NMVOCs are a organic compounds which may differ widely in their c hemical composition. These organic compounds are often grouped under the NMVOC label as the majority display similar behaviour in the atmosphere. NMVOCs are emitted to air as combustion products, as vapour arising from petrol and solvent use and several other sources.

Interest in NMVOC emissions has grown as their role in the photochemical production of ozone has been appreciated. The diversity of processes which emit NMVOCs is huge, covering not only many branches of industry, but also transport, agriculture and domestic sources.

The NMVOC inventory is summarised in Table 5.6. Only 34% of the NMVOC emissions arise from combustion sources (unlike SO₂ and NO_x where the contribution from the combustion sources is much higher). Of these emissions from combustion sources, it is the transport sector which dominates. Other major sources of NMVOC emissions are the use of solvents and production processes. Natural emissions of NMVOCs are included here as per UN/ECE guidelines (see Section 5.5.3). The NMVOC emission profile, Figure 5.10, shows a small overall increase in emissions between 1970 and 1989 with minor peaks in 1973 and 1979, followed by a steady reduction in emissions during the 1990s. The latter is largely a reflection of the increasingly stringent emission limits across a range of sectors.

1 UK emissions reported in IPCC format (Salway, 2001) differ slightly due to the different source categories used.

2 See Appendix 4 for definition of UN/ECE Categories

3 Including railways, shipping, naval vessels, military aircraft and off-road sources

The spatial disaggregation of NMVOC emissions in the UK is shown in Figure 5.11. A large proportion of emissions are caused either as a result of the activities of people in and around their homes (e.g. domestic solvent use or domestic combustion), or by widespread industrial activities such as small-scale industrial coating processes, dry cleaning shops, and small bakeries, which are present in towns and cities throughout the UK. Consequently the resulting emissions map is well correlated with population density.

The NMVOC map includes a large number of point sources, including oil refineries, crude oil terminals, large combustion plant, chemicals manufacture, iron and steel processes, whisky manufacture, large bread bakeries, and industrial solvent using processes. The domestic sources of NMVOC are distributed using population density statistics, and the sources arising from other industrial processes are mapped using information on the size and locations of industrial installations.

Unlike the maps presented previously for SO₂ and NO_x, the VOC map has little major road definition except where the major roads go through rural areas. This reflects the fact that NMVOC emissions are dependent on vehicle speed and are higher on minor and urban major roads than on the high speed motorways and major roads.

Solvent use and production processes are responsible for 27% and 12%, respectively, of the 1999 emission total. The estimates are derived either based on plant specific data provided by process operators or regulators or by use of appropriate emission factors combined with solvent consumption data or industrial production data. The NMVOC inventory has been subject to a continuous programme of review and improvement over the past decade (see Appendix 2 for a description of recent improvements), and estimates, especially those for the period 1988 – 1999, can be considered reasonably reliable.

The solvent use sector comprises both industrial and domestic applications, both being significant sources. Emissions from industrial solvent use reached their peak in 1973, then dipped to a low in 1982, before increasing again until 1989. Since 1989 emissions have fallen by 37% as a result of emission controls, technological changes, and reduced manufacturing output in some sectors. In comparison, domestic solvent emissions showed little trend until the mid 1980s when they increased sharply. Since 1991 however, emissions have fallen back by 20% due to a trend towards formulating products such as paints and aerosols with lower solvent contents.

The production processes sector includes emissions from the chemical industry, petroleum refining and food and drink manufacture as well as minor sources such as iron and steel production and road construction. Emissions from the chemical industry grew steadily until 1989, since when tightening emission controls have led to a reduction in emissions of 48%. The emissions from the petroleum refining sector show little trend over the period from 1970 until 1994, but since then emission controls and, latterly, refinery closures have led to emissions falling by 53% since the 1994 figure.

Emissions from the food and drink industry comprised 4.5% (78.9 ktonnes) of the total NMVOC emission in 1999. The largest source is whisky maturation although bread baking, animal feed manufacture, fat and oil processing and barley malting are also important. Emissions from the sector peaked in 1980 before falling again. Since 1987, however, emissions have been increasing by an average of 1% per year.

Total transport emissions are currently responsible for 31% of NMVOC emissions of which 27% are a result of road transport. From 1970 the emission rose gradually with increasing car numbers, to a peak in 1989. Since then it has declined substantially owing to the increased use of catalytic converters and fuel switching from petrol to diesel cars. Emissions from the road transport sector for 1999 are now substantially lower than in 1970.

Offshore oil and gas emissions have increased substantially since 1970 with the growth of the UK’s offshore industry and now constitute 9% of the 1999 emissions total. The most important sources of NMVOC emissions are tanker loading, flaring and fugitive emissions.

Emissions from gas leakage currently comprise around 1% of the total NMVOC emission. This source is declining as a result of the gas main replacement programme underway since 1990. The evaporative losses from the distribution and marketing of petrol rose between 1970 and the early 1990s reflecting the growth in road transport. Since then they have decreased, partly as a result of fuel switching to diesel, and partly as a result of increasing usage of petrol vapour recovery systems to prevent emissions from petrol terminals and service stations. They currently account for 6% of national NMVOC emissions.

The contribution from domestic heating has fallen by more than a factor of 6 over the period 1970-1999 as the use of coal for domestic heating has declined. It now accounts for just 3% of the UK emission.

NMVOC emissions from waste treatment and disposal contribute 1% to national emissions. Data from the Environment Agency (2000) shows emissions from municipal waste incinerators to be very small.

NMVOCs, in particular isoprene and mono-terpenes, are emitted from several natural and agricultural sources- such as forests. The entries under Land Use Change and Nature in Table 5.6 represent emissions from forests and forestry, heathland, pastures and crops.

As mentioned previously, the term NMVOC covers a wide range of compounds and although a total NMVOC inventory is sufficient for some purposes, in other cases greater detail is required concerning the nature and concentration of individual compounds. For example, when assessing the photochemical production of ozone, individual species have different ozone creation potentials hence information is required on the concentration of individual species (QUARG, 1993). Table 5.7 shows the emissions of the 50 most important NMVOC species disaggregated as far as possible by source. A more detailed speciation of all NMVOCs estimated by the NAEI (currently over 600) is given in Appendix 6.