Urban Air Quality

Smogswere common in many British cities in the late 19th and early 20th centuries, when domestic fires, industrial furnaces and steam trains were all emitting smoke and other hygroscopic pollutants by burning fossil fuels. The smogs were particularly bad during the winter months and when temperature inversions built up under high pressure, causing the pollutants to become trapped in the lower atmosphere and for water vapour to condense around these particles. One of the worst of these ‘pea-soup fogs’ was the London smog of the winter of 1952/53. Approximately 4,000 people died during the smog itself, but it is estimated that 12,000 people may have died due to its effects. As a result, the Clean Air Act of 1956 was introduced to reduce these emissions into the lower atmosphere. Taller chimney stacks and the banning of heavy industry from urban areas were just two of the measures introduced and, consequently, fewer smogs were recorded in the United Kingdom during the 1960s and 1970s. Research in the 1990s has shown, however, that another type of smog — photochemical — is now occurring in some urban areas as a result of fumes from car exhausts and the build up of other pollutants in the lower atmosphere which react with incoming solar radiation. The presence of a brown-coloured haze over urban areas is an indication of photochemical smog, and among its side effects are people experiencing breathing difficulties and asthma attacks.

What is photochemical smog?

Photochemical smog is a mixture of pollutants that are formed when nitrogen oxides and volatile organic compounds (VOCs) react to sunlight, creating a brown haze above cities. It tends to occur more often in summer, because that is when we have the most sunlight.

Primary pollutants The two major primary pollutants, nitrogen oxides and VOCs, combine to change in sunlight in a series of chemical reactions, outlined below, to create what are known as secondary pollutants. ,Secondary pollutants The secondary pollutant that causes the most concern is the ozone that forms at ground level. While ozone is produced naturally in the upper atmosphere, it is a dangerous substance when found at ground level. Many other hazardous substances are also formed, such as peroxyacetyl nitrate (PAN).

How is smog formed?

Below is a simplified explanation of the chemistry of smog formation. Nitrogen dioxide (NO2) can be broken down by sunlight to form nitric oxide (NO) and an oxygen radical (O):

1) NO2 + ‚ NO + O sunlight

Oxygen radicals can then react with atmospheric oxygen (O2) to form ozone (O3):

2) O + O2 ‚ O3

Ozone is consumed by nitric oxide to produce nitrogen dioxide and oxygen:

3) O3 + ‚ NO2 + O2 NO

Harmful products, such as PAN, are produced by reactions of nitrogen dioxide with various hydrocarbons (R), which are compounds made from carbon, hydrogen and other substances. The main source of these hydrocarbons is the VOCs.

What are the major sources of photochemical smog? While nitrogen oxides and VOCs are produced biogenically (in nature), there are also major anthropogenic (man-made) emissions. Anthropogenic sources: Nitrogen oxides are produced mainly from the combustion of fossil fuels, particularly in power stations and motor vehicles. VOCs are formed from the incomplete combustion of fossil fuels - Motor vehicles contributed a high percentage of VOC emissions.

Pollution Reduction Strategies

What steps are cities taking in order to reduce air pollution levels?

Read pages 384 and 385, follow some of the links on pfalevelgeog too…

Explain how air pollution levels in cities can be managed (6)

  • Make reference to London, Beijing and Mumbai (you can mention other cities too but you must mention these!)
  • Structure your answer to cover legislation, vehicle management, land use zoning.