smog. Residual NO
2
adds to the
general nastiness by obscuring vision with a brown haze.
Combustion of gasoline in an engine also produces carbon monoxide
(CO), carbon dioxide (CO
2
), water vapor, sulfur dioxide, and particulates.
Strictly speaking, these are not constituents of photochemical smog. They
are still a significant problem, however. Recent years have seen a growing
concern about the emission of CO
2
into the atmosphere because it may
contribute to a “greenhouse effect” and consequent global warming. Solid
proof of this phenomenon remains elusive, but the increasing likelihood
that today's cars and trucks are contributing to global warming may neces-
sitate the eventual supplantation of fossil-fuel burning internal-combustion
engines by other sources of power; no matter how clean it is in other
respects, an internal-combustion engine powered by a carbon-based fuel
will always produce CO
2
.
