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

PM2.5 emissions and receptor modelling
using a statistical Monte Carlo simulation approach consistent with the Tier 2
method in Good Practice Guidance for CLRTAP emission inventory reporting,
details of which are given in Passant (2003) and Passant et al. (2011). The
approach requires consideration of the available data on emission factors and
levels of activity, combined with expert judgement to estimate the uncertainty
parameters applicable to each source sector.
27. The estimated uncertainty in total UK PM2.5 emissions is estimated to be
between -20% and +30%. However, this estimate of uncertainty at the
UK level, when compared with inventory uncertainties for other pollutants
(Passant et al., 2011), masks the much higher uncertainties in emissions from
individual sectors and in emissions at specific locations and times. These are
difficult to quantify, but were addressed in Annex D of the report Evaluating the
Performance of Air Quality Models (Defra, 2010) 5 building on the work of Bush
et al. (2008).
28. Emission estimates for the combustion of fuels are generally considered more
reliable than those from industrial and agricultural processes and other noncombustion
sources such as quarrying, construction, tyre and brake wear,
and road abrasion. Many of these sources are diffuse or fugitive in nature,
e.g. emissions from coke ovens, metal processing, mining, quarrying and
construction. Emissions from these sources are variable in time and location
around a specific plant or operation and are difficult to measure and it is
likely that no entirely satisfactory measurements have ever been made. Even
emissions from combustion processes can be subject to high uncertainty,
especially in cases where PM emissions are very low and difficult to measure
(e.g. from gas combustion or emissions from vehicles with a diesel particulate
filter) or where the combustion process is not controlled or the fuel is of
variable quality. This may be especially true for combustion of solid fuels
including biomass. Inventories partly rely on emission factors and particulate
size distributions based on measurements or information from other countries
(e.g. the US) and the relevance of these data to UK emission sources can be
questioned. Vehicle exhaust emissions of PM are dependent on many factors,
including the driving cycle, engine technology and state of maintenance of the
vehicle, and inventories are only able to use nationally-averaged information on
these parameters.
29. Emissions of PM2.5 from non-exhaust traffic sources, such as tyre and brake
wear and road abrasion, are particularly uncertain. The PM2.5:PM10 mass fraction
for tyre wear of 0.7 used in the NAEI has been brought into question by recent
evidence suggesting that very little PM emitted from tyre wear occurs in the
PM2.5 size fraction. The NAEI uses PM2.5:TSP and PM10:TSP mass fractions for tyre
wear (and brake wear) taken from the EMEP Emissions Inventory Guidebook. 6
They are used in conjunction with TSP emission rates for tyre wear given by
the same source. The Guidebook itself takes the information from an earlier
review of measurements by the UNECE Task Force on Emissions Inventories. 7
However, the evidence given in this review for all non-exhaust traffic sources
covers a very large range. Recent studies reported in the literature suggest
that for normal all-weather tyres, the particles are primarily in two size ranges,
5 See http://uk-air.defra.gov.uk/reports/cat05/1006241607_100608_MIP_Final_Version.pdf.
6 See Table 3-4 in: http://www.eea.europa.eu/publications/emep-eea-emission-inventory-guidebook-2009/part-b-sectoral-guidancechapters/1-energy/1-a-combustion/1-a-3-b-vi-road-tyre-and-brake-wear.pdf.
7 See http://vergina.eng.auth.gr/mech0/lat/PM10/.
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