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Example

1. Introduction.

Emissions of VOCs from petrol stations are important for two reasons. The first is the direct health significance of benzene for which the UK has set objectives for the purpose of Local Air Quality management and the EU has set Limit Values. The second, and in many ways more significant in the long term, is the ozone creation potential of each component of the complex (and seasonally variable) range of VOCs in petrol. There are two main sources of petrol station emissions of VOCs. The first is displacement of vapour from the storage tanks during delivery. This has been addressed by fitting Stage 1 vapour recovery. The second is displacement of vapour from the tanks of vehicles during refuelling. Two methods of addressing this have been proposed. One is vapour recovery on the filling station (Stage 2 recovery), the other is fitting vapour traps to cars. There are also other sources including spillage and evapourative losses from storage tanks.

Since August 1995 Bristol City Council has been monitoring the concentrations of benzene, toluene and o, m and p xylenes on two supermarket petrol station forecourts. Both are fitted with Stage 1 recovery. In June 2002 further monitoring was commenced on another petrol station forecourt where Stage recovery was in operation. Initial results from this work were reported (Muir, 2002) suggesting that the use of Stage 2 recovery gave marked reductions in concentrations (and presumably emissions) of all the VOCs monitored.

The monitoring has been carried out using single Perkin Elmer passive diffusive samplers mounted on the dispenser support pillars at a height of 2 – 2.5 metres. The tubes are all mounted to the North East side of the forecourt on the outer rank of pumps. In so far as it has been possible to do so the tubes on the forecourts have been supplemented by tubes approximately 50 metres from the centre of the forecourt in both upwind and downwind directions. In so far as has been possible exposure of the tubes has been for periods of 14 days ± 1 day although in a few cases it has been necessary to accept exposure periods up to 21 days. Very few data have been lost from tubes exposed on forecourts.

This work seeks to present data from more extended monitoring to confirm this benefit and to examine some of the long term data from the other monitoring to illustrate the effects of legislative and other changes during the monitoring period.

2.1 Results and Discussion.

2.1.1 Background.

There have been a number of legislative measures taken to reduce emissions of benzene because of its potential genotoxic carcinogenic effects on the general populace. One of these was the requirement for fitting of Stage 1 recovery at petrol stations with a throughput greater than 1000 m³ per year by 31 December 1988. It should be noted that this measure also had the effect of reducing emissions of other VOCs with no identified direct health effects but with some measure of Photochemical Ozone Creation Potential (POCP), probably greater than that of benzene (Derwent et al, 1996). Stations with a throughput of 500 – 1000 m³ per year were required to install Stage 1 recovery by 31 December 2001 and smaller stations by 31 December 2004. The other measures have been a series of reductions in the maximum permitted benzene content of petrol.

Table 1 gives the POCPs for 7 VOCs present in petrol (red) and 4 either not present in petrol or present only small quantities (blue) as given in Derwent et al.

Table 1 Photochemical Ozone Creation Potentials for a range of VOCs

No firm proposals have yet been made for requirements for the installation of Stage 2 vapour recovery in the UK although DEFRA issued a consultation paper on the subject in 2002 (DEFRA, 2002). This proposed a 75% installation by 2010 for stations with a throughput > 200 m³ per year. It was also proposed that installation should be combined with planned refurbishment programmes to avoid additional costs. The Consultation Paper also stated that, at the time of publication, there were 120 stations fully compliant with Stage 2 vapour recovery and an unspecified number where the underground pipework had been installed and only required the installation of the above ground equipment. The proposal to make these requirements for the larger stations was justified on the grounds that emissions from the smaller stations formed only a small percentage of total emissions of VOCs and the cost of installation would be disproportionately large in relation to the benefits.

The current configuration of the 3 petrol stations involved in this study are given in Table 2.

Table 2 Configurations of petrol stations, December 2003.

These figures do not represent every stage of the monitoring programme because, as will be mentioned, one station was enlarged in 2001. Also the proportions of Unleaded:Super unleaded:Lead replacement:DERV dispensers have changed and over the years low sulphur fuels, both diesel and petrol have been introduced at the two stations in the original survey.

2.2 Results.

2.2.1 Long term data.

Figures 1 shows the annual average concentrations of all three VOCs at the two original stations. This shows that in 1995 concentrations of all three VOCs were higher at the larger station (Station 2) and continued to be higher until 1999/2000 when the maximum permitted concentration of benzene (rather than the actual benzene content) in petrol was reduced from 5% to 1%. At this point the concentrations of benzene at both stations fell. This fall is particularly apparent at the larger station. It is, however, noticeable that there is little if any effect of the concentrations of toluene or xylenes. It is also noticeable that after 2000 concentrations of all three VOCs began to rise at Station 1.

