How Is Photochemical Smog Formed?

photochemical smog only occurs in daytime

Photochemical smog is formed when emissions and other chemicals react with sunlight. The term "smog" is derived from smoke and fog. It is a type of air pollution where air particles turn into harmful substances such as ozone, peroxyacetyl nitrate and aldehydes due to the reaction between sunlight and automobile exhaust gases. Photochemical smog can cover an entire town or city.

Formation

Photochemical smog is an effect of the ultraviolet radiation from the sun on a polluted atmosphere that contains a lot of oxides of nitrogen and hydrocarbons, especially from motor vehicle exhaust. Photochemical smog only occurs in sunlight. The chances of formation of photochemical smog are increased when the weather is still and sunny and when pollutants have accumulated in the lower atmosphere.

Summer Photochemical Smog

Summer smog is common in cities that do not have proper emission disposal through wind; the smoke is thus trapped because of extreme heat conditions. Atmospheric chemical reactions happen quickly during the summer because of the availability of sunlight and high temperature conditions. When these conditions are accompanied by humidity, dense smog can be formed. When there is a temperature inversion in higher altitude, it can result in summer photochemical smog since a humid air layer is trapped below the warm air layer, holding up pollutant particles. Summer smog is common in cities that have mountains or hills around them.

Winter Photochemical Smog

In contrast, winter smog is fundamentally shaped by the extreme use of fossil fuels for heating homes and buildings. Winter photochemical smog formation is marked during the middle of the winter, when the weather conditions are extreme. During such extreme cold conditions, people in the cities use coal and other combustibles to generate heat, which results in smoke and pollutants in the atmosphere. Moist air can retain the emissions for a while. Cities that are surrounded by elevated land and that have large amounts of snowfall can witness formation of smog.

Effects of Toxicity

Toxicity is inherent in photochemical smog. Victims can experience breathing difficulties, fatigue and headaches as well as aggravated respiratory conditions. It can irritate the eyes and nose. Apart from health hazards, photochemical smog can also endanger the environment as well as cause decay in the structure of buildings.

Preventive Measures

If automobiles are provided with catalytic converters, it can reduce the emissions of carbon monoxide and nitrogen oxide. Such a catalyst can be a combination of platinum or rhodium or just platinum. Since platinum catalyses un-burnt hydrocarbon and oxygen reaction, resulting in production of carbon-dioxide and water vapor, the formation of ozone is nullified. Similarly, rhodium can also help by oxidizing all the carbon monoxide to carbon dioxide. If automobiles use unleaded petrol, the catalytic converters can be used.

References

• Dictionary: smog
• AUS-e-TUTE: The Chemistry of Photochemical Smog
• Middle Tennessee State University: Photochemical Smog