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

PM2.5 in the UK
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3.9 Composition
3.9.1 Overall composition of PM2.5
39. There have been few measurements of the overall composition of PM2.5 in the
UK. Yin and Harrison (2008) collected 24-hour samples of PM2.5 from three
sites in and around Birmingham between 2004 and 2006, sampling daily at a
central urban background site (12 months), a roadside site (six months, mainly
summer) and a rural site (six months, mainly winter). Samples were taken
sequentially. A pragmatic mass closure model was used to assign compounds
to the measured constituents. This mass closure model is a way of accounting
for the measured mass concentration of airborne particles by summing scaled
values of the concentrations of major chemical components in such a way as
to allow for those components that are not directly measured (Harrison et al.,
2003). The overall results for the central urban background site are shown in
Figure 3.17a. Three components account for a large proportion of the total
mass: nitrate, sulphate and organics. Iron-rich dusts are attributed to nonexhaust
traffic emissions, calcium salts are attributed to construction/demolition
dust and wind-blown soil, and sodium chloride to sea salt (although this should
also include resuspended de-icing salt). Elemental carbon relates to primary
combustion. Organic carbon (organics), both from combustion and natural
sources, may be either primary or secondary.
40. Yin and Harrison (2008) also reported the PM2.5 composition on high pollution
episode days (i.e. days with daily mean PM10 > 50 µg m -3 ). They reported a
substantial increase in nitrate, which accounts for just under half the PM2.5
on episode days (Figure 3.17b). The authors note that this emphasises the
importance of developing a better understanding of the sources of nitrate in
airborne PM. Putaud et al. (2010) also report higher nitrate contributions at
higher PM2.5 concentrations based on measurements carried out elsewhere
in Europe.
41. Measurements of chemical composition are important for source apportionment
and it is recommended that further measurements should be made of
chemical composition in a variety of locations, principally rural, urban
background and roadside, over different parts of the UK, to allow source
apportionment to be conducted.