The deposition of acidifying species can have adverse effects on buildings and vegetation, as well as acidifying streams and lakes and damaging the aquatic environment. Sulphur dioxide and nitrogen oxides from fuel combustion are major contributors to acidification (Review Group on Acid Rain-RGAR, 1997). Ammonia plays an important part in the long range transport of the acidifying pollutants by the formation of relatively stable particles of ammonium sulphate and ammonium nitrate. Although ammonia is a basic gas, deposition to soil surfaces and has an indirect effect on acidification. The biological transformation of NH₄⁺ to NO₃^- in soils (nitrification) and plant uptake both release acidity into the soil contributing to acidification. NH₃ deposition can also give rise to eutrophication- where nutrient enrichment gives rise to changes in ecosystems.

Tropospheric, or ground level, ozone occurs naturally and there are no significant ozone emissions from anthropogenic activities. Atmospheric levels can be increased in-situ by the photochemical reaction of precursor pollutants such as carbon monoxide, nitrogen oxides and volatile organic compounds. Specific NMVOC compounds and groups of compounds play a key role in ozone formation. Ozone episodes in which concentrations rise substantially above background levels occur in summer months when there are long periods of bright sunshine, temperatures above 20^o C and light winds. Ozone can affect human health and can damage plants and crops. The total 1999 UK emissions of acidifying gases and ozone precursors are summarised in Table 5.1.

The UK is committed to reducing acidifying gas and ozone precursor emissions and is a party to several protocols under the UN/ECE’s Convention on Long-Range Transboundary Air Pollution.

The VOC Protocol requires the UK to achieve a 30% reduction of anthropogenic VOC emissions by 1999 from a 1988 baseline. The 1999 inventory indicates that this has been achieved. Emissions excluding those from forests have fallen from 2475 ktonnes in 1988 to 1577 ktonnes in 1999 - a reduction of 36%. This reduction has been achieved largely as a result of emission controls for road vehicles and industrial processes, introduced by European Directives and the Environmental Protection Act 1990 respectively. Other factors have also had an impact:

• prohibition of the burning of crop residues in England and Wales since 1993, except in limited cases of exemption.

• a decline in the use of coal as a fuel by electricity generators, industry and domestic users in favour of gas;
• a decline in the use of petrol as a fuel for cars in favour of DERV;
• improvements in technology introduced for economic reasons, or in response to health & safety legislation (e.g. the introduction of more efficient dry cleaning machines with lower emission levels);
• measures introduced either voluntarily, or in response to pressure from end-users for improved environmental or health and safety performance (e.g. the formulations of many consumer products have been changed, resulting in lower levels of solvent in those products and therefore lower emissions of VOC during their use).

In 1996, the UN/ECE started negotiating a new multieffect, multipollutant protocol on nitrogen oxides and related substances. This was aimed at addressing photochemical pollution, acidification and eutrophication. The Protocol to Abate Acidification, Eutrophication and Ground-level Ozone was adopted in Gothenburg in December 1999, where it was signed by the UK. The multipollutant protocol incorporates several measures to facilitate the reduction of emissions:-

• Emission ceilings are specified for sulphur, nitrogen oxides, NH₃ and NMVOCs. These are summarised in the following table.

The Gothenburg protocol forms a part of the Convention on Long-range Transboundary Air Pollution. More detailed information on both of the Gothenburg protocol and the Convention on Long-range Transboundary Air Pollution may be found at the UN/ECE web site:- http://www.unece.org/env/lrtap/

Within the EU, the National Emission Ceilings Directive sets emission ceilings for 2010 for each Member State for the same 4 pollutants as in the Gothenburg Protocol. A number of Member States reduced their ceilings somewhat below the levels included in the Protocol. The UK reduced its SO₂ ceiling to 585 kilotonnes and its NO_x ceiling to 1167 kilotonnes.

Sulphur dioxide has long been recognised as a pollutant because of its role, along with particulate matter, in forming winter-time smogs. Estimates of sulphur dioxide emissions have been produced since the inception of the NAEI. Fuel combustion accounts for more than 95% of UK SO₂ emissions with the sulphur arising from the fuel itself. The SO₂ emission can be calculated from knowledge of the sulphur content of the fuel and from information on the amount of sulphur retained in the ash. Published fuel consumption data (DTI, 1998), published sulphur contents of liquid fuels (Institute of Petroleum, 1996) and data from coal producers regarding sulphur contents of coals enable reliable estimates to be produced.

The main sources of NO_x in the UK are also combustion processes. However, such emissions are complex since the nitrogen can be derived from both the fuel and atmospheric nitrogen. The emission is dependent on the conditions of combustion, in particular temperature and excess air ratio, which can vary considerably. Thus combustion conditions, load and even state of maintenance are important. The estimation of NO_x emissions is often based on relatively few measurements and, in view of the possible variation in emissions from apparently similar combustion plant, there is greater uncertainty in the estimates than for SO₂ .

Within the UK, the implementation of the EC’s Large Combustion Plant Directive and other associated policy measures has led to substantial reductions in acidifying pollutants from power plants and industrial sources. Emissions of NO_x from road traffic peaked in 1989 but by 1999 had substantially declined.

The inventories for SO₂, NO_x , HCl, NMVOC, NH₃ and HF are discussed in the following sections. Full details of the methodologies used to compile the inventories, changes to the methodology since the 1998 inventory and detailed time series for these pollutants are presented in the Appendices.