National Atmospheric Emissions Inventory

5 Acidifying gases and tropospheric ozone

5.1 Introduction

The deposition of acidic species can have adverse effects on buildings and vegetation, as well as acidifying streams and lakes and damaging the aquatic environment. Sulphur and nitrogen oxides from fuel combustion are the major contributors to acidification (RGAR, 1997). Ammonia plays an important part in the long range transport of the acidic pollutants by the formation of relatively stable particles of ammonium sulphate and ammonium nitrate. Although ammonia is a basic gas, it can also have a direct effect on acidification. The biological transformation of NH₄⁺ to NO₃^- in soils (denitrification) and plant uptake both release acidity into the soil contributing to soil acidification.

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. It can affect human health and can damage plants and crops.

The Total 1996 UK emissions of acidic gases are summarised in Table 5.1.

Table 5.1 Total UK Emissions of Acidifying and Ozone Creating Gases

Pollutant	Total 1996 emission (kt)
Sulphur dioxide	2028
Nitrogen oxides	2060
Hydrogen chloride	117
Non-methane volatile organic compounds (NMVOC)	2,111
Ammonia	339

The UK is committed to reduce acidic emissions and is a signatory to several protocols under the UNECE'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 1996 had achieved a 59% reduction from 1980 baseline levels, 9% ahead of the UNECE target level for the year 2000. The VOC Protocol requires a 30% reduction of VOC emissions by 1999 from 1988 baseline. The NO_x Protocol requires that total emissions of NO_x in 1994 should be no higher than they were in 1996; UK emissions were 11% lower in 1994 than in 1987 and had fallen by a further 10% by 1996. Negotiations are currently underway on a Second Nitrogen Protocol, which will include limits on ammonia as well as NO_x. The UNECE has also begun preparation for possible protocols on Persistent Organic Pollutants (POPs) and Heavy Metals (see Chapter 6).

The EU's Acidification and Ozone Strategy aims to significantly reduce the extent of areas in the EU where the tolerance of sensitive systems to acidity is exceeded, reducing the extent of affected areas in 2010 from a predicted 8.7 million ha to 4.5 million ha. The main elements of the strategy include:

the establishment of national emissions ceilings for each acidifying pollutant (SO₂, NO_x, and ammonia) for each Member State;
ratification of the UN protocol on further reductions of sulphur emissions;
proposal for a directive limiting the sulphur contents of heavy fuel oils of 1%;
review of the large combustion plant directive;
designation of the Baltic Sea and North Sea as so-called SO₂ control areas under the current revision of the Convention on Marine Pollution (MARPOL) which would set a limit on the sulphur content of fuels used by ships in these areas;

The EU is also preparing a strategy to combat tropospheric ozone pollution. This is based on the same methodology used to develop the acidification strategy and will include the definition of national emissions ceilings for the main ozone precursors NO_x and VOCs. A proposal for a directive setting national emissions ceilings for SO₂, NO_x VOCs and NH₃ is expected in 1999, and the EU and Member States are currently examining possible ceilings. For the UK, these are reductions of 74% in SO₂, 58% in NO_x, 61% in VOCs and 20% in NH₃ by 2010 from a 1990 base year.

Sulphur dioxide has long been recognised as a pollutant because of its role, along with black smoke, 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 99% 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, 1997), 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 source of NO_x in the UK is 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 acidification pollutants from power plants and industrial sources. Emissions of NO_x from road traffic peaked in 1989 but by 1996 had declined again by around 27%.

The inventories for SO₂, NO_x and NH₃ are discussed in the following sections. Full details of the methodologies used to compile the inventories, changes to the methodology since the 1995 inventory and detailed time series for these pollutants are presented in the Appendices.