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

PM2.5 in the UK
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53. Modelling results have illustrated how primary PM2.5 concentrations show
localised peaks in urban areas owing to local sources, superimposed on a
regional background. The sources which cause these peaks are potentially
subject to abatement and the dispersion of these sources are generally well
represented by models, except when close to roads with complex street
geometries. An important limiting factor in estimating concentrations and
human exposure is likely to be uncertainty in the emissions, including missing
sources.
54. The largest contribution to PM2.5 concentrations overall is secondary inorganic
aerosol (SIA). Secondary inorganic aerosol contributions are more smoothly
varying, resulting from advection on a range of scales up to continental and
illustrated by higher average concentrations in the south-east graduating to
much lower values over Scotland. Nitrate is the largest component of SIA over
the UK, and also the most spatially variable in space and time, depending as it
does on the variability of ammonia emissions and concentrations. More detailed
research is required to investigate the effect of temporal variations in emissions,
especially of ammonia, and explain the seasonal variation and the higher nitrate
levels observed in winter. Sulphate now makes a smaller contribution to PM2.5
concentrations than nitrate owing to major reductions in sulphur emissions in
the UK and in other countries and from shipping.
55. The ratio of urban increment to regional background points to future directions
for PM2.5 control. Control strategies should be considered for the regional
background where secondary inorganic aerosol is by far the largest component
according to models. It is also worth noting that given the exposure reduction
targets outlined in Chapter 1, the removal of the whole of the urban increment
would be required to satisfy them if nothing is done to address the regional
background.
56. Source apportionment from modelling shows how further reductions in SIA
depend on the control of emissions of SO2, NOx and NH3 in other countries,
and from shipping more generally, as well as in the UK. Scenario analysis
shows dependence on future emission ceilings in the EU-27 countries and also
the modelled reduction of sulphate resulting from the MARPOL agreement
of the International Maritime Organization (IMO), counteracted by enhanced
nitrate due to increasing shipping emissions of NOx. Modelling indicates a
complex non-linear response of SIA concentrations to reductions in precursor
emissions due to chemical interactions between pollutants, in particular the
high dependence on the availability of NH3 and the reversible and temperaturedependent
formation of ammonium nitrate. This needs to be borne in mind
when considering the effectiveness of further SO2 and NOx reductions, whilst
emissions of NH3 have remained more constant.
57. Modelling of the smaller secondary organic aerosol (SOA) component is
far more uncertain and speculative than for SIA, both in terms of precursor
emissions (see Chapter 4) and chemical processes, and further work is required
in this area. However, it is more difficult to control SOA and its precursors, of
which biogenic emissions are a large component. It is worth noting that the
oxidants for biogenic VOC precursors, namely hydroxyl ions (OH - ), ozone (O3)
and NO3 - , are all controlled by atmospheric chemistry, and will respond to
further reductions in carbon monoxide (CO), SO2, NOx and VOC emissions. It is