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Photochemical smog.

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Introduction

Photochemical smog, is the accumulation of pollutants caused by man, under the influence of light. Primary pollutants, which are directly produced from reactions, such as oxides of nitrogen (NOx), carbon monoxide and volatile organic compounds (VOC's). Whereas secondary pollutants are produced by the photochemical reactions of primary pollutants and the sun's energy, such as sulphuric acid, nitrogen dioxide and ozone. Primary pollutants are formed from the burning of fossil fuels in a coal fired power station and from the missions of car exhaust. Coal is a fossil fuel formed from the fossilisation of organisms, and therefore the sediment contains proteins. When fossil fuels are burnt, nitrogen compounds found in the fuel are oxidised and produce NOx. Thermal NOx is produced mainly due to the high temperature of combustion of the nitrogen and oxygen atoms. Sulphur compounds are also found in coal, when burnt at high temperatures these compounds are oxidised and forms oxides of sulphur (SOx) These oxides of nitrogen and sulphur are released as a waste product and are a primary pollutant in the formation of photochemical smog. ...read more.

Middle

However when primary pollutants are formed, and the hydrocarbons react with the oxides of nitrogen, an oxidation reaction occurs which breaks down the hydrocarbons into carbon dioxide and water in the troposphere. RCH2O2 + NO RCH2O + NO2 This reaction converts NO back to NO2 . This means that ozone is no longer required to form nitrogen dioxide because of the addition of the primary pollutants, and so ozone is not depleted and the concentration of ozone in the troposphere increases. Ozone breaks down due to the absorption of light energy to form an oxygen radical: hv O3 O2 + O This then reacts with water: O + H2O 2OH One molecule of O3 produces two OH radicals, which in turn produces one O3 molecule. Therefore a chain reaction occurs in which O3 is being readily used up, however two molecules of O3 are produced due to this and so the concentration of ozone in the troposphere increases. ...read more.

Conclusion

Inside the boiler natural gases are added. These react with the NOx to produce nitrogen, carbon dioxide and water vapour: CH4 + 4NO 2N2 + CO2 + 2H2O Any excess alkane and carbon monoxide is removed by oxidation. Longannet management decided to use gas reburn as their BPEO because it produces a cleaner waste gas which has minimal effect on the environment and as the oxidation of natural gas is exothermic, more electricity is produced. *** Research is being undertaken by chemists to understand which pollutants are present in the troposphere and how their concentrations vary. Monitoring stations have been set up to develop their research further. Chemists are researching reactions of photochemical smog and how quickly they occur in order to make predictions about pollution. This is achieved by measuring reaction rates under a variety of carefully controlled conditions. Further technological advances are being looked into to develop more accurate information on computers to predict the behaviour of pollutants. Practical research in laboratories using cutting edge technology such as probes that monitor concentrations are helping to further develop the knowledge of chemists about photochemical smog. ...read more.

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