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

PM2.5 emissions and receptor modelling
Table 4.2: Contribution of point sources to UK emission totals in the NAEI
(2006) (Bush et al., 2008).
Pollutant Point sources (%) Area sources
CO 24% 76%
NH3 2% 98%
NMVOCs 20% 80%
NOx 32% 68%
PM10 20% 80%
SO2 78% 22%
33. Equivalent figures for PM2.5 have not been estimated, but one might expect the
contribution from point sources to be slightly higher than for PM10 because these
mainly arise from combustion sources associated with higher PM2.5 fractions
than most area sources. This effectively means that the spatial distribution
of PM10, and most likely PM2.5, emissions cannot be known as accurately as
that of sulphur dioxide (SO2) emissions because a much smaller proportion of
emissions of PM2.5 come from point sources. Considering emissions at a finer
degree of resolution will lead to even higher levels of uncertainties. For example,
the movement of traffic and the emissions near a specific road junction may
be quite different to emissions occurring a few metres away on the same road
link. Emissions in different parts of a major industrial plant where many different
operations take place (e.g. an iron and steel works) can be highly variable but on
a 1 km x 1 km grid may be considered nominally as a single point source.
34. Sources where the spatial distribution of PM emissions are particularly
uncertain are domestic combustion, off-road machinery, shipping, construction,
agriculture and other fugitive releases of dust.
35. Although it is not possible to quantify the uncertainties in the spatial distribution
of emissions in terms of confidence levels, the NAEI has developed a fairly
sophisticated approach to provide an overall data quality confidence rating for
each pollutant map (Bush et al., 2008). This is aimed at ranking the confidence
rating for mapping emissions for different pollutants based on the quality ratings
of the various ‘grids’ used to spatially resolve the data. The quality ranking of
PM mapped emissions is relatively poor compared with the ranking for SO2 and
nitrogen oxides (NOx) but higher than for ammonia (NH3). This is because of the
relatively high contribution to emissions of PM from diffuse sources.
36. Another consideration is the temporal variability in emissions. Many combustion
sources follow a relatively regular pattern of activity by time of day and day of
week or month (e.g. emissions from road traffic, power stations and domestic
and industrial combustion), while others are far more sporadic in nature in terms
of temporal and spatial variability, such as emissions from off-road machinery
and construction, which can be transient in nature, starting and ending at
any time throughout a year. Emissions from other fugitive dust sources can
also be highly irregular and dependent on unpredictable changes in operating
and weather conditions, e.g. emissions from agricultural processes. Other
intermittent sources include natural and accidental occurrences, including forest
and grass fires, bonfires and building fires.
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