Benzene from traffic

4. Discussion

4.1 Petrol supply in Denmark
4.2 Crude petrol
4.3 Retail petrol
4.4 Trends of aromatics (BTX) in air

The major supply of petrol in Denmark originates from the two refineries in operation in Denmark, Statoil in Kalundborg and Shell in Fredericia. Both companies have all their crude oil stock from the North Sea area; Shell from the Danish sector, and Statoil from both the Danish and Norwegian sector. Although the production of crude petrol from both refineries are based on similar North Sea feedstocks some variation in products may be expected due to differences in production design and operation (e.g. cracking and reformation).

When it comes to retail petrol it is important to realise that retail petrol sold from petrol stations in the eastern part of Denmark (east of the Great Belt) to a major part is based on crude petrol from the Statoil refinery in Kalundborg. Besides Statoil itself, major companies like Shell and Hydro/Texaco presumably are supplied from the Stat-oil refinery. In the western part of Denmark (west of the Great belt) this situation is reversed as major companies like Statoil and Hydro/Texaco probably both get their supply of crude petrol from the Shell refinery. Q8 and "DK Benzin", on the other hand, are supposed to get most of their petrol from imported supplies.

Prior to the campaigns on studies of aromatics in petrol reported here NERI also did two similar campaigns in 1989-1990. The results of these two studies are shown in Figure 11 below (Iversen, 1999).

The average benzene content in retail petrol from 10 different petrol stations in east Denmark was 3-4% (vol./vol.) in all three qualities. For 92 RON and 95 RON qualities the data are from retail, unleaded petrol while for 98 RON the data are from both leaded and unleaded samples. For toluene the content is 10-12% (vol./vol.), lowest for 92 RON and highest for 98 RON, and the same for both periods.

In this study the samples of crude petrol were obtained directly from the Statoil and Shell refineries. Both companies reduced the content of benzene to about 1% during the summer of 1998. Statoil reduced benzene content from about 2% in 1995, whereas Shell reduced from about 4% in 1998. These reductions took place at the same time for all three qualities (92, 95 and 98 RON) at the two refineries, respectively.

Simultaneously, the content of C7+C8 aromatics (i.e. toluene, ethylbenzene and xylenes) showed some variations from period to period and between qualities. While the contents in the 92 RON quality from both refineries were observed to decrease slightly from 1997 to 1999, no similar trends were observed for the 95 RON and 98 RON qualities. The aromatics content generally increased with increasing octane number, and although some variations in the content of both the RON 95 and 98 RON qualities remained almost constant. Generally, the C7+C8 aromatics contents in all three qualities were higher in the Shell than the Statoil products, despite products from both refineries are based on North Sea crude oil feedstocks. Differences in operating designs and processing are supposed to cause this difference in composition.

4.3 Retail petrol

As was mentioned above Statoil supplies crude petrol for both Shell and Hydro/Texaco petrol stations east of the Great Belt. The benzene content in retail petrol from petrol stations in Roskilde closely related to the content in Statoil crude petrol. The reduction from about 2% to 1% during the summer of 1998 was observed for all three qualities. The benzene content in retail petrol seems to have been reduced also prior to 1997. The data presented in Figure 11 from two campaigns in show that it was higher (3-4%) in 1989-1990.

The changes of the aromatics content were different. For the Statoil crude petrol the aromatics content increased with octane number, but while the content in 92 RON seemed to decrease slightly, it remained more or less constant in 95 RON and 98 RON. This is not the same in the retail products, where the aromatics content is constant and almost the same in all three qualities except for the 98 RON quality, which is slightly higher. The average values presented here contains contributions from companies not getting supplies from Statoil, which affects the correlation to the Statoil crude products. Another factor, which must be considered is, that not all companies and petrol stations sell three different qualities all the time. 95 RON is often sold as 92 RON, and depending on actual supplies the same quality could possibly also be sold as both 92 RON, 95 RON and 98 RON for a limited time interval. This will also diminish the variation in aromatics content between the different qualities as shown in figure 4 and 6, chapter 2.

