Executive Summary
- The principal objective of this survey was to provide an indication of ambient levels of benzene in the immediate vicinity of petrol stations in the Greater London Area. The study, which was intended as a pilot exercise, has been commissioned by the Department of the Environment in response to a requirement to assess the implications and effectiveness of a recommendation made by the House of Commons Select Committee for Stage II controls to be fitted ahead of the deadline proposed within the EU Directive. The Stage II Directive covers VOC emissions during refilling of vehicles, which can be reduced by fitting recycling devices to petrol nozzles.
- The twelve petrol stations included in this study were chosen to provide a representative cross-section of retailer, station size, proximity to busy roads and type, if any, of VOC controls fitted. A total of six sampling locations were identified at each station. These included one site at the pumps, three sites at the sides and rear of the forecourt, and two sites at the adjacent roadside at a distance of approximately 100 metres to either side of the station. Monitoring was conducted at each location using passive diffusion tubes filled with Chromosorb 106. Diffusion tubes were exposed for six consecutive 2-week periods, for a total of twelve weeks.
- Continuous monitoring was also carried out at one of the stations using a Chrompack BTX analyser. This analyser has been validated against the Chrompack VOCAIR system used in the UK Hydrocarbon Network. The results indicated a reasonably good correlation with the diffusive samplers.
- Mean benzene concentrations at the roadside sites ranged from 1.6 to 4.7 ppb, whilst at the forecourt sites levels were higher and ranged from 1.6 to 8 ppb. Concentrations were higher still at the pumps, with levels ranging 2.7 to 19.1 ppb. The elevated benzene levels recorded at the forecourt and pump sites reflect the influence of emissions from a number of sources, whilst the higher levels measured at the pumps, are associated with emissions during refuelling.
- Elevated concentrations measured on the forecourt could not be directly attributed to specific emission sources such as the delivery tanks. Spatial variation in forecourt levels at individual stations probably reflected the influence of a variety of emission sources and differences in the orientation of the samplers in relation to the petrol pumps and prevailing wind direction. A simplistic approach to estimating the influence of road traffic emissions to measured forecourt levels indicated that, on average, road traffic emissions contribute approximately 44% to benzene concentrations measured at the front of the petrol station forecourt.
- The Air Quality Standard (AQS) for benzene published in the Consultation Draft of the UK National Air Quality Strategy (NAQS), which is set as 5 ppb as a rolling annual average, was estimated to be exceeded at nine of the twelve petrol stations, at a total of 15 monitoring sites. Exceedances were only measured at locations remote from the fuel pumps at five of these stations, at a total of seven sites. There were no exceedances of the AQS at any of the roadside locations.
- Exceedances of the AQS at the petrol pumps are of little significance with respect to personal exposure of the general public, as exposure will only occur over a relatively short period of time during refuelling of the vehicle. Of greater concern are the estimated exceedances of the AQS measured at the forecourt locations. A number of the petrol station forecourts were situated directly adjacent to residential housing, where longer term exposure of the public is likely.
- If it is assumed that emissions from the fuel pumps are the principal source of benzene emissions in the forecourt area, then the source strength will be proportional to the volume of petrol sales. By normalising the measured benzene levels at each station with respect to total petrol sales, an assessment of the effects of Stage I and Stage II controls can be made. This has indicated that Stage I controls are effective in reducing benzene concentrations within the forecourt area by up to 60%, although at some petrol stations fitted with Stage I controls, benzene levels in the forecourt area remained above the AQS. Stage II controls resulted in a dramatic reduction in concentrations measured at both the forecourt and the pumps (90% and 200% respectively), and levels were significantly below the AQS. However, only one Stage II station was included in the survey.
Report prepared by Stanger Science and Environment
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