4 Maps of Estimated Roadside Benzene & 1,3-Butadiene Concentrations

4.1 Current Background and Roadside Concentrations
Figure 4.1 shows a scatter plot of the estimated current roadside concentration (mapped background + estimated roadside enhancement) and diffusion tube and automatic benzene measurements. The numbers on the scatter plot correspond to the individual Site ID numbers (given in Appendix 1). The overall correlation is reasonably good (R²=0.40). Since these are the monitoring sites that were used to derive this plot, it does not provide an independent validation of the estimates. It does, however, provide a useful indication of the uncertainties of the overall estimates of roadside benzene concentrations.

An equivalent plot for 1,3-butadiene is not possible because of the small number of available sites. It is therefore assumed that the estimates of roadside1,3-butadiene have similar levels of uncertainty as the benzene estimates

The road link emissions data and the mapped estimates of background concentrations are then used to derive a concentration map of roadside concentration levels for both benzene and 1,3-butadiene (for all of the 15226 road links in the current GIS database) across the UK in 1996 and 2005.
Figure 4.2 and Figure 4.3 show examples of the estimates of current roadside benzene and 1,3-butadiene annual concentrations. The majority of roads already have current concentrations lower than the objectives for 2005. In 1996 a total of 75 roads have estimated benzene concentrations greater than 5 ppb and 324 road links have estimated 1,3-butadiene concentrations greater than 1 ppb.

The majority of the road links with the highest concentrations are either motorways or major trunk roads close to, or within, large cities. Several roads in central London, Birmingham, Manchester, Bristol and Glasgow have estimated roadside concentrations of benzene in excess of 5 ppb.

The methodology used to arrive at these results has used measurements from built up urban areas only, and it does not necessarily follow that the relationships found here will hold for other road types. It is likely that concentrations adjacent to major roads in more open rural areas will have lower concentrations. As a result it is more appropriate to consider the number of urban roads that exceed the air quality objectives.

Most of the 43 urban road links with current benzene concentrations above 5 ppb are in London.


4.2 Predicted Roadside Concentrations
4.2.1 Introduction
There is a considerable uncertainty in determining the relationship between the observed roadside enhancement and the emissions from the associated road links (kBz). This was clearly illustrated in Figure 3.1 and discussed in Section 3.3. The scatter on this plot indicates that the mean benzene concentration can be up to approximately 1 ppb higher than the estimated value.

In addition to this, it should be noted that the concentration maps indicate the mean concentration value taken across a particular calendar year. As explained in Appendix 2, this may give a different value to the maximum annual running mean taken across a period of one year (as specified in the NAQS). The maximum annual running mean taken across a period of one year cannot be determined from diffusion tube measurements alone. A statistical analysis between the annual "calendar" mean and the maximum running annual mean (taken from a period of one year) has been conducted (see Appendix 2). The results indicate that, for benzene, the maximum running mean across a period of one year is approximately 10% higher than the annual "calendar" mean.

These two factors can be combined to give the estimated annual "calendar" mean of 3.5 ppb as a concentration limit, within our analysis, equivalent to the NAQS objective. It has been assumed that the uncertainties in estimating likely exceedances of the NAQS objective for 1,3-butadiene are similar to those for benzene. The equivalent concentration limit for 1,3-butadiene is therefore 0.7 ppb.

The concentration estimates for urban road links are likely to have the lowest uncertainties because the model has been calibrated for this type of site. Road links in Northern Ireland are not broken down into different road classes, as this information is not available from our road link database. Where exceedances occur, these roads are listed in a separate class.

The following section estimates the predicted roadside concentrations, and relates the results to the NAQS. The values of 3.5 ppb and 0.7 ppb for benzene and 1,3-butadiene have been used to determine exceedance of the NAQS for the reasons given above. This ensures that a safety margin is incorporated to allow for uncertainties associated with the calculation methodology.