In Appendix A, Figure A1-A10 data on the content of benzene and C7+C8 aromatics in retail petrol from all five petrol stations in Roskilde are presented for the five periods, where samples were collected and analysed. These figures show that - for the five companies included in this study - the lowest overall BTX content was found in all three qualities from DK Benzin, whereas Statoil and Shell were very similar and had the highest content. Q8 and Hydro/Texaco - also being quite similar - had the highest content in 92 RON, but it was close to Statoil and Shell with respect to 95 RON and lower than these two companies with respect to 98 RON.

Data from the two campaigns in 1989-1990 (Figure 11) show that the average toluene content was about 10-12% (vol./vol.), 98 RON being highest, in retail petrol from 10 different petrol stations in east Denmark. The average toluene content in samples of retail petrol from five different petrol stations in Roskilde analysed in this study (data not reported) increased compared to the first study, in particular for the 98 RON quality. The average contents are 12,7%, 12,9% and 15,9% for 92 RON, 95 RON and 98 RON, respectively. The higher toluene (and possibly also C8 aromatics) content in actual (1999) retail petrol may have been introduced in order to compensate for the reduced benzene content to maintain the required octane number.

The results for the benzene content in crude petrol obtained in this project seem to be overestimated by 0,10-0,15% (absolute) compared to values for crude petrol provided by the Statoil and Shell refineries. There may be several reasons for that, but differences in analytical methodology (e.g. sampling, instrumentation, calibration) are probably the most significant. However, the purpose of this project has not been to control the benzene content to any exact value in neither crude nor retail petrol, but to study the trend for the reduction of benzene in petrol in relation to the new EU regulation of max. 1%, and demonstrate a probable corresponding decrease in the emission of benzene from traffic. Therefore, results presented in this study have only been subjected to internal quality control and no attempts have been devoted to intercompare accurately the methodology applied and results obtained with those from the refineries or elsewhere.

In conclusion this study has documented that the reduction in benzene content in retail petrol to about 1% as announced by all companies was introduced during the summer and second part of 1998. Since then the average content has remained constant at 1%. The analyses the content of C7+C8 aromatics have shown greater variations not only from company to company and from quality to quality but also from period to period. Overall, however, the average content of BTX in retail petrol showed a slightly decreasing trend during the period covered, while it seems to be slightly higher than in earlier campaigns in 1989-1990.

4.4 Trends of aromatics (BTX) in air

BTX in air

The available data on BTX from the two measurement sites have been analysed, especially for estimation of the trends. In order to remove the influence from the different meteorological conditions (dispersion) from year to year the method - inverse modelling -described in chapter 3 has been used. It has been assumed that the traffic was constant during the project period. This might not be true, but data were not available due to technical problems with the automatic traffic counters operated by the Road Directorate.

Trends at Jagtvej in Copenhagen

Calculating the diurnal emission profiles for several years provides estimation of the trends in the traffic contribution to air pollution. Concentrations depend on both emissions and meteorology, whereas the trend analysis of emissions is independent of the inter-annual variations in the meteorological conditions. Trend analysis of the average diurnal NO_x, CO emissions were performed for the years 1993-1999 and the benzene emissions for 1994-1999 at Jagtvej using the Filtered Least Square Method with zero intercept (cf. chapter 3.3). Results are shown in Figure 12 together with the measured annual average concentrations, where the background contribution was subtracted. Only weekdays were used for this analysis. The calculated emissions and measured concentrations show a similar long-term trend, but the inter-annual variation is different, illustrating the influence of meteorology on air pollution levels. It has to be taken into account that data are not available for complete calendar years all the years (see Figure 7, chapter 3), which means that not all annual averages are "true" averages.

CO and NO_x at Jagtvej

The emissions of NO_x and CO showed distinct decreasing trend, which mainly is due to increasing fraction of vehicles equipped with catalysts (approx. 60% in 1998).

Benzene at Jagtvej

The benzene concentration and emission decreased significantly at Jagtvej by a factor of 5. The most steep decrease was observed from 1994 to 1997. During the year 1997 to 1999 a weak decreasing trend was still observed for the emission. The concentration did not show a similar decreasing trend, probably due to bad dispersion and/or increasing traffic.

The trends at the monitoring station at Albanigade in Odense were similar, Figure 13, but the analysis was not so complete because data on BTX only were available from 1997-99

CO and NO_x at Albanigade

The emission of NO_x showed weak decreasing trends during the year 1997-1999. The CO emission is nearly constant.

