Photochemical Smog


Gifted Chemistry IB
Alternative Assessment
1997 March 19

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
inversion. Many of the components which make up these smogs are not only
respiratory irritants, but are also known carcinogens.

There are many conditions for the development of photochemical smog:

1. A source of nitrogen oxides and volatile organic compounds.

2. The time of day is a very important factor in the amount of photochemical
smog present.

• Early morning traffic increases the emissions of both nitrogen oxides (NOx)
and Peroxyacetyl Nitrates (PAN) as people drive to work.
• Later in the morning, traffic dies down and the nitrogen oxides and
volatile organic compounds begin to react forming nitrogen dioxide, increasing
its concentration.
• As the sunlight becomes more intense later in the day, nitrogen dioxide is
broken down and its by-products form increasing concentrations of ozone.
• At the same time, some of the nitrogen dioxide can react with the volatile
organic compounds (VOCs) to produce toxic chemicals.
• As the sun goes down, the production of ozone is halted. The ozone that
remains in the atmosphere is then consumed by several different reactions.

3. Several meteorological factors can influence the information of photochemical
smog. These conditions include :

• Precipitation can alleviate photochemical smog as the pollutants are washed
out of the atmosphere with the rainfall.
• Winds can blow photochemical smog away replacing it with fresh air. However,
problems may arise in distant areas that receive the pollution.
• Temperature inversions can enhance the severity of a photochemical smog
episode. Normally, during the day the air near the surface is heated and as it
warms it rises, carrying the pollutants with it to higher elevations. However,
if a temperature inversion develops pollutants can be trapped near the Earth\'s
surface. Temperature inversions cause the reduction of atmospheric mixing and
therefore reduce the vertical dispersion of pollutants. Inversions can last from
a few days to several weeks.

4. Topography is another important factor influencing how severe a smog event
can become. Communities situated in valleys are more susceptible to
photochemical smog because