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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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. The coal that is used contains compounds of nitrogen, formed from proteins contained in organisms. When the compounds are burnt they are oxidised to form what is called fuel NOx[1] 

                        N2(g) + O2                                     2NO(g)

                2NO(g) + O2                                 2NO2(g)

Secondly, at high temperatures of combustion, atmospheric nitrogen and oxygen combine to form thermal NOx.

VOCs tend to recycle NOx, as they react with NO to form NO2

CH3CH3 (ethane) + OH· ---> CH3CH3· (an organic radical) + H2O

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CH3CH3· + O2 ---> CH3CH2OO· (this is called a peroxy radical)

CH3CH2OO· + NO ---> CH3CH2O· +NO2
(here, NO2 is formed, and the resulting radical goes into the next reaction.

CH3CH2O· + O2 ---> CH3CHO + HO2·

HO2· + NO ---> OH· + NO2
(these reactions form more NO2)

Describe and explain the most favourable conditions for forming photochemical smog, and how the high concentrations of tropospheric ozone are produced

The most favourable conditions for photochemical smog to form are hot humid days, for example summer days. “In summer 1995 the first smog came early. The bright sunshine ...

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