Photochemical smog.

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  • Describe in terms of the production of photochemical smog, what is meant by primary pollutants and secondary pollutants in the atmosphere, and list the main primary and secondary pollutants produced as a result of motor vehicles.

Photochemical smogs are a mixture of smoke and fog derived from human activities. They take place in the troposphere, in the summer months. Photochemical smogs are highly oxidising. They contain a mixture of primary and secondary pollutants.

Primary pollutants are released directly into the atmosphere by, for example, combustion of fuels in cars and power stations. Secondary pollutants are formed when primary pollutants undergo further reactions. An example of a secondary pollutant is Ozone. Ozone is a secondary pollutant because it is formed in chemical reactions that take place when the sun shines on a mixture of primary pollutants.

The main primary and secondary pollutants released from motor vehicles are NOx (released from exhausts) of which contains mostly NO (95%) with only small amounts of NO2. Hydrocarbons are emitted from vehicle exhausts as unburnt fuel. Secondary pollutants due to these primary pollutants are O3, NO2, H2O2, PAN and partially oxidised VOCs HNO3.

  • Describe how primary pollutants are formed in the combustion of the fuel in a coal-fired power station.

Coal-fired power stations are major sources of pollution. They have three main inputs. These are, coal, cooling water to condense steam from the boilers, and very pure water for use in the steam turbines. The main outputs are electricity, waste heat, carbon dioxide, sulphur oxides, nitrogen oxides and ash.

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All living organisms contain sulphur compounds and these are the origin of sulphur found in coal. When coal burns, the sulphur compounds are converted to sulphur oxides (SOx). This is one primary pollutant.

Another primary pollutant is nitrogen oxide (NOx) contained in flue gases. Fuels contain compounds of nitrogen, formed from proteins contained in organisms. When the fuel is burnt, these nitrogen compounds are oxidised to form what is called fuel NOx. Also, at high temperature of combustion, atmospheric nitrogen and oxygen combine to form what is called thermal NOx. This is another primary pollutant.

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