3 Current Roadside Benzene & 1,3-Butadiene Concentrations

3.1 Roadside Concentrations
For the purposes of the methodology used here, the concentrations observed at the roadside have been broken down into "background" and "roadside enhancement" contributions. Hence "roadside enhancement" is defined as follows:-

roadside concentration = background concentration + roadside enhancement

For roadside, we have included all monitoring sites located within about 10m of the edge of the road.

This enables a simple relationship between the emissions from road traffic and the "roadside enhancement" contribution to be determined. Similar work has already been conducted for NO_x, and the relationship has been well characterised (see Stedman and Bush 1998).
In the case of NO_x the road traffic emissions data was taken from the NAEI (Salway et al 1997), which holds spatially disaggregated emissions data. The empirical relationship between the annual NO_x emissions from a major road link and the annual mean of roadside enhancement NO_x concentration was found to be:-

Roadside enhancement of NOx	=	kNOx . NOx emission from road link
(µg m-3 as NO2)		(kg NO2 m-1 y-1)

k_NOx is found to be 5.73 (Stedman and Bush). This coefficient was derived from a comparison of measurements of NO_x concentrations at roadside automatic monitoring sites and NAEI road link emissions estimates.

Similarly kBz and k13 can be defined as follows:

Roadside enhancement of benzene	=	kBz . benzene emission from road link
(µg m-3)		(kg m-1 y-1)
Roadside enhancement of 1,3-butadiene	=	k13 . 1,3-butadiene emission from road link
(µg m-3)		(kg m-1 y-1)

However, automatic measurements of roadside benzene and 1,3-butadiene concentrations have recently commenced, and monitoring is currently restricted to a small number of sites. Hence two alternative methods for determining an appropriate value for the coefficients k_Bz and k13 have been adopted. Both methods have been applied for benzene and the results can be compared, providing a robust estimate of kBz. Method 2 is the only method that can be used for 1,3-butadiene due to the lack of extensive monitoring data.

Method 1: Annual mean concentrations measured by both automatic and diffusion tube methods have been compared with road link emission estimates. This provides a value for kBz directly.

Method 2: A detailed comparison of the daily mean enhancement of roadside benzene and NO_x concentrations measured at Marylebone Road can provide an alternative estimate of kBz and an estimate of k13. Marylebone Road is the only kerbside monitoring site in the National Hydrocarbon Network, and hence is the only site for which this analysis can be carried out. London UCL is used as the background site.


3.2 Data Collection
Benzene data is primarily available from two sources- BTEX diffusion tube surveys and automatic point monitors. The automatic point monitors may be broken down into two categories depending on whether they also measure 1,3-butadiene. Detailed site information is also required, such as grid reference and distance to the nearest major road- this enables mapping and determines whether the location is truly roadside or background.

Although diffusion tubes are often used to monitor concentrations in the vicinity of industrial installations, the extensive number of surveys conducted by local authorities has meant that a large number of roadside and associated background sites are available. It has taken considerable time to compile this information as there is currently no national co-ordination of BTEX diffusion tube surveys. The available data is dominated by sites located in the London area, and details of all site locations are given in Appendix 1. Acknowledgements for the provision of data are given in Section 9 and Appendix 1.


3.3 Analysis of Annual Mean Benzene Concentration
Annual means of roadside and background concentration were calculated for each site. Where sites had less than an annual period of data, the average across the period was re-scaled (by using seasonal data trends from automatic sites) to give a value representative of an annual value. Background measurement sites were then paired with roadside sites. The roadside enhancement was then calculated for each site by subtracting the annual mean background concentration from the annual mean roadside concentration. The background concentration was taken from the map of estimated background concentrations and checked against nearby background measurements, where available. This enables the relationship between the road link emissions data and the "roadside enhancement" to be examined in Figure 3.1. The numbers on the scatter plot correspond to the Site ID numbers (given in Appendix 1).
Fig 3.1 Benzene, All Sites, Roadside Enhancement vs Road Link Emissions
Measurements from a total of 39 sites were included in this scatter plot. These are the sites for which NAEI road link emissions estimates are available, and for which it was possible to make unambiguous identification of this road link location.

Roadside enhancement of Benzene (µg m-3) = kBz . Benzene emission from road link (µg m-1y-1)

kBz is found to be 11.5, i.e. approximately twice that of kNOx (see Section 3.1).

There is considerable scatter in the data (R²= 0.37). In addition to any real differences, there are uncertainties within the measurement method, background concentration values and emissions estimates.

The value of kBz is initially surprising, as the factor relating emissions and concentrations (km) should be indicative of dispersion processes, and hence is expected to be similar to that for NO_x (assuming that the emissions estimates are reasonably accurate).