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

PM2.5 in the UK
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6. The non-linearity of the model response to emission reduction is the result of
complex interactions. It highlights the importance of introducing the emissions
into the model using realistic temporal profiles.
A2.2: WRF/CMAQ applications at the Centre for
Atmospheric and Instrumentation Research (CAIR),
University of Hertfordshire
7. CMAQ is not currently being used as a policy tool in the UK but is being
considered for regulatory applications as part of the Comparison of Simple
and Advanced Regional Models (CREMO) funded by the Environment Agency.
Defra is commissioning work to assess the potential of CMAQ to meet its policy
needs. The intention of this work will be to develop an operational version of
the model for UK policy applications. This and the following section provide
examples of applications for the years 2003, 2005 and 2006, which formed part
of the AQMEII, MEGAPOLI (FP7), Defra model inter-comparison and CREMO
projects. Further details on the model set-up and applications can be found in
Yu et al. (2008) and Chemel et al. (2010).
8. The focus of research has been on PM2.5 and modelled results are compared
with the observations from Harwell, London North Kensington and London
Bloomsbury. Overall, CMAQ underestimates PM2.5 concentration at the
selected sites but reproduces the temporal variations. There are important
considerations when comparing modelled and measured data for the results
presented here. First, the spatial grid resolution is 18 km x 18 km and is not
optimum for simulating pollutants that are, at least partly, generated on
smaller, local scales. This set-up has been used as a pragmatic approach to
develop a regional configuration to model hourly concentrations over multiple
years. Second, previous results (not shown here) have indicated sensitivity of
PM2.5 concentrations to boundary conditions, for example, in the comparison
conducted with boundary conditions from GEMS and from GEOS-Chem (global
air pollution model). Third, the treatment of aerosols is part of ongoing model
developmental work and significant changes will be introduced in the next
version of CMAQ. The results shown in Figure A2.2.1 are annual PM2.5 modelled
results for 2003 using CMAQ version 4.6 and annual PM2.5 modelled results for
2006 using CMAQ version 4.7.1. For 2003 the grid spacing was 15 km. The
2006 run conducted at 18 km grid spacing was part of the AQMEII model intercomparison
for a domain which covered all of Europe.
9. The daily average PM2.5 concentrations were calculated for a rural background
station Harwell (HAR) for 2003 and 2006, and for urban background stations
London North Kensington (LNK) for 2006 and London Bloomsbury (LBB)
for 2003. Modelled PM2.5 ground (first level) results are compared with
measurements from the selected stations for the year 2006 and 2003 for the
appropriate stations in Figure A2.2.1. This figure shows that overall CMAQ
values reproduce the temporal variations for all three stations but show about
a 30-40% underestimation especially for the urban stations. Further analysis is
required to understand the model response during peak occurrences, where it
correctly captures the timing but not necessarily the magnitude of the event.
This will include examination of the local and regional nature of PM2.5 and its
precursors, as well as the governing meteorological processes (simulated with
WRF).