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

PM2.5 in the UK
90
either nanoparticles generated transiently at high temperatures which represent
very little mass, or coarse particles, generally greater than 10 µm, from normal
attrition processes. 8 These studies suggest very little mass of PM from tyre
wear in the PM2.5 size range. There is clearly a need for this conflict between
available PM2.5 size fractions for national inventory reporting and the more
recent evidence to be resolved by those responsible for inventory guidebooks
and emission factor compilations. AQEG recommends this area is urgently
addressed for inventory compilers and modellers and that further
research is undertaken to improve available PM2.5 emission factors for all
non-exhaust traffic sources.
30. The national inventory of PM2.5 emissions is associated with higher levels of
certainty than is associated with emissions at specific locations because it is
largely derived from national statistics on, for example, total consumption of
coal or diesel. The NAEI maps national emissions on a 1 km x 1 km grid for
different sectors, but the spatial variability in emissions from many sources
cannot be known with as much certainty because its estimation relies on proxy
statistics such as household, employment and population census data to map
emissions from area sources (e.g. domestic and industrial process emissions). On
the other hand, emissions from some major combustion point sources such as
power stations can be assigned with reasonably high levels of accuracy because
the source location is broadly known and operators provide specific emissions
data for their operations on the site either from measurements or calculated
from known levels of activity at the site.
31. Emissions of PM2.5 from road transport can be spatially disaggregated with
reasonably high levels of accuracy using traffic flow information on individual
links of the road network. Even here though, assumptions have to be made
about the detailed composition of the fleet (e.g. the age composition and mix
of petrol and diesel cars) as well as about the way people drive, all of which can
be variable and increase the uncertainties in emissions at specific sections of the
network.
32. Overall, the uncertainties in the spatial distribution of emissions broadly depend
on the relative importance of point source emissions, which may be known
reasonably accurately, and the line sources (e.g. road, rail) and area sources,
such as domestic combustion and many fugitive sources, which are known
with much less accuracy. In providing the 1 km x 1 km maps of UK emissions,
the NAEI has considered the quality of the maps in terms of the contribution of
point and area sources to mapped emission totals for each pollutant. These are
shown in Table 4.2.
8 Workshop organised by RIVM and TNO for the Dutch Ministry for Infrastructure and the Environment: The policy relevance of wear
emissions from road transport, now and in the future. Amsterdam, 22 June 2011.