3 The Maps

AEAT-3133

3.1 Introduction
Maps of estimated background annual mean concentrations of a range of air pollutants for 1996 are shown in Figures 1 to 8. Each figure includes two graphs in addition to the map. The upper graph shows the relationship between measured air quality and low level emissions and the lower graph shows a comparison between measured and estimated concentrations. Each monitoring site is identified in these graphs by a unique id number (see Table A1 in Appendix 1). Figure 9 is a map of estimated summer mean ozone concentration for 1995 and Figure 10 shows the estimated number of days for which the running 8-hour mean national standard of 50 ppb was exceeded. There were more photochemical ozone episodes during the summer of 1995 than during the summer of 1996, so 1995 was chosen in preference to 1996 for the ozone maps. The ozone maps are therefore representative of levels during a summer with relatively high concentrations, rather than more typical concentrations. Details of the methods used to map ozone concentrations are given in section 4.

3.2 Summary of input data and coefficients
All of the maps except for the maps of ozone concentration were calculated using the following type of expression:

estimated concentration = rural map + k x emissions (kTonnes per 25 km² per year)

Table 2 summarises the rural map, emissions estimates and coefficients used for each map. The coefficients also given in units of sm^-1 (seconds/metres). For NO_x, for example, the coefficient k₁ was derived from the measured concentrations in ppb and emissions data in kTonnes per 25 km² area per year. The coefficient k₂ can be derived by expressing the measured concentration in gm^-3 and the emissions data in gs^-1m^-2. Chamberlain et al (1979) estimated the approximate box model coefficient for a 5 km x 5 km box using appropriate meteorology for the UK. They arrived at a figure of 20 sm^-1, and as can be seen, our coefficients are of similar order. The coefficient for SO₂ is smaller, suggesting that the low-level emissions estimates are too large in the vicinity of the monitoring sites used to calibrate the mapping coefficient. The coefficients for benzene and 1,3-butadiene are higher than the rest but are dependent on the percentage of total the VOC emissions represented by benzene and 1,3-butadiene.

Table 2. Summary of input data and coefficients used to calculate the estimated maps for 1996. Click on the figure number to view the figure.

pollutant	Fig No	units	interpolated rural map	emissions (kTonnes per 25 km2 per year)	coefficient, k1	coefficient, k2 (sm-1)
NO2	1	ppb	NO2	area + major road NOx	5.920	9
NOx	2	ppb	NO2 x 1.2	area + major road NOx	17.349	26
SO2	3	ppb	SO2	area + major road SO2	6.577	14
benzene	4	ppb	NO2 x 0.031	area + major road VOC	0.281	43[1]
1,3-butadiene	5	ppb	NO2 x 0.00538	area + major road VOC	0.0566	42[2]
CO	6	ppm	constant = 0.150	area + major road CO	0.03	25
PM10 (GB)	7	mgm-3	particulate SO4 + 6 + 0.00001 x grid easting[3]	minor + major road PM10	28.67	22
PM10 (Northern Ireland)	7	mgm-3	particulate SO4 + 6 + 0.00001 x grid easting[3]	area + major road SO2	5.91	-
Lead	8	ngm-3	NO2 x 1.344	area + major road NOx	19.431	15

[1]        Benzene assumed to be 1.55% to total VOC emissions (Salway et al, 1997)
[2]        1,3-butadiene assumed to be 0.23% of total VOC emissions (Hutchinson and Clewley, 1996)
[3]        See section 4.5


3.3 Summary of map reliability
Table 3 gives an indication of the reliability of each map in terms of the correlation coefficient between the measured and estimated values and the means of the measured and estimated concentrations at background monitoring sites (not roadside or kerbside).

Table 3. Summary of map reliability

Pollutant	units	correlation coefficient (r)	mean of measurements	mean of estimates	number of sites
NO2	ppb	0.881	20.08	19.75	57
NOx	ppb	0.822	40.09	38.05	57
SO2	ppb	0.726	6.01	5.84	50
benzene	ppb	0.614	1.104	1.146	12
1,3-butadiene	ppb	0.587	0.214	0.224	12
CO	ppm	0.454	0.578	0.539	30
PM10	mgm-3	0.0	24.10	24.40	26
Lead	ngm-3	0.970	36.46	31.02	10
O3 summer	ppb	0.960	26.51	26.98	28
O3 days	days	0.891	25.18	26.40	28

Click on the relevant figure below to view it

1 NO2

2 NOx

3 SO2

4 benzene

5 1,3-butadiene

6 CO

7 PM10 (GB and Northern Ireland)

8 Lead

INDEX