172 Ecology and Applied Environmental Science
9.1.1 Photochemical Smog
High concentrations of nitrogen oxides (especially monoxide) in the atmosphere of some cities, in conjunction with high concentrations of gaseous
hydrocarbons and under the influence of solar radiation, lead to a series of
photochemical reactions. Solar radiation supplies the energy for the breaking up of chemical bonds of nitrogen oxides and volatile hydrocarbons
(or other VOCs). Atomic oxygen and reactive free radicals are produced that
subsequently compose a variety of gaseous pollutants. This photochemical
smog is a special form of air pollution; its characteristic pollutants are ozone
(tropospheric), nitrogen dioxide, and various organic compounds, such as
peroxyacyl nitrates (PAN). The presence of photochemical smog is harmful for man—it causes eye and throat irritation and respiratory problems,
damage to plants, and reduced visibility.
The presence of temperature inversion that traps and accumulates gaseous
pollutants as well as the presence of strong solar radiation are necessary
preconditions for the complex reactions that make up photochemical smog.
Under normal conditions, temperature decreases higher up from the surface
of the earth; therefore the movement of warm air creates upward currents
that facilitate the escape of pollutants towards higher stratums. During
temperature inversion, a warmer and therefore lighter stratum of air is created at a height of a few hundred meters from the ground, which prevents
upward movements and the escape of pollutants; as a result, the trapping of
pollutants in the low stratums exposes them to solar radiation for sufficient
time for the photochemical reactions to come about.
The basin of Athens (Greece) is a characteristic example of air pollution that
started having as its most significant pollutants sulphur dioxide and smoke,
and gradually developed into photochemical smog. Non-photochemical
pollution, which was high until the 1980s, decreased subsequently thanks
to the smoke emissions control, the use of fuels with less sulphur content
and the introduction of lead-free gasoline. However, the multitude of
circulating cars burdens the basin’s atmosphere with great quantities of
unburned hydrocarbons and NO x , and in conjunction with temperature
inversion and the strong and long-lasting solar radiation, photochemical
smog was often created. During hours of high traffic, high concentrations
of smoke and carbon monoxide were observed in some sections of the city.
9.1.2 Effects on health
Our knowledge in relation to the air pollution phenomena and especially to
its effects on human health is insufficient, and even more so in relation to
the synergistic action of the pollutants. SO 2 , for example, when adsorbed
on the surfaces of suspended particulates, can cause serious damage to
9.1.1 Photochemical Smog
High concentrations of nitrogen oxides (especially monoxide) in the atmosphere of some cities, in conjunction with high concentrations of gaseous
hydrocarbons and under the influence of solar radiation, lead to a series of
photochemical reactions. Solar radiation supplies the energy for the breaking up of chemical bonds of nitrogen oxides and volatile hydrocarbons
(or other VOCs). Atomic oxygen and reactive free radicals are produced that
subsequently compose a variety of gaseous pollutants. This photochemical
smog is a special form of air pollution; its characteristic pollutants are ozone
(tropospheric), nitrogen dioxide, and various organic compounds, such as
peroxyacyl nitrates (PAN). The presence of photochemical smog is harmful for man—it causes eye and throat irritation and respiratory problems,
damage to plants, and reduced visibility.
The presence of temperature inversion that traps and accumulates gaseous
pollutants as well as the presence of strong solar radiation are necessary
preconditions for the complex reactions that make up photochemical smog.
Under normal conditions, temperature decreases higher up from the surface
of the earth; therefore the movement of warm air creates upward currents
that facilitate the escape of pollutants towards higher stratums. During
temperature inversion, a warmer and therefore lighter stratum of air is created at a height of a few hundred meters from the ground, which prevents
upward movements and the escape of pollutants; as a result, the trapping of
pollutants in the low stratums exposes them to solar radiation for sufficient
time for the photochemical reactions to come about.
The basin of Athens (Greece) is a characteristic example of air pollution that
started having as its most significant pollutants sulphur dioxide and smoke,
and gradually developed into photochemical smog. Non-photochemical
pollution, which was high until the 1980s, decreased subsequently thanks
to the smoke emissions control, the use of fuels with less sulphur content
and the introduction of lead-free gasoline. However, the multitude of
circulating cars burdens the basin’s atmosphere with great quantities of
unburned hydrocarbons and NO x , and in conjunction with temperature
inversion and the strong and long-lasting solar radiation, photochemical
smog was often created. During hours of high traffic, high concentrations
of smoke and carbon monoxide were observed in some sections of the city.
9.1.2 Effects on health
Our knowledge in relation to the air pollution phenomena and especially to
its effects on human health is insufficient, and even more so in relation to
the synergistic action of the pollutants. SO 2 , for example, when adsorbed
on the surfaces of suspended particulates, can cause serious damage to