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

PM2.5 in the UK
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the North Sea (although this is more than offset by increases in NOx from the
growth in shipping). This emphasises the dependence of trends in PM2.5 on
future projections of emissions outside the UK, and hence on the revision of
national emission ceilings under CLRTAP and the EU’s Air Quality Directive.
Another uncertainty in both models arises from the linear extrapolation of the
response of SIA concentrations to changes in emissions (see Section 5.4).
48. Trends in primary components are more consistent between the two models,
but UKIAM estimates lower concentrations than the PCM model, and both may
underestimate where emission inventories are incomplete.
49. For both models there is still a substantial contribution from other components
which is highly uncertain. The UKIAM model used the same treatment of
urban and rural dusts and sea salt developed for the PCM model, so these are
effectively the same; and both adopt SOA modelling undertaken independently
and assumed to be unchanged over time. The UKIAM model uses modelled
water content from the EMEP model but does not allow for any change in
this component in conjunction with soluble SIA components. Further work is
needed on the contribution of all these additional components.
50. In conclusion, there are significant differences between the preliminary
projections to 2020 from the two models, both in source apportionment
and trends, and in assumptions about future emissions. These need further
investigation, ideally with extension to other models, taking note of the
uncertainties and gaps in knowledge indicated in this report concerning, inter
alia, emission inventories and projections. When considering the multiple
components of PM2.5, each of which presents different problems to quantify,
model validation is not possible in the absence of new speciated measurements
to give a detailed breakdown of overall PM2.5 concentrations.
Table 5.4: Population-weighted mean contributions to annual mean PM2.5 in the UK in 2009
and projections to 2010 and 2020 from the PCM model (µg m -3 ). The % reductions between
2010 and 2020 are also shown.
2009 2010 2020
Reduction
2010 to 2020
sea salt 0.67 0.67 0.67 0.0%
residual 1.00 1.00 1.00 0.0%
secondary inorganic aerosol 4.05 3.94 3.34 15.2%
secondary organic aerosol 0.86 0.86 0.86 0.0%
regional primary 1.14 0.90 0.80 12.1%
rural dust 0.51 0.51 0.51 0.0%
urban dust 0.62 0.62 0.62 0.0%
point sources 0.07 0.06 0.06 0.3%
non-traffic area sources 1.02 0.99 0.78 20.5%
traffic area sources 0.75 0.73 0.38 47.3%
Total 10.70 10.29 9.04 12.2%