Air quality indicator for Sustainable Development 2008 provisional results

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The air quality indicator is one of the 68 indicators of the 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'.

Headline results

* Urban background particulate levels averaged 20 micrograms per cubic metre (ug m-3) in 2008 compared to 22 ug m-3 in 2007. These levels have fluctuated in recent years, although there has been an overall decreasing trend since 1993, the first year for which data were available.

* Roadside particulate levels averaged 28 ug m-3 in 2008 compared to 29 ug m-3 in 2007. There has been a general downward trend since the series began in 1997, although this decline has slowed since 2001 and has been subject to increased fluctuation.

* Urban background ozone levels averaged 60 ug m-3 in 2008 compared to 57 ug m-3 in 2007 and 44 ug m-3 in 1992. These levels have shown an overall increasing long term trend since 1992, the first year for which data were available.

* Rural ozone levels(*) averaged 70 ug m-3 in 2008 compared to 68 ug m-3 in 2007 and 59 ug m-3 in 1987. There is no clear long term trend.

* In urban areas, air pollution in 2008 was recorded as moderate or higher on 27 days on average per site, compared with 24 days in 2007, and 59 days in 1993. This series has shown a high degree of year-on-year variability.

* In rural areas, air pollution in 2008 was moderate or higher for 47 days on average per site, compared with 28 days in 2007. This series has also fluctuated significantly over time.

* These results are provisional and are therefore subject to change. Final results we be available in the spring.

(*) Measured as the daily maximum 8-hour running mean

Background

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 of pollutants are moderate or higher.

(a) Particulates and ozone

Part (a) of the indicator measures annual exposure to particulates and ozone (see Figure (a) below). It was introduced in the light of increasing evidence suggesting that long-term exposure to even low levels of particulates (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.

Results

Results and underlying data are show in Figure (a) and Table A.

* Urban background particulate levels were on average 20 micrograms per cubic metre (ug m-3) in 2008 compared to 22 ug m-3 in 2007. These levels have fluctuated in recent years, although there has been an overall decreasing trend since 1993, the first year for which data were available.

* Roadside particulate levels averaged 28 ug m-3 in 2008 compared to 29 ug m-3 in 2007. Similar to urban background levels, there has been a general downward trend since the series began in 1997, although this decline has slowed since 2001 and has been subject to increased fluctuation.

* Urban background ozone levels averaged 60 ug m-3 in 2008 compared to 57 ug m-3 in 2007 and 44 ug m-3 in 1992. These levels have shown an overall increasing long term trend since 1992, the first year for which data were available. This may be due to the reduction in emissions of nitrogen oxides, which act to destroy ozone close to its source.

* Rural ozone levels averaged 70 ug m-3 in 2008 compared to 68 ug m-3 in 2007 and 59 ug m-3 in 1987. There is no clear long term trend.

* Both particulate and ozone concentrations are strongly influenced by weather, which will contribute to the fluctuations seen across the time series.

Figure (a): Levels of ozone and PM10: 1987-2008 (provisional), UK

Notes: Ozone index shows annual mean of the daily maximum 8 hour running mean. PM10 index shows annual average.

(b) 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. The bandings are based on 5 pollutants consisting of carbon monoxide, nitrogen dioxide, ozone, particulates (PM10) and sulphur dioxide. These are recognised as the most important for causing short term health effects. At the moderate level, the effects of pollution may start to be noticeable to sensitive people.

Changes to the automatic monitoring network

The EU Directive on Ambient Air Quality and Cleaner Air for Europe (2008/50/EC) has resulted in introduction of new equipment at some monitoring sites to enable reference method equivalent measurements of PM10 and PM2.5. The composition of sites in the monitoring network has also been reviewed with more emphasis being placed on roadside locations, a reduction in the number of sites monitoring carbon monoxide (CO) and sulphur dioxide (SO2), and an increase in the number of sites measuring nitrogen dioxide (NO2) and ozone (O3).

Historically, the composition of sites used for this part of the indicator was based on a sub-set of 67 sites that were identified in 2001, with a view to improving year-to-year consistency in the calculation. Therefore, since 2001, only data from these sites were considered for inclusion. However, following the changes to the network, many of the original 67 sites no longer measure CO or SO2, and many of the newly introduced sites are not included within the original sub-set. Due to the gradual decline in the number of indicator sites producing useful data and also to the availability of additional sites, this year a new method was used to calculate part (b) of the indicator. This involved using all available sites, but applying the data capture criteria detailed in the tables above. The criteria relate to ozone and PM10, as these pollutants cause the vast majority of pollution days and are considered to pose the greatest risk to health. This method results in 65 sites being used in the calculation 2008, as opposed to just 32 if only indicator sites were used. This therefore reflects changes in the monitoring network, provides better geographical coverage, and is more closely aligned with EU reporting requirements. Historical data have been revised to reflect this change, although there is very little impact on trends, as seen in Figure (b)

Results

Results and underlying data are presented in Figure (b) and Table B

* In urban areas, air pollution in 2008 was recorded as moderate or higher on 27 days on average per site, compared with 24 days in 2007, and 59 days in 1993. This series has shown a high degree of year-on-year variability.

* In rural areas, air pollution in 2008 was moderate or higher for 47 days on average per site, compared with 28 days in 2007, and 21 days in 1987. This series has also fluctuated significantly over time.

* There is no clear trend in the number of either urban or rural pollution days. Fluctuations are likely to be due largely to the effects of variability in weather from year to year.

Figure (b): Number of days when air pollution is moderate or higher: 1987-2008, provisional (UK)

Causes of air pollution

* As ozone causes the vast majority of rural air pollution, a breakdown by pollutant of part (b) of the indicator is not presented for rural sites.

* At urban sites in 2008, two of the five pollutants, ozone and particulates, caused approximately 96 per cent of the pollution days, either separately or in combination with each other (see Figure (c) below). Between 1993 and 2008, the average number of days of pollution at urban sites caused by particulates, solely or in combination with other pollutants, has fallen from an average per site of 43 days to 7 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 18 days 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. Since 2001, sulphur dioxide has not caused any pollution days, either solely or in combination with other pollutants.

* Carbon monoxide has not contributed to the number of pollution days since the series began. Nitrogen dioxide has impacted very rarely, at a maximum of 1 day.

* Since 1999, ozone has caused more days of poor air quality in urban areas than have particulates, as pollution by particulates has declined. However, the number of days caused by ozone pollution has no clear overall trend. A proportion of the ozone experienced in the UK originates from releases of precursor pollutants which form ozone that are blown over from mainland Europe. Weather conditions will also contribute to volatility of the series, with more ozone produced on hot, sunny days, as was the case during 2003 and 2006.

Figure (c) : Average number of days when levels of ozone, particles and sulphur dioxide are moderate or higher at urban sites (UK)

Note: for the purposes of this chart, where a day is caused by more than one pollutant, it is counted for each pollutant i.e. there is double counting.

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