Estimating NO2 concs in the UK - chapter 5 5 Mapping roadside NO2 and NOx concentrations
5.1 Our approach

We have considered that the annual mean concentration of NOx at a roadside location is made up of two parts: the background concentration (as described in section 4) and a roadside enhancement.

There are several stages to mapping roadside NO2 concentrations in 2005 and 2009:

5.2 The relationship between roadside NOx concentrations and emissions

The NAEI provides estimates of NOx emissions for a total of 15226 major road links in the UK (Goodwin et al, 1997). Figure 5.1 shows a comparison of the roadside enhancement of NOx concentrations at ten automatic monitoring sites with NOx emission estimates for the individual road links alongside which these monitoring sites are located (roadside enhancement = measured roadside concentration - mapped background concentration). The sites used in the analyses presented in this report are listed in Appendix 1. The identification numbers on the scatter plots can be used to identify each site. The sites chosen for this analysis are those that are in built up areas and there is clearly a strong dependence of the roadside enhancement of concentration with NOx emission at these sites.

roadside enhancement of NOx (ppb) = kNOx . NOx emission from road link (kg NO2 m-1 y-1)
The value of kNOx is 3.0 (or 5.73 if the roadside enhancement of NOx is expressed in mgm-3).

Roadside NOx monitoring sites at locations with a more open aspect, such as those adjacent to motorways or major roads in rural areas, do not conform to this relationship. The NOx emissions from vehicles travelling on these roads are generally more effectively dispersed than the emissions on built up urban roads. We have therefore restricted our mapping to urban major roads (a total of 7508 road links), where the relationship is reliable.

Figure 5.2 shows the good agreement between the measured and estimated total roadside NOx concentrations at the ten roadside sites (mean of measurements = 142 ppb, mean of estimates = 138, r2 = 0.89)

5.3 Current roadside NO2 concentrations
We have calculated the annual mean roadside NO2 concentration in 1996 by applying the non-linear function for roadside sites shown in Figure 3.1. The use of this function to convert from NOx to NO2 clearly introduces an additional uncertainty to our estimates but the agreement between the estimates and measurements of annual mean NO2 at automatic monitoring sites shown in Figure 5.3 is still reasonably good (mean of measurements = 35 ppb, mean of estimates = 34 ppb, r2 = 0.64). The resulting maps of estimated roadside NO2 concentrations in 1996 are shown in Figure 5.4. Additional detail is shown in Figure 5.5 (London) and Figure 5.6 (Birmingham and Manchester).

5.4 Predicting annual mean NO2 concentrations for 2005
We have derived estimates of annual mean roadside NO2 concentration in 2005 from the estimates of roadside NOx concentrations for 2005. The estimates of roadside NOx concentration in 2005 were derived from estimates for 1996 by applying the NOx emission reduction factors listed in Table 2.1.

Figure 5.7, Figure 5.8 and Figure 5.9 show the estimated annual mean urban roadside NO2 concentrations for 2005. The links for which the estimated concentration is above 21 ppb are highlighted. Figure 5.7 shows all of the 761 road links with concentrations higher than 21 ppb. This represents about 10% of the total number of urban major road links and a total length 670 km. Only 10% (76 road links) are estimated to have concentrations greater than 26 ppb. The majority of these links (495, representing 387 km) are in the Greater London area. Figure 5.8 shows these links, with the lower map showing the central area in more detail. The remainder are generally confined to the most heavily trafficked roads in other big cities and Birmingham and Manchester are shown in Figure 5.9.

5.5 Predicting annual mean NO2 concentrations for 2009
Roadside NO2 concentrations have also been estimated for 2009, in order to assess compliance with the EU 'Daughter Directive' limit value for annual mean concentration. The limit value is 40 mgm-3, which is essentially the same value as the NAQS objective of 21 ppb. We have estimated the NO2 concentrations in 2009 for comparison with this limit value, which will come in to force on 1 January 2010.

Figure 5.10, Figure 5.11 and Figure 5.12 show the estimated annual mean urban roadside NO2 concentrations for 2009. Fewer urban road links are expected to have a concentration greater than 21 ppb in 2009 than in 2005. A total of 170 links representing 120 km of road will have concentrations greater than the limit value. Only 10% (18 road links) are estimated to have concentrations greater than 25 ppb. 121 of the links with projected concentrations greater than 21 ppb (representing 83 km) are in the London area.

5.6 Predicting roadside NO2 concentrations for individual monitoring sites
The emissions projections that have been used to calculate the maps of estimated roadside NO2 concentrations for 2005 and 2009 can also be applied to individual site measurements for 1997 (or most of 1998) to provide projections which do not have the additional uncertainty associated with the mapping methods. Projections based on measured annual mean NOx concentrations at roadside sites are listed in Table 5.1 below.

An annual mean NO2 concentration of 21 ppb is currently exceeded at most roadside monitoring sites. Projected concentrations in 2005 at sites on smaller roads are lower than 21 ppb but not for the more heavily trafficked roads. Projected concentrations for 2009 are still higher than 21 ppb for the busiest roads in London. This is fully consistent with the results of the mapping analysis.

Table 5.1. Projected annual mean NO2 concentrations for roadside sites based on 1997 (or 1998) measurements for the business as usual scenario (ppb).

Site 1997 2005 2009
  NOx NO21 NO22 NOx NO2 NOx NO2
Sutton Roadside 77 26 25 39 17 30 15
Tower Hamlets Roadside 156 37 36 78 25 60 22
Haringey Roadside 98 31 28 50 20 39 17
Camden Roadside 145 37 35 72 24 56 21
Exeter Roadside 79 23 25 39 17 30 15
Bristol Old Market 123 32 32 61 22 47 19
Bath Roadside 137 33 34 66 23 51 20
Glasgow Kerbside 162 37 37 80 25 62 22
Lincoln Roadside 149 35 35 72 24 55 21
Norwich Roadside 43 19 18 23 13 18 12
London Marylebone Road 202 49 42 100 29 78 25
Hove Roadside 62 19 22 31 15 24 14
1 measured NO2 concentration
2 NO2 concentration estimated from measured NOx concentration



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