Air quality

Review and Assessment of Sulphur Dioxide

Information about air quality in solihull

9 Review and Assessment of Sulphur Dioxide

9.1 The Government

has adopted three standards for sulphur dioxide (SO₂):- A 15-minute mean of 266µg/m³ (100 ppb) as an air quality standard for sulphur dioxide, with the objective for the standard not to be exceeded more than 35 times per year by the end of 2005 (approximately equivalent to the 99.9^th percentile of the total annual 15-minute values).

A 1 hour mean of 350µg/m³ (132 ppb), with the for the standard not to be exceeded more than 24 times per year by the end of 2004 (approximately equivalent to the 99.7^th percentile of the total annual hourly values).

A 24-hour mean objective of 125µg/m³ (47 ppb), with the for the standard not to be exceeded more than 3 times per year by the end of 2004 (approximately equivalent to the 99^th percentile of the total annual daily values).

For the 15-minute and 1-hour objectives, the focus of the review and assessment should be on any non-occupational exposure at near ground level outdoor locations.

For the 24-hour objective, the focus of the review and assessment should be on non-occupational exposures at near ground level outdoor locations, including:

• Background locations,
• Other locations where people may be regularly exposed, such as in the vicinity of housing, schools or hospitals.

9.2 Sulphur dioxide

is a compound formed by the combination of one atom of sulphur and two atoms of oxygen. At normal temperature and pressure, it is a gas. It dissolves in water to give an acidic solution which is readily oxidised to sulphuric acid. The predominant source of sulphur dioxide in the United Kingdom is from the combustion of sulphur-containing fossil fuels. Because of its acidic nature it is an irritant when inhaled and it may cause breathing difficulties to people exposed to it, particularly those suffering from asthma. It can also damage vegetation.

9.3 Although released by volcanoes and marine organisms

most of the sulphur dioxide found in the atmosphere, near ground level in the United Kingdom, is likely to have resulted from human activity.

9.4 The main sources of sulphur dioxide

for non-occupational exposures are emissions arising from the combustion of sulphur-containing fossil fuels. Coal and heavy fuel oils generally contain higher proportions of sulphur than light fuel oil or diesel. Petrol and, in particular, natural gas contain lesser amounts of sulphur.

9.5 Specific sources of sulphur dioxide in Solihull are:

• Emissions from coal and oil fired combustion plant,
• Emissions from many industrial processes,
• Plumes of pollution from large combustion plant such as power stations located outside the Borough.

Sources of Sulphur Dioxide in the West Midlands

Figure 9.1

9.6 Sulphur Dioxide Levels in 1999.

Air pollution is measured at a number of locations across the West Midlands conurbation. Some of the monitoring stations are owned and operated by the Government, as part of the Automatic Urban and Rural Network (AURN). Others are owned by Local authorities, but are affiliated to the AURN. Also included is a monitoring station at Birmingham Airport. In this report, the levels of sulphur dioxide, measured at AURN stations in neighbouring authorities, are included to demonstrate the relationship between the levels, across the conurbation.

35th Highest 15-Minute Values in the West Midlands Sulphur Dioxide, 1999

Figure 9.2

Figure 9.2 shows the maximum 15-minute average values recorded, in 1999, at the monitoring stations in the West Midlands. In all cases the values are well below the air quality objective. Similarly, the 24^th highest hourly values for 1999, shown in Figure 9.3, are approximately one quarter of the air quality objective.

24th Highest Hourly Values in the West Midlands Sulphur Dioxide, 1999

Figure 9.3

Finally, the 3^rd highest daily values for 1999, shown in figure 9.4, also meet the relevant objective with ease.

3rd Highest Daily Values in the West Midlands Sulphur Dioxide, 1999

Figure 9.4

Solihull MBC has a mobile air quality station that has been in operation since 27th April 2000. It has been located at two urban background sites: Coppice Junior School (off Damson Lane) from 27th April 2000 to 15th October 2000, and Marston Green Infants' School from 30th October 2000. Figures 9.5, 9.6 and 9.7 show the monthly averages, the 24th highest hourly mean and the 3rd highest 24 hour mean respectively, for SO₂ as recorded at the station.

SO2 Monthly Averages May 2000 - Mar 2001

Figure 9.5

SO2 24th Highest Hourly Value

Figure 9.6

SO2 3rd Highest Daily Value

Figure 9.7

The trends in the maximum concentrations of sulphur dioxide have shown a progressive improvement throughout the 1990s. Although the concentrations of air pollutants vary from year to year, due to varying weather patterns, the frequency and magnitude of the maximum values have fallen.

Trends in West Midlands Air Quality Sulphur Dioxide:35th Highest 15 Minute Value

Figure 9.8

9.7 Future Trends in Sulphur Dioxide Levels

Most sulphur dioxide emissions are associated with large combustion plant, such as power stations and major industrial processes. The West Midlands conurbation no longer has any of these major sources of sulphur dioxide within its boundaries.

Whilst the emissions from power stations are normally dispersed effectively into the atmosphere, occasionally the pollution is trapped near ground level and spreads across country to affect communities at some considerable distance from the source of the pollution. Pollution from power stations in Nottinghamshire, Shropshire or Staffordshire can affect the West Midlands conurbation.

Prior to the early 1990s, plumes of sulphur dioxide from large combustion plant, often those located outside the area, would be carried across one part of the conurbation or another several times each year. These short pollution episodes were of sufficient frequency, magnitude and duration to cause the sulphur dioxide levels in Solihull to exceed the 15-minute standard for sufficient time to breach the 15-minute air quality objective.

Over the past few years, the frequency with which these pollution episodes occur has fallen. The reason for this improvement is not clear, but it may be due to a combination of tighter emission controls, developments in the power generation industry and/or changing meteorology. The electricity generators are now being asked to reduce their emissions of sulphur dioxide by some 60% by 2005, from the 1996/7 levels (Air Quality Management, May 2000). If these are achieved, then the maximum and average levels of sulphur dioxide are expected to continue to decline (Figure 9.9).

Trends in West Midlands Air Quality Sulphur Dioxide: 35th Highest 15 Minute Value

Figure 9.9

Given that the sulphur dioxide levels already meet their objective now and that the trend graph suggests a continuing improvement through to 2005, the sulphur dioxide objective should still be met in 2005.