Photochemical Smog

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Photochemical Smog

Historically, the term smog referred to a mixture of smoke and fog, hence the

name smog. The industrial revolution has been the central cause for the increase

in pollutants in the atmosphere over the last three centuries. Before 1950, the

majority of this pollution was created from the burning of coal for energy

generation, space heating, cooking, and transportation. Under the right

conditions, the smoke and sulfur dioxide produced from the burning of coal can

combine with fog to create industrial smog. In high concentrations, industrial

smog can be extremely toxic to humans and other living organisms. London is

world famous for its episodes of industrial smog. The most famous London smog

event occurred in December, 1952 when five days of calm foggy weather created a

toxic atmosphere that claimed about 4000 human lives. Today, the use of other

fossil fuels, nuclear power, and hydroelectricity instead of coal has greatly

reduced the occurrence of industrial smog. However, the burning of fossil fuels

like gasoline can create another atmospheric pollution problem known as

photochemical smog. Photochemical smog is a condition that develops when primary

pollutants (oxides of nitrogen and volatile organic compounds created from

fossil fuel combustion) interact under the influence of sunlight to produce a

mixture of hundreds of different and hazardous chemicals known as secondary

pollutants. Development of photochemical smog is typically associated with

specific climatic conditions and centers of high population density. Cities

like Los Angeles, New York, Sydney, and Vancouver frequently suffer episodes of

photochemical smog.

One way in which the production of photochemical smog is initiated is through

the photochemical reaction of nitrogen dioxide (NO2) to form ozone. There are

many sources of photochemical smog, including vehicle engines (the number one

cause of photochemical smog), industrial emissions, and area sources (the loss

of vapors from small areas such as a local service station, surface coatings and

thinners, and natural gas leakage).

Vehicle engines, which are extremely numerous in all parts of the world, do not

completely burn the petroleum they use as fuel. This produces nitrogen dioxide

which is released through the vehicle exhaust along with a high concentration of

hydrocarbons. The absorption of solar radiation by the nitrogen dioxide results

in the formation of ozone (O3). Ozone reacts with many different hydrocarbons to

produce a brownish-yellow gaseous cloud which may contain numerous chemical

compounds, the combination of which, we call photochemical smog.

Both types of smog can greatly reduce visibility. Even more importantly, they

pose a serious threat to our health. They form as a result of extremely high

concentrations of pollutants that are trapped near the surface by a temperature

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