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Describe, in terms of 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.

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Introduction

Chemistry Open Book 2003 Describe, in terms of 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 The combustion of fossil fuels in vehicles releases incompletely burnt chemicals and oxidised species known as primary pollutants into the atmosphere. Many of the primary pollutants undergo further reaction under the influence of sunlight. The produces of these photochemical reactions are called secondary pollutants. Primary pollutants consist of oxides of nitrogen. In the atmosphere nitric acid, which is produced by high temperature combustion cylinders in engines, is oxidised to the brown gas nitrogen dioxide NO2, a major constituent to smog. Primary NOX (NO + NO2), CO Secondary: O3, NO2, H2O2, PAN (peroxyacetyl nitrate), 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 have three main inputs, coal, cooling water and very pure water. ...read more.

Middle

< 435nm .NO2 . NO + .O. But once formed these radicals go on to react with NO to form NO2.. O3 +NO3 O2 + NO2. The more NO2 is available to produce more oxygen atoms and more ozone will therefore be formed. Further reactions with hydrocarbons and other volatile organic compounds can lead to an increase of ozone in the troposphere. (Fig 1) O. + H2O 2.OH These .OH radicals can abstract H from the hardocarbons to form alkyl radicals .OH + RCH3 H2O + RCH2 (Fig 1) Photochemical smog occurs in the lower portion of our atmosphere, known as the troposphere. (Fig 2.) If there is light prevailing wind, the polluted air mass is transported from the urban areas, where it was created, to rural regions. In fact, the highest ozone readings are often recorded over rural areas because the chemical reactions producing the secondary pollutants take place quite slowly. Fig 2 This graph shows the ozone concentration peaks in the middle of the day. Fig 3 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 The BPEO (best practical environmental option) ...read more.

Conclusion

and by delaying the combustion of coal. Reduction of 30 to 55 percent of NOx can be achieved with low-NOx burners. Advanced stage combustion technologies use overfire air and gas or coal reburning to achieve even greater reductions of NOx . Fig 6 Longannet chose to use sea water scrubbing as a BPEO because there was no solid waste created plus at Longannet the key raw material is sea water and is readily available because it is situated on the bank of the Fifth of Forth. As far as the reduction of NOx is concerned Longannet is fitted with both low NOx burners and gas reburn, therefore making it one of the cleanest power stations in the world. Outline the part played by chemists in the research on photochemical smog formation. There are currently several fronts of research to discover and explain the complexity of the chemistry involved in photochemical smog. Monitoring of tropospheric pollutants is an important first step is to know which pollutants are present in the troposphere Fig 7 There are now several monitoring stations across the country recording pollutant concentrations. Studying individual reactions in the laboratory allows chemists to make predictions about pollution how they occur. ...read more.

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