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

PM2.5 in the UK
124
5. At the regional scale, both Eulerian (grid-based) and Lagrangian (“particlefollowing”)
models are used. These model systems typically have output
resolution of a few hundred kilometres or more and in some cases are able to
assimilate measured air pollution data, both surface measurements and satellite
data. They generally require meteorological fields and can incorporate complex
chemical reaction schemes. Models based on these frameworks can calculate
short-term concentrations, however, their use for long-term averages can,
dependent on computer resources and efficiency, be limited by model run times.
Examples of regional-scale models used in the UK include the Eulerian models
EMEP4UK (Annex 2, A2.5) and CMAQ (A2.1-A2.3), and the Lagrangian model
NAME (A2.4). Most regional-scale air quality policy model development within
the EU and UNECE has utilised the RAINS/GAINS integrated assessment model
which is based on source–receptor relationships derived from the EMEP model.
Many studies are also conducted in Europe using other Eulerian models
(e.g. Chimere and EURAD).
6. At the national scale, models include nested versions of regional numerical
models complete with complex meteorological and chemical algorithms (e.g.
EMEP4UK (A2.5) and CMAQ (A2.1-2.3), the trajectory model PTM (A2.6),
the straight-line trajectory model FRAME (A2.7), the semi-empirical model
PCM (A2.8) and UKIAM (A2.9), which combines input of source–receptor
footprints from models at different scales. These models are not able to resolve
concentration gradients at local scales (e.g. below 1 km), however, PCM and the
urban component of UKIAM (BRUTAL) include roadside enhancement factors.
PCM, UKIAM and BRUTAL are designed to calculate long-term averages only.
7. Local- or urban-scale models have very high spatial resolution and typically
resolve the concentration distribution at roadside locations; they can include the
impacts of local buildings, for example in algorithms, to estimate the impact of
street canyons. Their overall domain size is usually limited to tens of kilometres
by underlying assumptions including spatially uniform meteorology and
stationarity (i.e. dispersing plumes do not evolve in time but are recalculated
hour by hour). An example of a local model used in the UK is ADMS-Urban
(A2.10). In principle, local-scale models can be nested in regional-scale models
and this approach is likely to become more common in future.
8. As there is a wide diversity of emission sources of PM, some of which are poorly
specified (e.g. fugitive emissions), some emission processes and categories are
omitted from models. This is generally compensated for by adding additional
concentrations to represent unknown emissions or components.
9. Whilst many of the processes determining PM2.5 concentration are understood
and can be represented reasonably well in models (e.g. the generation of
inorganic aerosols, deposition processes, etc.), there remain some processes
where there is incomplete understanding and/or knowledge. This limits the
representation of these processes in models; examples include the generation of
secondary organic aerosol, treatment of bound water and, relevant only to local
models, initial dispersion from road traffic.
10. Annex A2 summarises the principal models used in the UK (listed in Table
5.1) to predict and understand PM2.5 concentrations, and highlights different
aspects of modelling PM2.5. Annex 2 shows models are currently used for a