Trends in air quality in the UK.

Many problems are encountered when assessing trends in air quality, including changes in methods of measuring pollutants over time, different sampling timeframes, and the number and distribution of monitoring sites. Nevertheless, reasonable projections have been established for trends in sulphur dioxide (SO2), particulate matter, nitrogen dioxide (NO2), and ozone levels in the UK, which allow some estimate of future levels. The major source of SO2 is coal-fired power stations. Since the Clean Air Act of 1956, SO2 levels have decreased substantially. Nevertheless, a small but progressive increase in SO2 emissions is predicted over the next 15 years, but this is unlikely to exceed current UK air quality standards on a frequent basis. Particulate pollution (as black smoke) has also decreased dramatically since the 1960s, when the main source was fossil fuel burning. In the 1990s, the main source of particle emissions is from heavy goods vehicles. If measures currently under consideration in the UK are implemented, annual emissions may decrease to below 60 kT over the next decade, but a progressive increase is predicted thereafter. No significant changes have occurred with respect to oxides of nitrogen (NOx) and ozone levels over the last 20 years. It is predicted that NOx emissions will decline in the future due in part to the introduction of catalytic converters, while trends in ozone levels will depend on substrate supply (NOx and hydrocarbons) and weather conditions. Despite short-term trends downwards in air pollutant levels, trends may reverse in the next millennium, and continued efforts must be made to develop new ways of reducing ambient air pollutant levels.