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

3.10 Summary and recommendations
3.10.1 Key points from measurements
Concentrations and composition of PM2.5
(a) There are limited measurements of the composition of PM2.5 in different
types of location and in different areas of the UK. This restricts the ability to
understand sources of PM2.5.
(b) Until recent years there have been few measurements of PM2.5
concentrations using reference equivalent instruments. Data are now
becoming available for a comprehensive network across the UK which
should help understanding of PM2.5.
(c) Regional (rural) background concentrations are particularly important, even
in urban areas. In urban areas in central and southern UK at least 60-80%
of the urban background PM2.5 will on average be derived from the regional
background.
(d) The regional background concentrations are dominated by secondary
PM2.5, primarily as ammonium nitrate and ammonium sulphate, but also
as organic particles. In central and southern UK around 60% of the urban
background PM2.5 will be secondary particles.
(e) High PM2.5 concentrations are frequently associated with air transported
into the UK from continental Europe.
(f) Wintertime PM2.5 concentrations are much higher and will make the
greatest contribution to the annual mean. It may thus be more effective to
target measures to those sources contributing to the elevated wintertime
concentrations.
(g) Winter episodes can give rise to high PM2.5 concentrations over a few days.
These are due to a build-up of local emissions under stagnant weather
conditions and are especially prominent in areas with continued domestic
solid and liquid fuel burning.
(h) There is evidence that PM10 episodes associated with air arriving from
continental Europe, especially during the spring, are composed of fine
particles (PM2.5) and not coarse particles (PM2.5-10), with nitrate playing a
particularly important role. This nitrate appears to be largely associated with
ammonium, derived from ammonia emissions.
(i) Black carbon is a major component of the PM2.5 associated with road traffic
emissions and domestic (oil and solid fuel) combustion.
(j) Primary emissions from road traffic, including the non-exhaust component,
make a significant contribution to the urban background increment of
PM2.5. They also contribute to regional secondary PM through the oxidation
of emissions of nitrogen oxides.
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