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

57. The map of the annual mean PM2.5 in 2009 at background locations is shown in
Figure 5.1 (Chapter 5). This map has been calibrated using measurements from
TEOM FDMS instruments (see Chapter 2) within the national network for which
co-located PM2.5 and PM10 measurements are available for 2009, the first year
for which PM2.5 measurements from an extensive network of sites in the UK are
available. The models for PM10 and PM2.5 are designed to be fully consistent.
Each component is either derived from emission estimates for PM10 or PM2.5
or the contributions to the fine and coarse particle size fractions are estimated
separately. This enabled an additional check that the calibration parameters
for the two pollutants are reasonably consistent. Measurements from national
network sites without co-located PM10 instruments have been used as an
additional verification dataset. The results from the annual mean model could
then be directly compared with the annual mean limit value in order to carry out
air quality assessments and policy analyses.
58. An important application of the PCM model is its ability to provide a source
apportionment of the observed PM2.5. Figure 5.10 (Chapter 5) illustrates this
ability for a number of background locations in 2009. The importance of
secondary PM is clearly illustrated and dominates all other PM components,
particularly traffic exhaust emissions.
59. Figure A2.8.1 presents a verification plot for the PCM model estimates of the
annual mean PM2.5 concentrations. The model performance indicated is entirely
satisfactory, with the vast majority of the points in the scatter plot falling within
±50% of the observations.
Modelled (µg m -3)
25
20
15
10
5
0
0 5 10 15 20 25
Measured (µg m-3) Annex 2: PM modelling in the UK
FDMS calibration sites
Paroistol common sites
FDMS PM2.5 only sites
Partisol PM2.5 only sites
Verification sites FDMS
Verification sited TEOM x 10
x=y
x=y + 50%
x=y - 50%
Figure A2.8.1: Verification plot for the PCM model estimates of PM2.5 for
background sites in 2009.
60. An interesting feature of the PM observations over the years has been the
magnitude of the traffic and London increments, as for example shown by the
differences between London Marylebone Road and London North Kensington,
and between London North Kensington and Harwell, Oxfordshire. These have
pointed to the large influence that long-range transport has on observed PM
levels in London. The PCM model has provided a view on the contributions
played by a number of PM components to the traffic and London increments, as
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