Box 10B Air quality in London

London is a good example of how air quality standards have reduced traditional air pollution in Western European cities over the last 30 years. Annual mean sulphur dioxide concentrations have dropped dramatically from the 300 to 400 µg/m3 of the 1960s to 20 to 30 µg/m3. Concentrations of suspended particulate matter (smoke) levelled to 30 µg/m3. These levels are well below WHO annual guidelines. Both are the result of the switch in fuel type from fuel oil and coal to smokeless coal and natural gas as well as of the introduction of pollution control technologies in the manufacturing industry. A factor which has contributed to the drastic reduction of these pollutants in the city centre is also the relocation of these activities out of the city. However, high pollution episodes are still a problem during the winter. Exceedances of sulphur dioxide short-term guidelines (500 µg/m3 10-minute means, and 350 µg/m3 one-hour mean) are still a regular occurrence.

A drastic reduction in lead-in-air concentrations also occurred in the second half of the 1980s as a result of the phase-out of permissible lead levels in petrol which entered into force in 1986. Annual concentrations are now below WHO guidelines at background sites, and roadside concentrations do not exceed the upper limit of WHO guidelines at the central London site (Figure 10.3).

Carbon monoxide concentrations show mixed patterns across different monitoring sites. The number of episodes of the 8-hour WHO guideline exceedances in central London are of major concern. Emissions from road traffic, which account for 95 per cent of total carbon monoxide emissions, are trapped by ground-based temperature inversions, building up high localised concentrations. High traffic volumes are also the cause of increased concentrations of nitrogen oxides. While a number of sites show stable trends in nitrogen oxides concentrations, other sites indicate marked increases. During 1989, 16 sites in central London exceeded the EC 98 percentile limit of 200 µg/m3, and all 64 sites thoughout London but two had an estimated 50 or 98 percentile greater than the EC guide values of 50 and 135 µg/m3. Measurements of ambient tropospheric ozone concentrations at a number of sites in central London show no clear trends, but they indicate the frequent occurrence of exceeding the hourly mean value of 214 µg/m3 (100 ppb), the upper limit of the WHO guideline range.

Emissions from motor vehicles are the most significant source of carbon monoxide, nitrogen oxides and secondary pollutants such as ozone. These are expected to increase further due to the increased number of cars. Greater London had 2.7 million motor vehicles in 1988 (one car per three inhabitants) and together with the metropolitan area accounts for over 6 million cars (35 per cent of all UK cars). According to OECD estimates, road traffic in London has increased by 70 per cent during the last 20 years. A 7 per cent increase has occurred since 1989 alone.

Source: UNEP/WHO, 1992


Figure 10.3 - Annual mean concentration of SO2, smoke and NOx, London, 1973­89