Theory

• Air quality in urban areas in often poorer than in rural areas. Particulate air pollution is caused by the release of particles and noxious gases into the atmosphere. Emissions of particles can occur naturally but they are largely caused by the combustion of fossil fuels. A combination of dust, soot and gases are produced from vehicles and industry, having a negative impact on human health in cities.
• Air pollution varies with the time of year and with air pressure. Concentration of pollutants may increase five- or six-fold in winter because temperature inversions trap them over the city. The mixture of fog and smoke particulates produces smog (common in European cities in the 19th century e.g. December 1952 - 4000 deaths in London)
• A concern today is the increase of photochemical smog. Photochemical oxidants (ozone and peroxyacetyl nitrate) are associated with damage to plants and discomforts to people (headaches, coughs, eye irritation, chest pains). The action of sunlight on nitrogen oxides and hydrocarbons in vehicle exhaust gases causes a chemical reaction which results in the production of ozone.
• Photochemical smog is a particular hazard during anticyclonic conditions because once air has descended, it is relatively static owing to the absence of wind. Such weather systems tend to be stable and can persist for weeks during the summer months.