Production of Photochemical Smog.

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Atmospheric Pollution Caused by

Coal-Fired Power Stations and Motor Vehicle Engines

Production of Photochemical Smog

Photochemical smog comprises of ‘primary’ and ‘secondary’ pollutants, which are extremely oxidising.

Primary pollutants are pollutants created as by-products of chemical processes such as combustion in car engines and industrial factories, and released directly into the atmosphere. These pollutants produced from motor vehicles include nitrous oxides, (NOx) carbon monoxide, (CO) and volatile organic compounds (VOCs) such as hydrocarbons.

Secondary pollutants are formed when primary pollutants undergo further reactions. Secondary pollutants from motor vehicles include ozone, nitrogen dioxide (NO2), Hydrogen Peroxide (H202), Peroxylacetyl nitrate (PAN), incompletely oxidised VOCs and Nitric Acid (HNO3). Ozone is formed when the sun shines on a mixture of primary pollutants. (This indicates the origin of the name ‘photochemical’ smog.)

Hydrocarbons are expelled from car exhausts as unburnt fuel, and can be released by evaporation from petrol and diesel fuel.

Formation of Primary Pollutants

Primary pollutants are often formed by oxidation under high heat during combustion in coal-fired power stations and in car engines. Most NOx emissions are from thermal NOx formed from nitrogen and oxygen atoms in air. Fuels burnt also contain nitrogen compounds, which are oxidised to form NOx. Carbon monoxide (CO) is formed by a similar process when carbon compounds in the fuel are oxidised making CO.

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Formation of Photochemical Smog

Photochemical smog usually occurs during summertime during high pressure, anticyclonic periods, (in the UK, from as early as May to August.) Ozone mixes with primary pollutants in the lower troposphere. Smog tends to form over cities due to higher pollutant concentration, but can disperse over large areas.

Although there is a background concentration of ozone in the troposphere, these naturally occurring pollutants are a negligible contribution.

Production of high concentrations of Tropospheric ozone

For the production of photochemical smog, the troposphere symbolises a huge reaction vessel, where ...

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