Sources of Exposure to Sulphur Dioxide

6. From the time of the Industrial Revolution until the early 1960s, the main source of sulphur dioxide emissions in towns and cities was the domestic, commercial and industrial burning of coal. However, this pattern of emission in the United Kingdom has changed significantly since the 1960s. Following the Clean Air Act of 1956 and subsequent moves to the increased use of energy sources such as natural gas and electricity, emissions in towns have fallen considerably. The generation of electricity by combustion of fossil fuels has now become concentrated in large power stations with tall chimneys, situated mainly in rural areas rather than close to towns as was formerly the case. Restructuring of the industrial base and increased energy ellicicncy have also contributed to a decrease in emissions over the past two decades.
7. The consequence of this changing pattern has been an overall decrease in sulphur dioxide emissions in the United Kingdom by some 50% since 1970, with fossil fuel power stations accounting for 66%, of the total in 1993. In addition, the distribution of pollution has changed. In the past, emissions were derived predominantly from many low-level sources, which resulted in elevated long term average concentrations in towns and cities, with particularly high peak concentrations occurring during cold still weather conditions in winter, a characteristic of smog episodes. Now that emissions are dominated by a relatively small number of large rural point sources with tall chimney stacks, long-term average concentrations in towns are much lower and, because these chimneys generally emit their pollutants above the low-lying temperature inversion layer, the very high concentrations characteiistic of cold winter smog conditions no longer occur. In contrast, the pattern of sulphur dioxide ail pollution in most of the United Kingdom is now characterised by short-term peak concentrations, typically lasting a few hours, at some point downwind of a power station where the plume reaches ground level. These episodes tend to occur as day-time air turbulence breaks up night-time temperature inversions, bringing the emissions to the ground. However, it should be noted that there are still some areas of the United Kingdom, most notably Belfast, where coal is burned in significant quantities, and here the patterns of emission and air pollution are similar to those prevailing in the rest of the country some years ago, although the absolute levels are now much lower.
8. The other principal sources of sulphur dioxide are the combustion of coal and oil by industry and commerce and the use of diesel fuel in motor vehicles. In contrast to other pollutants, such as nitrogen oxides, motor vehicles are a relatively unimportant source nationally, being responsible for only about 2%, of the total sulphur dioxide emissions. However, the combustion of diesel fuel can make a significant contribution to background levels in urban areas. The sulphur content of diesel fuels is currently restricted to 0.2% or less by weight and this is to be reduced further to less than 0.05% by 1996. By comparison, the sulphur content of petrol is typically about 0.04%.
9. The relative contributions of the different sources and absolute levels of emissions of sulphur dioxide for the years 1970 and 1993 are contrasted in Table 1. Sulphur dioxide is predominantly emitted from large point sources, and exposure to the highest concentrations occurs mainly outdoors.

Table 1 Estimated United Kingdom emissions of sulphur dioxide by emission source, 1970 and 1993, thousand tonnes per year

Source	1970		1993
	Estimated emissions *	Percentage of total **	Estimated emissions *	Percentage of total **
Power stations	2,913	45	2,089	66
Domestic	522	8	113	4
Commercial/public service	257	4	88	3
Refineries	213	3	156	5
Iron and steel	435	7	92	3
Other industrial combustion	1,804	28	509	16
Non-combustion processes	40	1	12	-
Extraction and distribution of fossil fuels	-	-	2	-
Road transport	44	1	59	2
Railways	25	-	2	-
Civil aircraft	1	-	2	-
Shipping	129	2	51	2
Waste treatment and disposal	4	-	4	-
Agriculture	41	1	10	-
Total	6,428	100	3,188	100

*      Rounded to nearest thousand tonnes

**    Rounded to nearest 1%

Source: Digest of Environmental Protection and Water Statistics, Number 17 (1995)