 What is Photochemical smog? Nitrogen oxides
(NOx) and volatile organic compounds (VOCs) are emitted into the atmosphere from many natural and
anthropogenic processes. In the atmosphere, these substances enter a complex
network of photochemical reactions induced by ultraviolet
light (UV-light) from the sun. These reactions lead to the formation of
ozone (O3), peroxyacetyl nitrate (PAN) and peroxybenzoyl
nitrate (PBN ) and a number of other substances in the troposphere. The
compounds share the term “photochemical smog” because they are
generated by photochemical reactions and constitute a major part of the
smog above many cities and industrialized areas. The
photochemical smog compounds are toxic to humans, animals and plants and
they degrade many materials. The smog can be
observed as a red brownish cast in the air above many cities. |
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| Photochemical smog is generated when NOx and VOCs appear simultaneously in the atmosphere in the presence of UV-light from the sun. The two pictures show photochemical smog over Bangkok (left) and Seattle (right). Note the characteristic red-brownish color from NO2. | |
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Which compounds contribute to photochemical smog formation? NOx is a common term for NO and NO2. The substances are emitted from combustion processes, e.g. coal combustion in power plants, gasoline or diesel combustion in cars and trucks, wood and gas combustion in stoves and cookers and waste incineration. VOCs comprise a major group of organic compounds which evaporate readily into the atmosphere under ambient temperature and pressure conditions. The group includes substances such as benzene, ethanol and trichloroethane, and mixtures such as gasoline and turpentine. VOCs are also emitted from combustion processes.
Damage to plants Tropospheric ozone is absorbed by plants through the leafs. In the leaves, ozone degrades chlorophyll. The loss of chlorophyll inhibits photosynthesis and hence growth of the plant. This is critical for the natural balance of plants and animals in the environment and for human beings, because reduced photosynthesis leads to reduced output from agriculture. Damage to humans At low concentrations, photochemical smog can cause reduced functionality of the lungs, chest constriction and eye, nose and throat irritation. At higher concentrations, smog can cause coughing and decreased ability to concentrate. Damage to materials and products Ozone attacks natural rubber, cellulose, synthetic polymers (plastics)
etc. and reduces the life time of textiles, car tires, windscreen
wipers, electrical insulation, etc. Ozone, PAN and PBN attack dyes and
causes undesired bleaching of textile colors, for example. |
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