Air pollution is not a recent problem, it has occurred throughout history through the burning of coal, wood and vegetation. However over the twentieth and twenty first centuries extreme forms of urban pollution take place during short periods of time, these occurrences are known as pollution episodes (Jacobson, 2002). Stable atmospheric conditions, along with low wind speeds and high pressure systems, can lead to the build-up of various pollutants.
Winter pollution episodes are associated urban coal burning and vehicle emissions. The infamous London smog of 1952 was caused by smoke and sulphur dioxide from domestic heating and industrial emissions. However now in the UK they are more closely linked to gases from vehicle emissions. This type of air pollution occurs in urban areas where there is a large input of pollutants. The formation of winter smog’s occur during particular weather conditions, anti-cyclonic pressure systems, along with low wind speeds give calm and stable atmosphere (DEFRA, 2010). The winter weather brings cold surface temperatures that lead to the formation of a temperature inversion. This phenomenon occurs when a warm air mass causes a lid effect preventing mass of cold air from rising and dispersing (Environment Canada, 2011). Put this with the low wind speeds and cold calm conditions, the air can become stagnant and the pollutants accumulate. Water vapour in the atmosphere can condense onto the particles in the air, leading to fog like conditions, and therefore a loss of visibility. This is where the term smog originated. Pollutants particularly at ground level can cause many health problems. Smog particles can cause respiratory and cardiovascular diseases. According to the World Health Organisation (WHO) the particles can be so small that they are inhaled into the lungs. The deeper into the lungs the particles go the more severe the health problems (WHO, 2008). High levels of Nitrogen oxides can lead to the inflammation of the airways, any long term exposure can affect lung functions and the respiratory systems. People that are particularly susceptible to the health problems are the elderly, very young and people that suffer with asthma and other respiratory diseases. The environment also suffers during pollution episodes. They can lead to leaf and needle damage as well as reduced growth. Sulphur dioxide can reduce photosynthesis and degrade chlorophyll in the leaves.
Summer episodes are associated with hot weather, with an abundance of sunshine. These smogs are caused by nitrogen oxides, volatile organic compounds and unburnt hyrdocarbons from road veicles and large combustion engines, these are called primary pollutants. Unlike winter pollution episodes no fog actually occurs here, this type of smog forms a brown-red-yellow haze. The colour comes from the primary pollutants as as they absorb blue and green light, leaving the coloured haze behind. Jacobson (2002) states that the sun plays a key role in summer pollution episodes, photochemical reactions take place in the atmosphere forming secondary pollutants. Ozone is a secondary pollutant, it is not emitted by any source but is formed by photoylsis (reactions between chemicals and sunlight) of other chemicals that have been emitted into the atmoshpere. The UV rays that form ground level O3 can also deplete it; the reaction can go both ways. Throughout the day primary pollutants are released into the atmosphere. These pollutants peak twice daily, during rush hour times, as this is when the majority of chemicals are emitted through vehicle emissions. Secondary pollutants peak around midday, this is the time when the Earth receives the strongest of the sun’s rays causing the photochemical reactions to occur. Furthermore it has been noted that ozone can accumulate in areas downwind from where it was formed. The moving air mass carries the ozone out of the cities to other urban and rural areas. Ground level ozone can be high in both urban and rural zones during pollution episodes. Ozone is hazardous at ground level; it causes irritation to both the eyes and nose. Very high levels can damage airways, reduce lung functions and increase respiratory illnesses (WHO, 2008). Furthermore ozone damages many plant species. This leads to a loss of yield, damage to forests and a reduction in biodiversity (WHO, 2008).
Pollution episodes usually last for a few days, however severe episodes can last for over a week, until the weather conditions change. The pollutants can be swept away by winds into the background, be washed out of the air by rainfall or diffuse upwards.
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References
Anon. (last modified 2011). Winter smog, Environment Canada Available: http://www.ec.gc.ca/Air/default.asp?lang=En&n=AFF4D58F-1 [accessed 23/02/2011].
Anon. (last modified 2011). Sources and impacts of air pollution, Department of food and rural affairs. Available: http://ww2.defra.gov.uk/environment/quality/air/air-quality/impacts/ [accessed 23/02/2011].
Jacobson, Mark Z. (2002). Atmospheric Pollution: History, science and regulation. Cambridge University Press: Cambridge.
World Health Organisation (Last modified 2008). . AAir Quality and Health Fact Sheet number 313. Available: http://www.who.int/mediacentre/factsheet/fs313/en/ [accessed: 25/02/2011].
