3. Sources of ozone precursors

3. Sources of ozone precursors

Figure 2 presents the trend in annual emissions of VOC and NO_x in the period from 1980 to 1995 in the EU15 Member States (Mylona, 1996; Olendrzynski, 1997). Emissions from biogenic sources were excluded from this inventory. The data presented are the latest (1997) officially reported emissions under the LRTAP Convention and are made available by UN-ECE. For 1990 and 1994 the data were supplemented by emissions from the CORINAIR programme (Grösslinger et al. 1996; Ritter 1997). This programme is performed by the Member States and supported by EEA's Topic Centre on Air Emissions (ETC/AE). The CORINAIR inventory is based on data gathered by national experts in individual countries. For each pollutant it gives the contribution of individual countries to the total European emissions as well as emissions per main source group, per capita and per km². The results are estimates of actual emissions with significant uncertainties in several cases.

Figure 2 suggests that emissions of non-methane VOC and NO_x increased until the late 1980s but are now decreasing. Between 1990 and 1994 NMVOC emissions from the EU15 countries decreased from approximately 14,000 ktonnes to 12,700 ktonnes, i.e. a reduction of 9%. In the same period the total Pan-European emissions decreased by approximately 14%, demonstrating a higher decrease in central and East European countries. This may be partly due to the economic restructuring process in this part of Europe. Similar emission reductions are found for NO_x. These show a 8% decrease (from 13,500 to 12,400) between 1990 and 1994, whereas the Pan-European emissions declined by 14% over the same period.

Figure 2: Anthropogenic emissions of NOx (as NO2) and VOC (1980-1995). Source: UN-ECE / Corinair

These emission reductions can be evaluated in the framework of the 5th Environmental Action Programme (EC, 1992b). The 5EAP includes emission abatement targets of the ozone precursors. The VOC target is a 30% reduction in the year 2000 from the 1990 emission levels. The NO_x target also uses 1990 as its reference year, and 2000 as the year to achieve a reduction of 30%. It also aims at a stabilisation in 1994 on the 1990 levels. Figure 3 and 4 illustrate the emission reductions achieved so far. Many countries managed to realise a significant reduction. However, given the current reduction rate, it is unlikely that the remaining required reductions of more than 20%, according to the 5EAP targets, will be met in 2000. In a study on cost optimised abatement strategies Amann et al. (1997,1998) show that the EU15 countries need to reduce their emissions far beyond the 5EAP target in order to achieve a substantial reduction of the exceedances resulting from 1990 emissions. Their combined AOT60/AOT40 scenario D7 indicates that, averaged over all EU15 countries, emissions of NO_x and VOC need to be reduced by 52 and 58% respectively from the 1990 level in order to reduce existing exceedances of AOT60 and AOT40 by 60% and 35%, respectively, while at the same time AOT60 should nowhere exceed 3000 ppb.h and AOT40 nowhere 10 000 ppb.h (cf. Table 1.). See section 5.4 for more details on AOTs.

Emissions from vegetation and soil contribute to the concentrations of hydrocarbons and NO_x in the atmosphere and therefore to formation of ozone. In the EU15 averaged over a full year the contributions are of the order of 20 and 7 percent for VOC and NO_x respectively. On a hot summer day the fraction of NO_x emissions from soils may be over 25% of the emissions from combustion processes (Stohl et al., 1996). As biogenic VOC emissions also increase strongly with temperature, the biogenic VOC share may, in some regions, be the major fraction of the atmospheric burden of hydrocarbons during episodes. Model calculations (Simpson, 1995) indicate that in areas with major isoprene emissions the concentrations of NO_x are limiting for the production of ozone. Thus, uncertainties in isoprene emissions are generally not very important for the evaluation of long-term (e.g. 6 months) ozone scenarios (Simpson, 1995). On hot days, ozone concentrations may be elevated as a result of enhanced biogenic VOC emissions.

Tentative model calculations by Stohl et al. (1996) imply that daily ozone maxima in summer were about 8 μg.m^-3 higher when soil emissions of NO_x were included. These issues are obviously important in the discussion on chemical regimes (see Chapter 2) and more work may be needed to resolve the importance of emissions from soils and vegetation.

Emissions of CO, a less reactive precursor of ozone, were 44 ktonnes in the European Union in 1994, and decreasing; in 1990 the emission was 53 ktonnes. The largest contribution to these emissions are from road transport, in 1994 about 65%. (Grösslinger et al. 1996; Ritter 1997).

Figure 3: Emissions of VOC expressed as a percentage of the 1990 levels. Source: UN-ECE / Corinair

Figure 4: NO_x emissions expressed as a percentage of the 1990 levels. Source: UN-ECE / Corinair

Figure 5 and Figure 6 present the 1990 emissions of VOC and NO_x from the 5EAP source sectors. In 1990 as an average over the EU15, the transport sector accounted for 45% of total anthropogenic VOC emissions, which principally arose in urban areas. Similarly, in the case of NO_x, the largest fraction originated from transport with an almost stable share of 64% between 1990 and 1994. The second largest sector is industry (approximately 35%) for VOC, whereas for NO_x it is the energy sector contributing about 19%.
Figure 5: Anthropogenic emissions of VOC per 5EAP target sector in 1990 for each Member State. Source: Corinair, EEA-ETC/AE

Figure 6: Anthropogenic emissions of NO_x per 5EAP target sector in 1990 for each Member State. Source: Corinair, EEA-ETC/AE

Figure 7 is included to illustrate a major part of the activity growth in the transport sector; i.e. the number of passenger cars with and without a three way catalytic converter and the total distance travelled (car-kms). Note that the total number of cars consists of both diesel and gasoline cars, while the latter category only can be supplied with a three way catalyst. The figure shows that the increase in the total distance travelled is steeper than the trend in the number of cars. In 1995 the fraction equipped with a three way catalyst was 29% averaged over the EU15. The data indicate that although overall emissions of VOC and NO_x are declining, the increased use of motorised vehicles partially offsets the gains from improved car technology. This growth also tends to counterbalance the reductions achieved by the stationary 5EAP sectors. It appears that increased traffic intensity currently prevents movement towards the emission reductions necessary to meet air quality objectives of O₃.

