Benzene and 1,3-butadiene concs at the rdside - chapter 22 Mapping Background Benzene & 1,3-Butadiene
2.1 Current Background Benzene and 1,3-Butadiene maps
Maps of estimated annual mean background benzene and 1,3-butadiene concentrations for 1996 are presented in Figures 2.1 and 2.2. These maps have been taken from an earlier report on background concentration maps (Stedman 1998), where details of the mapping methods can be found. The general approach to mapping background concentrations has been described in Stedman (1998) and Stedman etal (1997).
Measured annual mean background concentrations have been considered to be made of two parts :
A contribution from relatively distant major sources, such as large conurbations. Measurements from rural monitoring sites well away from local sources are generally found to provide an indication of the spatial variation of concentrations due to these distant sources. A rural map can be interpolated from measurements at these rural sites.
A contribution from more local emissions. We have found that estimates of emissions in an area of 25 km
2 centred on a background monitoring site location can be used to derive this local contribution.
The difference, diff, between measured ambient pollutant concentrations at urban automatic monitoring sites (not roadside or industrial sites) and the underlying rural concentration field is calculated where monitoring data are available. A regression analysis is then performed to find a coefficient, km, for the relationship between diff and estimated emissions in the vicinity of the monitoring sites:
This coefficient, which is the equivalent of an empirical box model coefficient, can then be used to derive a map of annual mean concentrations from a combination of a rural map and emissions inventory estimates. Thus automatic monitoring data is used to derive the relationship between ambient air quality and emissions inventories.
The three inputs required to construct a map of total background concentration are therefore a rural map of concentration, an emissions inventory and automatic measurements in urban areas.
Maps of background benzene and 1,3-butadiene could ideally be derived from a combination of rural and urban ambient measurements and emission inventories for benzene and 1,3-butadiene. While sufficient urban measurement data are available for these species, rural automatic measurements of benzene and 1,3-butadiene are only available for one monitoring site (Harwell) and emission inventory maps of these two hydrocarbon species are not yet available.
Maps of rural concentrations of benzene and 1,3-butadiene have therefore been derived from a map of rural NO2 concentrations by multiplying by the benzene to NO2 and 1,3-butadiene to NO2 ratios measured at the Harwell site (0.031 and 0.00538 respectively, with all concentrations expressed as ppb).
The implicit assumption in using rural NO2 measurements to estimate rural benzene and 1,3-butadiene concentrations is that traffic emissions are a dominant source of these pollutants on a regional scale.
The contribution to background concentrations from local emissions has been derived from a map of low level (area plus road sources) VOC emissions (Goodwin etal 1997). Future work will be able to incorporate benzene and 1,3-butadiene emission maps directly, once they are available from the National Atmospheric Emissions Inventory (NAEI). The following empirically derived regression coefficients were used to calculate the map of background concentrations.
estimated concentration
=
rural map +
km.emissions
(ppb)
(ppb)
(kTonnes of VOC/25 km2/year)
Benzene:
km = 0.281
1,3-Butadiene:
km = 0.0566
The highest background concentrations are in city centre locations as expected. The maximum values on these 1996 maps are 2.6 ppb for benzene and 0.5 ppb for 1,3-butadiene.
2.2 2005 Background Benzene and 1,3-Butadiene maps Maps of the projected annual mean background concentrations of benzene and 1,3-butadiene for 2005 have been calculated from the 1996 maps (Figures 2.1 and 2.2). The 2005 maps shown in Figures 2.3 and 2.4 have been calculated by scaling the 1996 maps by the changes in emissions that current policies are likely to deliver- calculated by the NAEI road transport model (pers. comm. Murrells 1998). The recently published local emissions inventories for a number of UK cities (Hutchinson and Clewley 1996, Buckingham etal 1997a, 1997b, Buckingham etal 1998) have indicated that road traffic emissions generally dominate the emissions of these pollutants in urban areas. Taking these inventories together, it is reasonable to assume that road traffic emissions currently contribute 90% of the benzene and 1,3-butadiene observed at urban background locations and background concentrations for 2005 have been projected on this basis. Estimates of urban road traffic emissions for both current and future years are available from the NAEI road transport model (pers. comm. Murrells 1998) and it is reasonable to assume that the roadside enhancement of concentrations is directly attributable to traffic emissions. Table 2.1 lists the emission reduction factors for emissions in 2005, relative to 1996. These emissions reduction factors are appropriate for calculation of concentrations for both urban background and for the roadside.
Table 2.1 Emission reductions that current policies are likely to deliver, expressed as the ratio 2005:1996.
Emission Reduction Factors
Background
Roadside
Benzene
0.31
0.23
1,3-Butadiene
0.33
0.25
The non traffic emissions contributing the remaining 10% of background concentration are assumed to remain unchanged.
The background maps for benzene and 1,3-butadiene indicate that exceedances of the NAQS objectives at background locations are unlikely in 2005. The maximum estimated background concentrations are 0.8 and 0.2 ppb for benzene and 1,3-butadiene respectively.
