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

Modelling PM2.5 and the future
17. The model calculations are consistent with rural background levels which are
lowest in the north and west of the British Isles because of the greater geographical
separation from the large UK and European PM sources. However, background
levels do not decline to zero in either rural areas with negligible emissions or in
marine air masses arriving at the North Atlantic Ocean coastline of the British Isles.
18. The models show that the contributions to rural background levels include the
following:
• vehicular and stationary sources on the regional scale, including shipping;
• short- and long-range formation and transport from UK PM precursor sources;
• long-range transboundary formation and transport of primary and
secondary PM;
• intercontinental-scale PM formation and transport; and
• natural background sources which are estimated in some models and which
include sources such as secondary organic aerosol (SOA) from oceanic
dimethyl sulphide (DMS), Saharan dusts, forest fires and volcanic eruptions.
19. Figure 5.3 shows a calculation using ADMS-Urban of annual average PM2.5
concentrations across London for 2008, and includes an inset for concentrations
in central London at higher resolution. In this study, as the ADMS-Urban model
considers the rural background to be constant across the model domain and
the local generation of secondary particulates is negligible, the variations in
concentrations in Figure 5.3 arise solely from primary emissions in the urban area.
These comprise a number of contributions including emissions from road vehicles
(combustion-derived particles, brake and tyre wear particles and resuspended
road dusts), and also other combustion sources. The background annual average
concentration rises towards the centre of the urban area with local peaks close to
roads resulting from exhaust and non-exhaust road traffic emissions. This general
pattern is typical for all significant urban areas in the UK.
Figure 5.1: Concentrations of total annual mean PM2.5 as calculated by the
PCM model (µg m -3 ) for 2009 (left) and 2020 (right).
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