Benzene at Albanigade

The emission and the concentration of benzene decreased slightly during the year 1997-1999. However, it must be realised that data are not available for complete calendar year, see Figure 7, chapter 3.

Figure 12 Trends of annual averages of NOx, CO and benzene concentrations and calculated traffic emissions in Copenhagen (street canyon, Jagtvej).

Figure 13 Trends of annual averages of NOx, CO and benzene concentrations and calculated traffic emissions in Odense (street canyon, Albanigade)

The benzene content in the major part of petrol sold east of the Great Belt (from Statoil), e.g. in Copenhagen was reduced from approx. 3.5% in 1994 to approx. 2 % by the end of 1995; in July 1998 the benzene content was reduced to approx. 1%. The benzene content in the major part of petrol sold west of the Great Belt (from Shell), e.g. in Odense was approx. 3.5% until July 1998 and then reduced to approx. 1%. The content of other aromatic compounds in petrol was mainly unchanged. The reduction in benzene concentrations in air was mainly caused by the reduced benzene content in petrol and the increasing number of cars with three way catalysts. The fraction of cars with catalysts was in 1999 60-70% depending on the location. It is assumed that the highest fraction is present at the most busy streets, where the newest part of the car fleet operates.

Benzene: CO ratio

Another aspect of this reduction on the pollution composition is illustrated in Figure 14; it shows the annual averages of the ratios between benzene and CO (ppb/ppm), estimated as the slopes of the linear regression lines. A clear reduction of the benzene/CO ratio was observed between 1995 and 1996 for Jagtvej and another decrease at both sites during 1998, when both refineries reduced their content of benzene in petrol to 1%. In Copenhagen the ratio between benzene (ppb) and CO (ppm) was 4.3 in 1994 -95. In the period 1996 - 97 the ratio was 2.4. The reduction in this ratio corresponds to the relative reduction in the benzene content of approx. 40%. A similar analysis of data from a street in London (Marylebone Road, http://www.aeat.co.uk/netcen/aqarchive/my1.html) shows a ratio between benzene and CO of approx. 1.4. This is significantly lower than that derived from the Danish data, indicating different fuel qualities. The toluene/CO ratio did not change so drastically (Palmgren et al., 1999), which shows that benzene was removed specifically. A small year-to-year decrease of the toluene/CO ratio in the period 1994-1999 can be explained by reduced evaporation losses from newer cars with direct injection engines.

A benzene contribution from the other aromatic VOCs could not be identified, and conversion of these species into benzene does not represent an important source at the moment compared to the contribution from the benzene in petrol.

Vehicle categories

Automatic traffic counts have been carried out at Jagtvej by the Danish Road Directorate in 1994/95 and classified in light and heavy vehicles as 1 hour averages. Supplementary manual traffic counts provided additional split-up of traffic in four vehicle categories: passenger cars, vans, trucks and buses. The passenger cars dominate the traffic with a diurnal variation significantly different from the other vehicle categories (Figure 15). This analysis was only carried out at Jagtvej.

The emissions from different vehicle categories were estimated by expression (3) in chapter 3. In order to make the system better defined, trucks and buses are treated jointly in the case of NO_x emissions and all diesel vehicles (incl. vans) are treated jointly for estimation of CO and benzene emissions.

Analysis in 1994/95

Because the traffic counts were available only from the 1994/95 period and the traffic may have changed since, the results obtained for the emission profile for the year 1994 were used as a basic profile. The emissions calculated for the other years were decomposed in a part proportional to 1994 and a residual. The least square estimations were made separately for each decomposed part of the total emission profile for each year except 1994. The least square solutions for the residuals were subtracted from the solutions obtained for the "proportional part" of the profiles.

Emission factors

The estimated emission factors are shown in Table 2. Because the contribution from diesel vehicles (the category "other vehicles") to CO and benzene emissions is small, the uncertainties of the estimations are very high. This analysis has not been repeated due to lack of accurate traffic data.

The benzene emission factor for cars has been reduced to more than 1/3 or by approx. 70% from 1994 to 1997. This is more than the reduction of benzene in petrol from approx. 3,5% to 2% in the same period. The additional reduction of the benzene emission factor can be explained by the increasing number of cars equipped with catalysts.