Figure 1 Annual average concentrations of benzene, toluene and xylenes on 2 petrol station forecourts in Bristol, August 1995 to December 2003.

Figures 2 to 5 illustrate the 3 monthly average concentrations of benzene and toluene at these two stations during this period. These periods equated as closely as was possible the periods January to March, April to June, July to September and October to December for each year. In the case of 1995 all the data were averaged as one period because monitoring only commenced in late August.

Figure 2 3 Monthly average concentrations of benzene at Station 1, 1995 to 2003.

Figure 3 Monthly average concentrations of benzene at Station 2, 1995 to 2003.

Figure 4 Monthly average concentrations of toluene at Station 1, 1995 to 2003.

Figure 5 Monthly average concentrations of toluene at Station 2, 1995 to 2003.

These figures show clearly that at both stations there was a reduction in concentrations of benzene coincident with the reduction in the maximum permitted content of petrol. In actual fact this reduction became noticeable during November 1999. The difference between the two stations is that at Station 2 this reduction was maintained whereas at Station 1 the increase in the number of dispensers (and, presumably, the throughput) resulted in a return to concentrations only slightly lower than before the reduction in the benzene content of petrol.

These observations are reflected in the measured concentrations of toluene. Here there are no noticeable differences in concentrations when the benzene content of petrol reduced but, whereas at Station 2 concentrations of toluene have remained broadly similar to earlier concentrations at Station 1 they have increased markedly. The same pattern is observed with data for xylenes but this is not illustrated here.

2.2.2 Stage 2 recovery.

Figures 6 and 7 shows the average concentrations of benzene, and toluene at the two stations with Stage 1 vapour recovery (stations 1 and 2) and the station fitted with Stage 2 vapour recovery (station 3) for the period June 2002 to December 2003 with corresponding data from a roadside site. These show that there is a marked and consistent reduction in the concentrations of both benzene and toluene with the fitting of Stage 2 recovery. This is also reflected in the concentrations of the xylenes (not illustrated).

Although the concentrations of benzene, toluene and xylenes are not a direct measure of the emissions of these substances from the individual petrol stations it seems reasonable to regard them as an indicator of the emissions. Reasoning by analogy, although sometimes a dubious procedure, would suggest that emissions of other constituents of petrol will be reduced by Stage 2 recovery. From the POCPs given in Table 1 it is apparent that the VOCs with the greatest POCPs are the higher aromatics in petrol rather than benzene and the aliphatic constituents of petrol so even if this analogy was shown to be inappropriate the benefits of Stage 2 recovery in relation to tropospheric ozone formation would be substantial.

Figure 6 Benzene concentrations, June 2002 to December 2003.

Figure 7 Toluene concentrations, June 2002 to December 2003.

Station 3 is close to a busy road and there is another large filling station on the opposite side of this road. A diffusive sampler has been exposed alongside this road since 1995, in part because of the presence of the petrol stations but also because of higher than expected concentrations of benzene at another nearby site on a very heavily trafficked road. Data from this site are included in Figures 6 and 7 and show that although the concentrations of benzene and toluene are much lower on the forecourt of Station 3 than on the other forecourts they are still higher than those measured by the roadside.

Conclusions.

1. The reductions in the maximum permitted concentrations of benzene in petrol resulted in a reduction in measured concentrations on petrol station forecourts, presumably as a result of reduced emissions of benzene.
2. These reductions in concentrations could be offset by increased emissions resulting from an increase in the number of dispensers on a station and consequent increased throughput of petrol and increased emissions.
3. These reductions in concentrations of benzene were not accompanied by reductions in the concentrations of other VOCs.
4. The concentrations of all 3 VOCs were much lower on the forecourt of a petrol station fitted with Stage 2 vapour recovery, although they were still higher than at a nearby roadside site.
5. The fitting of Stage 2 vapour recovery, although not a complete answer in itself, has the potential for contributing to the reductions of emissions of VOCs in line with the Gothenberg Protocol.

The views expressed in this paper are those of the author and should not be taken to represent the views and policies of Bristol City Council.

References.

DEFRA, Petrol Vapour Recovery Stage II – Consultation, April 2002.

R G Derwent, M E Jenkin and S M Saunders. Photochemical Ozone Creation Potentials for a large number of reactive hydrocarbons under European conditions. Atmospheric Environment, 30, 181-199, 1996.

Muir D, Stage 2 Vapour Recovery on Petrol Filling Stations - Is it Worthwhile?, Clean Air, 32, 2, Autumn 2002.

Last Updated

13th January 2005