Sulphur Dioxide

Emission Sources and Trends

The main sources of Sulphur dioxide (SO2) emissions are electricity generation, industrial and domestic fuel combustion. Total SO2 emissions have decreased substantially, in line with changes in fuel use and commitments to international agreements within the UNECE Convention on Long Range Transboundary Air Pollution (CLRTAP) and the European Union ("Sulphur Protocols"). The UK is also a signatory to the Gothenburg Protocol (UNECE, 1999). This commits the UK to reducing emissions of S to 625 Gg-SO2 (313 Gg-S) by 2010. In addition, the UK is also a signatory to the EU National Emission Ceilings Directive (Directive 2001/81/EC), which requires 2010 emissions of SO2 to be 585 Gg (293 Gg-S) or lower.

Heavy industry declined from the late 1970s through to the 1990s as the UK underwent a change from an economy dominated by manufacturing to a service-based economy. As a result there were substantial reductions in the levels of heavy industry in the UK, most notably in the iron and steel sector (DECC, 2009). This gives rise to the emission reductions observed in the 1970s. Substantial reductions were seen from the early1990s onwards as there was a switch from coal to gas in the domestic, industrial and electricity generating sectors. In 1992 the use of gas was responsible for only 2% of electricity generation, but by 2000 this had grown to 34%.

Emissions of sulphur still show a steady decline due to legislation associated with restricting sulphur emissions from large combustion plants, which has resulted in the installation and use of emissions abatement equipment. Flue gas desulphurisation abatement equipment is highly effective at removing suplhur from stack emissions, and its use has ensured that emissions have continued to fall, even though coal use has increased since 2000.

Sulphur emissions from shipping - International shipping is a large source of sulphur and activity levels of shipping in general have been increasing with time as other sources have been significantly reduced. As a result, current trends suggest that shipping is set to become one of the most important contributors to UK sulphur emissions in future years.

Impacts and Recovery

Background level concentrations of SO2 in the UK have fallen so much that there is no longer a threat to plant health.

The UK has one of the most diverse lichen flora in the world. Historical records and field observations clearly document the widespread loss of lichen species across many areas of England from the last century, with air pollution levels and changes in land use being the most likely causes. There are large differences in the extent of the loss of different species, largely reflecting their sensitivity to SO2; indeed, methods were developed of mapping SO2 concentrations using the range of lichen species found at different sites (Hawksworth & Rose, 1970).

Decreases in SO2 concentrations over the past two to three decades have had detectable effects on vegetation, including substantial increases in the distribution of many lichen species, improved tree growth in certain areas and increased likelihood of sulphur deficiency in crops. Rose & Hawksworth (1981) examined lichen populations in north and west London, and found several species, such as Evernia prunasti, Parmelia caperata, Parmelia subaurifera and Usnea subfloridana, that were rare or extinct in the area prior to 1970.