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

were for daily PM2.5 nitrate and for daily PM10 sea salt and sulphate, and some
were for daily PM10 EC/OC (for the last five months of 2008).
13. In general, the model underpredicts hourly PM2.5 measurements when using
estimates of modelled PM2.5 mass without particle bound water (Figure A2.3.1).
A similar conclusion can be drawn for total PM10 concentrations. Despite the
underprediction of PM10, the model replicates the proportion of each species
reasonably well, slightly overpredicting observed NO3 - and SO4 2- fractions and
underpredicting others such as Cl - , EC, SOA and OC (Figure A2.3.2). The model
results for fine mode PM nitrate (Figure A2.3.3) and POC (Figure A2.3.7) are
in reasonable agreement with measurements, although some of the peak
concentrations were not well predicted. Some of the coarse mode particles were
underpredicted and require further evaluation, especially in the case of nitrate
and sea salt. EC, SO4 2- (Figure A2.3.5) and SOA were also underpredicted.
14. Improving the predictive capability of the model for some components (EC)
can be achieved in a straightforward way by improving emissions estimates.
Improving the predictive capability for others (Cl - and coarse mode nitrate) may
prove to be more difficult because there is a combination of possible reasons for
the model underprediction.
15. An examination of the chemical composition of the PM2.5 mass in 2008 revealed
that 58% was estimated to be particle bound water (PBW) (Figure A2.3.1).
This is a model estimate of ambient PBW and contrasts with measurements,
where PBW is mostly removed by conditioning in the field or in the laboratory
and water content is typically reduced to around 10% (Harrison et al., 2004,
and Green et al., 2009). As a consequence, including modelled PBW without
correcting for the measurement conditions can undermine attempts to
evaluate the model for total PM2.5 or PM10 as well as for hygroscopic particles
such as SO4 2- and possibly NO3 - and SOA. These results demonstrate the
importance of PBW when quoting PM concentrations and it would be worth
considering quoting the assumptions made alongside the concentrations from
measurements and models.
Hourly predictions of PM2.5
Annex 2: PM modelling in the UK
16. Speciated PM analysis was conducted against measurements from the AURN
and London Air Quality Network (LAQN) during 2008, using CMAQ (v4.7)
and the WRF meteorological driver. The WRF-CMAQ model was operated
with 23 vertical layers (up to approximately 15 km above ground) and two
nesting levels, downscaling from 81 km grid resolution over Europe to 9 km
grid resolution over the UK. The emissions from EMEP, NAEI and the European
Pollutant Emission Register (EPER) were processed into hourly 3-D gridded
chemical species and used the CB05 chemical scheme which included the
aqueous and aerosol (AERO5) extension.
17. In 2008 CMAQ PM10 and PM2.5 predictions were assessed against measurements
from AURN and LAQN, including 70 PM10 sites (three rural, 12 suburban and
55 urban background) and 29 PM2.5 sites (two rural, six suburban and 21 urban
background). In comparing the hourly average concentrations we have removed
the model’s predictions for water. The reason for doing this is that CMAQ water
estimates can be high (58%), as demonstrated by the modelled estimate of
particle bound water (PBW) at ambient conditions in Figure A2.3.1a.
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