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

PM2.5 in the UK
20
5. The exact form of the inlet collection efficiency curve for different particle sizes,
typically determined by an impactor that removes the larger particles through a
combination of drag and inertia, is another variable whose effects are generally
smaller. Figure 2.1 illustrates an indicative size-selection curve for PM2.5. The
curve has a midpoint rather than a step change at 2.5 µm, and variations in the
curve can allow through a larger or smaller fraction of particles with a diameter
of say 3 µm, with a consequent change to the result. The correct form of the
collection efficiency curve for regulatory purposes is the one obtained from the
reference method inlet system described within European Standardisation body
CEN standard EN 14907:2005.
collection efficiency (%)
100
90
80
70
60
50
40
30
20
10
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
particle size (aerodynamic diameter) (µm)
Figure 2.1: Indicative size-selection curve for a PM2.5 inlet.
6. It is worth noting that fibrous filters downstream of the size-selective inlet are in
practice highly efficient at collecting all the airborne particles that reach them.
Small particles that might be expected to pass through gaps between the fibres
adhere to them by diffusive processes.
7. Further measurement problems have arisen as PM concentrations have
decreased in recent decades. The filters used to collect the particles are
themselves susceptible to changes in mass due to, for example, absorption of
water or loss of fibres, and these changes have become more significant as PM
concentrations have become lower. It is difficult to correct for these effects as
they can vary between different filter types and batches in ways that are not
fully understood. Other artefacts such as the absorption of reactive gases onto
the filter or the PM are also possible.