Figure 7: The development in the total number of passenger cars, total distance travelled (car-kms), and the fraction of cars equipped with catalytic converter. Source: Eurostat.

Figure 8 shows the per capita emissions of the main ozone precursor species in 1990. In this year, the consumption pattern of the average European caused an emission of 39 and 37 kg of VOC and NO_x respectively. The VOC and NO_x emission rates and in particular their emission ratios are vital parameters in determining the ozone formation potential. VOC and NO_x have different chemical atmospheric lifetimes and their concentrations change by mixing with other air masses and deposition processes.

Figure 8: Emissions of VOC and NO_x allotted per capita in EU15 countries. Source: Corinair, EEA-ETC/AE

Although the Ozone Directive obliged measurements of NO_x and recommended measurements of VOCs at so-called "additional measurement points" this has not led to a substantial data base on NO_x and VOC data. Some other air quality measurements are available that can support the changes in the VOC/NO_x emission ratios. Over the 1970s and 1980s several cities report an upward trend in NO_x: e.g. the yearly mean levels doubled in Amsterdam over the 70s and 80s coinciding with the doubling of the total distance travelled in the Amsterdam area over this period. Many cities in Belgium, France, UK, Germany and The Netherlands report similar NO₂ values in the beginning of the 1990s; in winter 30-100 μg.m^-3 and in summer 20-85 μg.m^-3 (EEA, 1996). In recent years exceedances of the WHO NO₂ guideline (150 μg.m^-3 24-h mean) were observed in several countries including UK and Germany (Sluyter, 1995).

At only few sites VOC records were collected over sufficient long periods to allow for trend analysis. These are: Moerdijk (NL), close to an industrial area near Rotterdam, the rural site Birkenes (N), and the 'clean' top of the Jungfraujoch. (Roemer et al, 1998; Beck et al, 1996; Solberg et al., 1994; Mahieu et al., 1997; Zander et al., 1997). At Moerdijk a significant downward trend of traffic and refinery related VOCs of minus 2 to 5 percent per year was observed over 1981-1991. The Birkenes data show the opposite pattern. Here a significant upward trend in the concentrations of anthropogenic VOCs is detected. At Jungfraujoch the acetylene concentration was unchanged over the whole data record. Acetylene emissions are almost entirely from the exhaust of motor vehicles, and this could be taken to indicate that on a hemispheric scale emissions from this source have neither increased nor decreased over the last decade. Given the increased number and use of motorised vehicles this is a curious observation. The pattern may well be obscured by decreased acetylene emissions per km driven as a result of improved car technology.

The reports recording trends in VOC all indicate a changing hydrocarbon mix. The results can, however, hardly be generalised because the two lowland sites from which detailed results are available experience a very different mix of emission sources and meteorology. It will take more time and effort to establish the relationship between emissions from particular sources and the resulting ozone concentrations.

3.1 Legislation aiming at emission reductions

To reach the various EU emission targets the Commission has developed several policies and measures. These are summarised in the following sections.

An integrated approach to air pollution problems was followed in the Commission's first Auto-Oil Programme completed in 1996 (COM(96)248 final). The Programme anticipated likely Community objectives for air quality (focusing on nitrogen dioxide, carbon monoxide, particulates, benzene and ozone), calculated the emission reductions which could be expected from road transport in order to comply with those objectives by 2010 and identified cost effective packages of measures to be introduced in the year 2000 to achieve those reduction targets. Following the completion of the Programme, the Commission presented a series of legislative proposals on limiting emissions from passenger cars, light commercial vehicles and heavy duty vehicles, environmental specifications for petrol and diesel fuels and random roadside testing for heavy duty vehicles. The Directives on passenger cars and light commercial vehicles and quality of fuels were adopted by Council and Parliament in September 1998 and will result in reductions in emissions per vehicle of the order of 70% as compared to current standards. The Council and the European Parliament subsequently agreed not only on the standards to be applied from the year 2000, but also on many of the standards for 2005.

The Commission further addresses emissions of the ozone precursor species in a programme for the control of chemical substances. This programme is activity, technology and source oriented and includes the so-called Solvent Directive (Directive on the limitation of the emissions of organic compounds due to the use of organic solvents in certain processes and industrial installations). This Directive which was adopted by the Commission in November 1996, aims at the reduction of emissions of VOCs from stationary sources. The Directive sets emission limit values for all emission sources, including diffuse emissions.

The Directive on Integrated Pollution Prevention Control (IPPC), adopted in September 1996, is one of the instruments to reach the targets of 5EAP and it aims at an integrated approach to reduce emissions to air, water and soil from stationary sources. The Directive does not set specific emission limit values, but it requires authorities responsible for issuing permits to base emission limits in the permits on BAT (Best Available Techniques).

Finally, the Directive to reduce emissions from storage and distribution of petrol has been adopted (Stage 1), requiring reduction of emissions of VOC in the complete chain of petrol storage and distribution, from refineries to petrol filling stations.