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 1998 UK emissions of acidic gases and ozone precursors are summarised in Table 5.1.

Table 5.1 Total UK Emissions of Acidifying and Ozone Precursors

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

Under the Second Sulphur Protocol, the UK must reduce its total SO2 emissions by 50% by 2000, 70% by 2005 and 80% by 2010 (all from a 1980 baseline). The UK is well on track to meet these targets, and by the end of 1998 had achieved a 67% reduction from 1980 baseline levels, 17% ahead of the UN/ECE target level for the year 2000.

The VOC Protocol requires the UK to achieve a 30% reduction of VOC emissions by 1999 from 1988 baseline. The 1998 inventory indicates that a 26% reduction has been achieved.

The NOx Protocol requires that total emissions of NOx in 1994 should be no higher than they were in 1987; UK emissions were 16% lower in 1994 than in 1987 and have fallen substantially since 1994.

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.

Table 5.2 Emissions Ceilings for 2010 (kTonnes)

• The protocol gives emission limits for sulphur, nitrogen oxides and NMVOCs from stationary sources.
• The protocol indicates limits for CO, hydrocarbons, nitrogen oxides and particulates from new mobile sources
• Environmental specifications for petrol and diesel fuels are given.
• Several measures to reduce NH₃ emissions from the agriculture sector are also given.

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 political agreement was reached by Ministers in June 2000 on the proposed National Emission Ceilings Directive. This 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 NOx 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 97% 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 1998 had substantially declined.

The inventories for SO₂, NO_x , HCl, NMVOC and NH₃ are discussed in the following sections. Full details of the methodologies used to compile the inventories, changes to the methodology since the 1997 inventory and detailed time series for these pollutants are presented in the Appendices