4.3.2 Comparison with NAQS Objectives
Estimates of roadside concentration for 2005 have been calculated by scaling the current background and roadside enhancement concentrations by the emissions reduction factors given in Table 2.1. Figure 4.2 and Figure 4.3 show that no roadside locations are expected to have benzene or 1,3-butadiene concentrations exceeding the NAQS objectives by 2005. The highest predicted roadside concentrations for 2005 are 1.9 ppb and 0.5 ppb for benzene and 1,3-butadiene respectively.

Since these concentrations are significantly below the NAQS objectives it is instructive to calculate an estimate of the earliest year by which these objectives are likely to be met. It is clearly important that an adequate safety factor is incorporated into these estimates. Therefore the earliest years for which benzene and 1,3-butadiene are below 3.5 and 0.7 ppb respectively have been identified.

Tables 4.1 and 4.2 shows the number of road links with annual mean concentrations greater than or equal to 3.5 ppb of benzene or 0.7 ppb of 1,3-butadiene along with the maximum concentration for each year from 1996 to 2005. In addition to listing the total number of road links, this table also shows a breakdown into urban/built up roads and rural/non-built up roads. Motorways have been included with the rural roads.

Table 4.1 Number of Road Links Exceeding Benzene NAQS

Benzene	All Road Types		Urban Roads		Rural and Motorways
Year	# of Road Links	Max. Conc. (ppb)	# of Road Links	Max.Conc. (ppb)	# of Road Links	Max Conc. (ppb)
1996	771	7.7	571	7.7	200	6.2
1997	423	6.9	292	6.9	131	5.6
1998	201	6.1	129	6.1	72	4.9
1999	62	5.3	38	5.3	24	4.3
2000	17	4.6	15	4.6	2	3.6
2001	2	3.9	2	3.9	0	3.1
2002	0	3.3	0	3.3	0	2.6
2003	0	2.7	0	2.7	0	2.2
2004	0	2.3	0	2.3	0	1.8
2005	0	1.9	0	1.9	0	1.5
	Total # of road links		Urban road links		Rural/M.way road links
	15226		7508		7718

Table 4.2 Number of Road Links Exceeding 1,3-Butadiene NAQS

1,3-Butadiene	All Road Types		Urban Roads		Rural and Motorways
Year	# of Road Links	Max. Conc. (ppb)	# of Road Links	Max. Conc. (ppb)	# of Road Links	Max. Conc. (ppb)
1996	1616	2.1	1288	2.1	328	1.5
1997	1113	1.9	876	1.9	237	1.3
1998	645	1.6	493	1.6	147	1.2
1999	327	1.4	249	1.4	78	1.0
2000	140	1.2	104	1.2	36	0.9
2001	35	1.1	30	1.1	5	0.7
2002	12	0.9	12	0.9	0	0.6
2003	1	0.8	1	0.8	0	0.5
2004	0	0.6	0	0.6	0	0.4
2005	0	0.5	0	0.5	0	0.4
	Total # of road links		Urban road links		Rural/M.way road links
	15226		7508		7718

If benzene is considered, the mapped results indicate that all roadside locations will be below 3.5 ppb of benzene (expressed as an annual fixed mean) by the year 2002. 1,3-Butadiene roadside locations will be below 0.7 by the year 2004.

The two road links with the highest concentrations are A4202 Park Lane and A4 Talgarth Road in London. Apart from these two links the benzene objective should be achieved by 2001 and the 1,3-butadiene objective by 2003.

The latest years with significant exceedances of the objectives are 2000 for benzene and 2002 for 1,3-butadiene, with 13 and 12 urban links exceeding respectively. These links include several sections of the A4, including Talgarth Road, Cromwell Road and Knightsbridge, several sections of the A501 Marylebone Road, along with A4202 Park Lane, A3 Kensington Park Road and A3200 Stamford Street in central London. The only urban links outside central London are the A406 North Circular Road at Brent Cross and the A46 in Leicester city centre.

The five motorway road links with estimated roadside annual mean 1,3-butadiene concentrations greater than 0.7 ppb in 2001 include three sections of the M25 near Heathrow, and short sections of the M8 in Glasgow and the M62 near Manchester. Note that these concentrations are probably overestimates as indicated above.