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

Conclusions and future directions
other countries. From the UK perspective, only about half the exposure
of the UK population to SIA is due to UK emissions, with around 33%
arising from other countries and 17% from shipping. Future effects
will depend on the control of emissions of SO2, NOx and NH3 in other
countries and from shipping as well in the UK.
(c) Control of SIA is uncertain because of the complex non-linear response
of SIA concentrations to reductions in precursor emissions owing to
chemical interactions between pollutants; in particular, the formation
of ammonium nitrate is reversible, temperature dependent and highly
dependent on the availability of NH3. This needs to be borne in mind
when considering the effectiveness of further SO2 and NOx reductions,
with emissions of NH3 likely to remain broadly constant. Regional
ammonia control combined with NOx and SO2 control are therefore
likely to be critical future factors in future control of PM2.5. There are also
trade-offs to be considered as, for example, abatement of SO2 may lead
to increase in nitrate aerosol.
9. Looking forward, there is a clear potential policy imperative in terms of meeting
future exposure reduction targets for PM2.5. The UK is likely to be required to
meet a reduction target of around 2 µg m -3 in three-year average concentrations
across the UK network of urban background sites, currently roughly 13 µg m -3
(see Chapter 1), between 2010 and 2020. While the reductions required to
meet targets appear to be relatively small, they will still present a substantial
challenge, especially in view of the proportion subject to UK control. There
are significant non-linearities in PM chemistry that mean that changes in
precursor gas emissions can have non-proportional effects on the observed PM
concentrations. Interaction between pollutants means that changes in one can
affect another; for example, reductions in SO2 and NOx over the next decade
are expected to reduce ammonium (NH4 + ) concentrations even though NH3
emissions are projected to remain relatively constant (with a greater proportion
of the NH3 redeposited by dry deposition).
10. The chemistry of secondary organic PM formation is poorly understood and it
is not clear which sources should be targeted to reduce PM concentrations (see
Table 6.1). Though modelling suggests that the bulk of SOA is biogenic in origin
implying limited capacity for reduction, there is considerable uncertainty in the
modelling of such complex chemistry with semi-volatile compounds.
11. There is a general and important challenge in the development of emission
inventories fit for modelling PM2.5 concentrations. Are inventories that have
traditionally been constructed for reporting to international bodies following
prescribed methods and procedures suitable for use in air quality models? The
answer is no because of the importance of the temporal and spatial variability
of emissions of primary PM2.5 and secondary precursor gases from many
varied sources and because of the high uncertainty in the methods used for
quantifying emissions from, in particular, the many diffuse fugitive dust sources.
Another reason is the absence of certain sources from reported inventories, such
as wind-blown dust, resuspension of road dust and biogenic sources.
177