Whether pollution levels are going up or down over the long term is a question that is often asked. The shorter term variations described earlier in this brochure can often obscure these trends, or make analysing them difficult.
Automatic monitoring of air pollution started in the early 1970's, but there are only a few sites with a data record that goes back that far. Most of the rural sites that measure ozone were set up in the late 1980's, and urban site numbers started to increase in the early 1990's. Non-automatic measurement of smoke and sulphur dioxide has a 35 year data history, but this is restricted to these two pollutants. The data record for other pollutants is therefore relatively short. Since concentrations are also subject to the shorter term variations that were discussed in the previous sections, it can be seen that the task of reliably identifying long term trends can be quite difficult - it is considered that a data record lasting at least five years is required to do this.
Ozone
Fig. 14 shows exceedences of the UK air pollution standard for six sites, showing data from 1987, where available. Rather than showing any long term trend, it demonstrates that the strongest influence on the annual average is the character of a particular summer. 1989 and 1990 experienced hot summers, and therefore higher numbers of ozone standard exceedences.
The graph also echoes the pattern of ozone concentrations shown in the map in Figure 12. The sites in Southern England and Mid-Wales have exceeded the standard more often than those in Scotland and Northern Ireland. Also the urban sites have by far the lowest range of exceedence numbers, because of the ozone scavenging effect, where levels are depleted by reaction with nitric oxide produced by vehicles. More detailed studies suggest that overall average background ozone levels have, in general, been increasing by about 1-2% per year since the middle of the century. It is very difficult to be sure about such small trends without good quality data that extend over a long time.
Smoke and Sulphur dioxide
Measurements from the UK's large scale smoke and sulphur dioxide sampler network extend right back to the early 1960's. Fig.15 shows the overall annual average from the National Survey sites up to 1981 and the overall average from the Basic Urban Network sites from 1982 to 1993. The picture that the data from this network gives is very encouraging. Levels of sulphur dioxide have fallen substantially over the last 30 years. This is because much less coal is used for heating houses, smoke control legislation has been introduced and because cleaner fuels and technologies are now being used in industry.
These conclusions are backed up by measurements from the automatic networks, where thirteen sites now have sufficient data for statistically valid trends to be calculated. All of the trends are negative - i.e. they show that levels are falling across the UK.
Nitrogen Dioxide
There are indications from the Diffusion Tube Survey that nitrogen dioxide levels in the UK are also falling. 1996 results showed a decrease in levels in intermediate and kerbside sites of 3%, averaged across the UK. This trend continued in 1997, when levels decreased by a further 4% at kerbside sites, and by 6% at urban background sites (again, averaged across the UK.)
As with sulphur dioxide, this evidence is backed up by data from the automatic networks. Sixteen sites now have enough data over the last five years for trends to be calculated, and all but two of the sites show that levels are going down.
Other pollutants
Twelve sites measuring carbon monoxide have sufficient data for a trend to be calculated. Of these, seven show a decrease in their 98th percentile value - i.e. the value that 98% of the recorded data is less than or equal. This means that the incidence of high values at these sites is going down, which, in turn, indicates that the emissions of carbon monoxide at these locations is decreasing. This is probably due to the increased proportion of 'clean' catalyst-equipped vehicles which emit smaller quantities of carbon monoxide.
The data record for particles is not as long as for the pollutants described so far. There are now eight sites that have a good five year data record, and four of these show significant downward trends. Whilst this is encouraging, it is not enough to draw conclusions about a national trend.
Site prepared by the National Environmental Technology Centre, part of AEA Technology, on behalf of the UK Department of the Environment, Transport and the Regions
