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

PM2.5 in the UK
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93. While there are substantial similarities between the output of the PCM model
and the CMB results, there are also significant differences which warrant
examination. The most important is the industry/commercial/domestic category
which requires close study to resolve the discrepancies. While the sources
contributing to this category are not identical for PCM and CMB, they should
nevertheless not diverge by such a large factor. The other major area of
discrepancy is with secondary organic aerosol, which is unlikely to be explained
by the different sampling periods. As indicated earlier in this chapter, predictive
models for secondary organic aerosol are still at a relatively early stage of
development, and given the agreement between the CMB model and elemental
carbon tracer method in estimating secondary organic aerosol by receptor
modelling, it seems most likely that the numerical model used to predict
secondary organic aerosol for use in PCM is underestimating the SOA mass.
94. The results of the PCM and CMB models in Figure 4.11 provide food for
thought in relation to abatement strategies. The very large contribution
of secondary inorganic particles suggests that large improvements in air
quality could be achieved through abatement of the precursor gases sulphur
dioxide and NOx. However, a study of the relationship of measured sulphate
concentrations at European sites to their precursor sulphur dioxide (Jones and
Harrison, 2011) suggests that due to the non-linearity of the relationship,
substantial reductions in sulphur dioxide will be needed to achieve relatively
small gains in relation to atmospheric sulphate. Specifically, Jones and Harrison
(2011) predicted that a reduction in sulphate concentrations of 1 µg m -3 at
Harwell and London North Kensington would require a reduction in sulphur
dioxide emissions from sources affecting the two UK sites of 55% and 49%
respectively. Measured data for nitrate are far less adequate than those for
sulphate and hence establishing relationships between nitrate aerosol and either
emissions or concentrations of NOx is more difficult than for sulphur dioxide.
Numerical models rely upon substantial parameterisations in order to describe
the atmospheric formation of nitrate aerosol and its subsequent behaviour
and hence much still needs to be done to generate reliable predictions of how
abatement of NOx would benefit concentrations of nitrate in air. Traffic exhaust
emissions of PM2.5 are likely to continue to decrease as a result of new Euro
standards requiring diesel particle filters on new vehicles. However, there are at
present no regulations affecting non-exhaust particles from road traffic, which
currently account for around 50% of the traffic contribution to PM10, although
they contribute substantially less to PM2.5. Other source categories which require
substantially more information before abatement policies can be formulated
are wood burning and cooking. Current knowledge of the magnitude of
their contribution to airborne concentrations is wholly inadequate and hence
the possible benefits of abatement policies are unpredictable. Another
component of particulate matter making a significant contribution to total PM2.5
concentrations is secondary organic aerosol (SOA).
4.6.3 Use of carbon-14 as a tracer of contemporary carbon
95. A further, complementary way of evaluating particulate carbon sources is by
analysis of the radiocarbon ( 14 C) content of airborne particulate matter, which
yields insights into the proportion of the carbonaceous material derived from
fossil and contemporary carbon sources. Heal et al. (2011) applied radiocarbon
analysis to a total of 26 samples of PM2.5 collected at the EROS urban