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Photochemical smog.

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Introduction

* 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. ...read more.

Middle

The still air means that there is much less mixing with high altitude air and the pollutants are trapped near ground level. If there is a light prevailing wind (shown in the diagram below), the polluted air will be transported from the built up urban area it was generated and will move along rural areas. This diagram shows the formation of photochemical smog The troposphere acts as a huge reaction vessel, with a vast array of chemical reactions taking place all the time. Most of these reactions involve radicals. Some reactions take place when molecules absorb energy from sunlight (this has to be of a specific frequency) and undergo a photochemical reaction. This is indicated by the symbol hv where v is t O that creates ozone therefore more ozone is produced. * Describe the chemistry of the processes chosen as BPEOs at Longannet for minimising sulphur dioxide and NOx emissions, and suggest why the Longannet management made those choices. ...read more.

Conclusion

Studying individual reactions in the laboratory To make predictions about pollution, chemists need to know what reactions take place and how quickly they occur. Many of these reactions involve broken down fragments of molecules called radicals. Reactions with radicals happen very quickly but other reactions happen very slowly. Chemists measure the length of time of these reactions to predict the rate at which a reaction will proceed for any set of conditions. 3. Modelling Studies The information on rates of reactions is used in computer simulation studies to reproduce and predict the behaviour of pollutants during a smog episode. The more accurate the information used, the more closely the model simulates the observed behaviour. 4. Smog Chamber Solutions These are laboratory experiments on a large scale. Primary pollutants are mixed in a huge clear plastic bag called a smog chamber and exposed to sunlight under carefully controlled conditions. Probes monitor the concentrations of various species as the photochemical smog builds up. The chamber has to be big to minimise any 'surface effects' where the reactions take place on the walls of the container instead of the gas phase. 1 ...read more.

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