6 Comparison with Benzene Target Value

6.1 Introduction
A target value for benzene of 1 ppb (expressed as an annual running mean) has been set by EPAQS as a long term target level. The following section shows a comparison between this target value and estimated roadside concentrations. The locations of the road links exceeding this value are also highlighted. In addition, hypothetical emission reduction scenarios have been used to estimate the potential impact on the number of road links exceeding the target value.

The method of estimating future benzene roadside and background concentrations is similar to that used for calculating exceedances of the 5 ppb standard. In this case, the threshold used to indicate exceedance of the 1 ppb target value is 0.7 ppb (the rationale for this is explained in Section 4.2.1).

6.2 Estimated exceedences in 2010
6.2.1 Business as Usual - "Scenario a"
Figure 6.1 indicates the estimated number of road links exceeding the target value, from 1996 to 2010 using the business as usual scenario (scenario a- see Section 5). The road links are broken down into urban, rural/motorway and Northern Ireland classes. It is clear from the plot that there are expected to be exceedances of the target value in 2010.

Figure 6.2 illustrates the estimated maximum concentration from 1996 to 2010. It is evident that although there are exceedances in 2010, the maximum concentration (1.3 ppb) approaches the target value. It is also clear from Figure 6.2 that if a "business as usual scenario" is assumed, further reductions in the maximum benzene concentration after 2010 are estimated to be small.

It is possible to specify the location of the urban road links exceeding the target values more precisely. Table 6.1 gives an indication of the number of road links exceeding the target value on a town by town basis, and also classifies the exceeding road links on a concentration basis. It is clear that relatively few roads account for the higher concentrations. Consequently reduction measures at a relatively few locations would have a significant impact on the maximum concentration estimated for 2010. However, further reductions would involve an increasing number of road links, and hence considerably more stringent measures to reduce emissions. The total number of road links in the database is 15,226 (the breakdown into different classes is given in Table 4.1).

It is important to note that the errors associated with the methodology used for these projections are significant, and are large when compared to the narrow concentration ranges under consideration here. As a result, small changes in the threshold used to indicate compliance with the target value (in this case 0.7 ppb) will have a large impact on the number of road links estimated to be compliant in 2010 (see Table 6.1).

Table 6.1 Location and Concentration frequency distribution of the Road Links Exceeding the 0.7 ppb Target Value for Benzene in 2010

Location* # of Urban Road Links Exceeding 0.7 ppb Benzene (2010) Inn./Cen. London 303 Outer London 48   Leicester 13 Liverpool 13 Birmingham 9 Leeds 8 Manchester 8 Bristol 6 Sheffield 5 Coventry 2 Glasgow 2 Nottingham 2 Bolton 1 Newcastle 1 Total 421

Concentration Range (ppb) # of Road Links (Urban and Rural/Motorway) 1.3 - 1.4 1 1.2 - 1.3 2 1.1 - 1.2 9 1.0 - 1.1 13 0.9 - 1.0 43 0.8 - 0.9 145 0.7 - 0.8 314 Total 527 of which: Rural/M.way 106 Urban 421

* The location of the road links specified here is only to intended to be indicative. For example, road links in Warsall have been included under Birmingham for convenience- however the division of the London areas has been done accurately.

The location of the 50 road links with the highest estimated concentrations of benzene in 2010 are given in more detail in Appendix 3 (Table A3.1). Sites in the London area clearly dominate the table, with 35 of the 50 sites located in "Inner/Central London", 5 in "Outer London" and 3 on the M25. This is as expected; London has already been identified as the region most likely to have exceedances of the annual mean NO₂ objective in 2005 (Stedman J R and Bush A J, 1998).

6.2.2 Emission Reduction Scenarios
It is possible to apply different emission reduction scenarios to the projected concentrations. In this way it is possible to investigate the impact that changes in policy may have on the number of road links exceeding the target value in 2010.

Three scenarios have been used in the previous sections, as given in Section 5. The "business as usual" scenario has been considered above, in Section 6.2.1. Two further hypothetical scenarios hL and iL are considered here and compared to scenario h (which has been applied to roadside benzene concentrations). Scenarios hL and iL are defined as follows:-

Scenario hL	Reduction in emissions from traffic:	30% In Inner/Central London
		10% In Outer London
		Business as usual in all other locations
Scenario iL	Reduction in emissions from cars:	30% In Inner/Central London
		10% In Outer London
		Business as usual in all other locations

The following table (Table 6.2) summarises the findings from applying Scenarios hL, iL and h to the benzene emissions.

Table 6.2 Estimated Benzene Concentrations and Exceedances of the Target Value for Scenarios hL, iL and h.

	Estimated Number of Road Links Exceeding 0.7 ppb
	Built Up Road Links Only		All Road Links
	2005	2010	2005	2010
Scenario a	1811	421	2236	527
Scenario hL	1414	117	1839	205
Scenario iL	1499	142	1924	232
Scenario h	609	36	735	39
	Estimated Maximum Benzene Concentration (ppb)
	Built Up Road Links Only		All Road Links
	2005	2010	2005	2010
Scenario a	1.9	1.3	1.9	1.3
Scenario hL	1.7	1.1	1.7	1.1
Scenario iL	1.7	1.1	1.7	1.1
Scenario h	1.4	0.9	1.4	0.9

All scenarios result in a number of road links exceeding the target value in 2010. If only the built up road links are considered for Scenarios h, hL and iL, then of exceeding road links, the majority are located in "Inner/Central London", followed by "Outer London". The following table (Table 6.3) indicates the location of the exceeding road links for Scenarios h, hL and iL.

Table 6.3 Location of the Road Links Estimated to Exceed 0.7 ppb of Benzene in 2010 for Scenarios hL and iL.

Location*	# of Urban Road Links Exceeding 0.7 ppb Benzene (2010)
	Scenario h	Scenario hL	Scenario iL
Inner/Central London	30	30	54
Outer London	4	17	18
Liverpool	0	13	13
Leicester	2	13	13
Birmingham	0	9	9
Leeds	0	8	8
Manchester	0	8	8
Bristol	0	6	6
Sheffield	0	5	5
Other	0	8	8
Total	36	117	142

* The location of the road links specified here is only to intended to be indicative.

From Table 6.3 it is evident that for scenarios hL and iL there are many road links exceeding the threshold value of 0.7 ppb in the major cities and towns. By combining estimates from the different scenarios it may be concluded that reducing emissions by 30% (whether from cars or from all road traffic) appears likely to meet the 0.7 ppb threshold in 2010 for all major towns and cities with the exception of London..

One of the results of significant reduction in benzene emissions from road traffic is an increase in the relative importance of non-traffic sources. The identification of Leicester and Liverpool as the cities with a large number of exceedances reflects the relatively high estimates of non-traffic emissions in these cities, relative to other cities. The spatial distribution of benzene emissions from industrial sources within the NAEI has, at best in part, been constructed from surrogate activity statistics. The spatial distribution of these sources is therefore considerably less certain than for sources such as road traffic.