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

Concentrations and composition of PM2.5
verify modelled regional background concentrations and confirm the modelled
spatial pattern of declining rural concentrations from the south-east of England
to the north-west of Scotland. They will also allow the urban enhancement
to be determined more precisely. These new sites will need to be sufficiently
far from urban areas to define the true rural background uninfluenced by the
urban area, but not too far, as they would then not represent the regional
background applicable across the urban area. The latter will be a particularly
important consideration where there is a strong gradient in concentrations, such
as that suggested in models on moving from the south coast of England to the
Midlands. In these situations it may well be appropriate to define the urban
enhancement as the difference between the urban background concentration
and the rural background averaged across two or more rural sites located on
different sides of the urban area. It is recommended that an expansion of
the rural PM2.5 monitoring network to allow the rural background to be
properly defined is given urgent consideration.
3.6 Relationship with wind direction and air mass trajectory
3.6.1 Relationship of hourly mean PM2.5 concentrations with wind
direction
22. The Openair software package (Carslaw and Ropkins, 2011) using the R
language (R Development Core Team, 2011) was used by Laxen et al. (2010) to
examine the relationship of PM2.5 concentrations with wind direction and wind
speed (in polar plots) for urban background sites in 2009. They found a broadly
consistent pattern across the UK, with a strong easterly predominance to the
higher PM2.5 concentrations, with a tendency for the highest concentrations
to occur with higher wind speeds ~10 m s -1 . This analysis has been repeated
here for 2010 data and the results show a similar association of the highest
concentrations with easterly and south-easterly winds (Figure 3.11), with
the highest concentrations often being associated with higher wind speeds.
The findings reinforce the view that urban background PM2.5 concentrations
are dominated by regional rather than local sources, and that PM from
sources in continental Europe, probably as secondary PM, significantly affects
concentrations in the UK.
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