Air quality improves in the UK 
The air quality indicator is one of the 68 indicators of the UK Government’s Sustainable Development Strategy. It measures annual levels of pollution from particulates (PM10) and ozone (O3), the two pollutants thought to have the greatest health impacts, as well as the number of days on which levels of any one of a basket of five pollutants were ‘moderate or higher’.
The main results are:
* Urban background particulate levels averaged 21 microgrammes per cubic metre (µg m-3) in 2007 compared to 24µg m-3 in 2006. These levels have fluctuated in the last 5 years, although there has been an overall decreasing trend since 1993, the first year for which data were available.
* Roadside particulate levels averaged 31 µg m-3 in 2007 compared to 32 µg m-3 in 2006. There has been a general downward trend since monitoring started in 1997, although this decline has slowed since 2001.
* Rural ozone levels averaged 67 µg m-3 in 2007 compared to 74 µg m-3 in 2006 and 68 µg m-3 in 1993. There is no clear long term trend.
* Urban background ozone levels averaged 57 µg m-3 in 2007 compared to 61 µg m-3 in 2006 and 44 µg m-3 in 1992. These levels have shown an overall increasing long term trend since 1992, the first year for which data were available.
* In urban areas, air pollution in 2007 was recorded as moderate or higher on 23 days on average per site, compared with 41 days in 2006, and 59 days in 1993. This series has reflected a high degree of year-on-year variability over time, and this has again been apparent for 2007.
* In rural areas, air pollution in 2007 was moderate or higher for 26 days on average per site, compared with 56 in 2006. This figure has also varied significantly over time.
* These results are provisional and are therefore subject to change. Final results we be available in the spring.
An air quality “headline” indicator was introduced in 1999 in support of the UK Sustainable Development Strategy . When this strategy was updated in 2005, a new air quality indicator was included, better reflecting the effects on health of long term exposure to lower levels of pollution. The indicator is split into two parts covering; (a) annual exposure to pollutants and (b) the number of days when levels pollutants are moderate or higher.
Particulates and ozone
Part (a) of the indicator measures annual exposure to particulates and ozone. It was introduced in the light of increasing evidence suggesting that long-term exposure to even low levels of particulate (PM10) may have a significant effect on public health. The annual mean values for particulates are a useful measure of overall exposure to particulates at all concentrations. The annual average measures of PM10 have been included to reflect this.
The impact of long term exposure to low levels of ozone is currently less clear, but if there is no lower limit on the levels which have a health impact then the parameter used in the indicator gives the best representation of the overall annual impact of the short term effects of ozone pollution. The production of ozone is strongly influenced by the weather, more being created on hot, still, sunny days. There is an upward trend in urban background ozone levels in the UK, in common with rising hemispheric ozone levels, but this is not particularly evident in the rural ozone index. There is a more marked increase in urban areas, due to the reduction in urban emissions of nitrogen oxides, which tend to destroy ozone close to their emission source.
Days with moderate or higher air pollution
Part (b) of the indicator measures days of moderate or higher pollution according to the Air Pollution Information Service bandings used in air pollution forecasting. At the moderate level, the effects of pollution may start to be noticeable to sensitive people. There is no clear trend in the number of either urban or rural pollution days, due to the effects of variability in weather patterns from year to year.
The bandings are based on 5 pollutants consisting of carbon monoxide, nitrogen dioxide, ozone, particulates and sulphur dioxide. These are recognised as the most important for causing short term health effects. The main causes of days of moderate or higher air pollution at urban sites are ozone and particulates (PM10). Sulphur dioxide also used to make a significant contribution but has now fallen to relatively very low levels. Carbon monoxide and nitrogen dioxide have very rarely reached moderate or higher levels since the urban index began in 1992.
Causes of air pollution in urban sites
Three of the five pollutants, ozone, particulates and sulphur dioxide caused over 99 per cent of the pollution days, either separately or in combination with each other. Between 1993 and 2007, the average number of days of pollution at urban sites caused by particulates, solely or in combination with other pollutants, fell from an average per site of about 43 days to 11 days per year. Particulates come from numerous man-made and natural sources, and can be generated in the UK or transported from abroad. UK emissions of particulates have been reduced substantially in recent years, but the number of pollution days can still fluctuate from year to year due to variations in weather conditions, as demonstrated by the unusually high figure of 17 in 2003.
The average number of pollution days at urban sites caused by sulphur dioxide, solely or in combination with other pollutants, was 20 days per site in 1993. In 2007 sulphur dioxide did not cause any pollution days, either solely or in combinations with other pollutants.
Ozone causes the great majority of pollution days in rural areas. Since 1999 it has also caused more days of poor air quality in urban areas than particulates have, as pollution by particulates has declined. The number of days caused by ozone pollution has fluctuated in both rural and urban areas, with no clear overall trend. The hot summers in 1999, 2003 and 2006 led to the greatest number of days of moderate or higher ozone pollution since this series began in 1987. A proportion of the ozone experienced in the UK originates from releases of pollution that are blown over from mainland Europe.
The series can be volatile from one year to the next, reflecting the variability in levels of ozone, more of which is produced in hot, sunny weather, as was the case during 2003 and 2006.
Changes to the UK automatic monitoring network
The proposed EU Directive on Ambient Air Quality and Cleaner Air for Europe is expected to come into force in May/June 2008, and provides a new regulatory framework for PM2.5. This Directive requires measurements of the concentrations of very small particles (PM2.5) as well as measurements of larger particles (PM10), and also requires that measurements are comparable in all EU member states. Some changes to the UK automatic monitoring network are needed in order to fulfil these requirements. These changes include the number of sites monitored, the pollutants and locations to be monitored and the monitoring techniques to be used.
These changes will be phased in over a period which began on 2nd October 2007 and will be completed by the end of 2008. New equipment is being introduced at some monitoring sites to enable reference method equivalent measurements of PM10 and PM2.5 to be made. The composition of sites in the monitoring network has also been reviewed with more emphasis being placed on roadside locations.
Related categories: Environmental legislation Pollution control technologies
