In urban areas like London motor vehicles, amongst other sources, are the main cause of air pollution. Photochemical reactions produce a cloud of toxic chemicals including ozone and a variety of caustic agents.

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Altaf Korimbocus       Centre number: 14342

Salters Chemistry Open-book Paper

In urban areas like London motor vehicles, amongst other sources, are the main cause of air pollution. Photochemical reactions produce a cloud of toxic chemicals including ozone and a variety of caustic agents.  This is often covering whole cities a photochemical smog.  This photochemical smog consists of two criteria of pollutants: primary and secondary pollutants.

Primary pollutants are emitted directly from their sources into the atmosphere.  Examples of primary pollutants include: carbon monoxide (CO), NOx, sulphur dioxide (SO2), and various hydrocarbons (HxCy), also known as volatile organic compounds (VOC). The prime source of air emissions generated by motor vehicles such as truck diesel emissions is CO, NOx and HxCy

Secondary pollutants can be produced over a period of time when photochemical reactions take place in the atmosphere.  An example of a secondary pollutant is ozone (O3), which is one of the products formed when NOx react with hydrocarbons (from motor vehicles), in the presence of sunlight.  Other secondary pollutants are NO2, H2O2, PAN (peroxyacetyl nitrate), partially oxidised hydrocarbons and HNO3.        

Primary pollutants can also be produced when coal-fired power stations combust fuels.  Coal is made up of many different organisms that were once alive, and all organisms contain compounds of sulphur, therefore when power stations combust coal to produce electricity the sulphur compounds oxidises to form the primary pollutant SOx.  Subsequently the gas will be released into the atmosphere. When coal burns in an enclosed space, oxygen is gradually depleted and carbon dioxide is increased. The changes in both of the these gases increasingly cause the combustion process to change from one of complete combustion to one of incomplete combustion, resulting in the production of increasing amounts of CO. Adapted from http://www.bartleby.com/65/ai/airpollu.html

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Photochemical smog is a result of photochemical reaction involving primary pollutants. The time of day also can affect the production of photochemical smog

Adapted from:

http://royal.okanagan.bc.ca/mpidwirn/atmosphere.html

The graph above illustrates the diurnal variations of NO, NO2 , and O3 typically detected in a photochemical smog situation. The morning rush hour peak in the NO emissions is followed by the gradual conversion to NO2 and the subsequent rise of O 3 which decays as the sun goes down in late afternoon.

For photochemical smog to occur there must be abundance of atmospheric pollutants, previously mentioned, and certain environmental conditions. To ...

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