Air quality expert group - Fine particulate matter (PM2.5) in ... - Defra

PM2.5 in the UK
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Correlation
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all data
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Distance (km)
Figure 3.10: Correlation coefficients of hourly mean PM2.5 concentrations
(µg m -3 ) at 11 urban background sites in the Thames Valley area of southern
England as a function of distance between the sites (Laxen et al., 2010). The
shading represents 95% confidence interval.
3.5.2 Differences in annual mean PM2.5 concentrations by site type
21. Laxen et al. (2010) in their summary of 2009 annual mean PM2.5 concentrations
reported that urban background concentrations in major urban areas were
around 3-6 µg m -3 above the rural background, 3 while concentrations alongside
busy roads were around 1-2 µg m -3 above the urban background, rising to 7-8
µg m -3 above the urban background at the kerbside (within 1 m of the kerb)
of busy roads. Thus in the south of England, where the rural background is
around 10-11 µg m -3 , the urban background concentrations are around 13-17
µg m -3 . This is the pattern illustrated in the transect of modelled background
concentrations across London in Figure 5.6 in Chapter 5. The kerbside
concentrations would then be up to 20-25 µg m -3 . Clearly the rural background
is a dominant contributor to PM2.5 concentrations at urban background sites,
but as Laxen et al. (2010) observed, only two rural sites provided data in 2009
(Auchencorth Moss and Harwell). The same was true for 2010. Two sites are
not considered sufficient to define the spatial distribution of rural background
concentrations across the UK. Laxen et al. (2010) recommended that seven
additional PM2.5 monitors should be set up at rural sites as a minimum and
that these could usefully be located at existing ozone monitoring sites. AQEG
agrees that there are too few rural monitors to properly define the important
rural background contribution to PM2.5 across the UK. Additional sites will help
3 The term ‘rural background’ is treated as being synonymous with ‘regional background’. Rural monitoring sites must be located away
from local sources.