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

Table 5.3: Population-weighted annual mean PM2.5 concentrations (µg m -3 ) calculated using
the PCM model for different parts of the UK.
2009 2010 2015 2020
Scotland 6.83 6.61 6.14 5.85
Wales 8.95 8.62 8.02 7.64
Northern Ireland 7.27 6.93 6.35 5.98
Inner London 15.05 14.47 13.26 12.59
Outer London 13.91 13.37 12.28 11.67
Rest of England 10.80 10.39 9.61 9.13
UK 10.70 10.29 9.51 9.04
5.4.2 Secondary inorganic aerosol
Modelling PM2.5 and the future
29. The secondary inorganic aerosol (SIA) in the form of sulphate (SO4 2- ), nitrate
(NO3 - ) and ammonium (NH4 + ) particulates is formed from precursor emissions
of SO2, NOx and NH3. Most of the SIA is within the PM2.5 range, and gives a
contribution to population exposure several times greater than primary PM2.5
emissions, although there are questions as to whether this component is as
toxic as some of the primary material. The modelling of SIA components is
complicated by the chemical interaction between pollutants, resulting in the
formation of ammonium sulphate and ammonium nitrate, as well as a coarser
nitrate component of SIA. The formation of ammonium nitrate particulate
matter is reversible, with models frequently assuming an equilibrium between
gaseous and particulate states depending on ambient NH3 and nitric acid
(HNO3) concentrations. This system is extremely difficult to model given the
highly variable emissions of NH3 spatially and temporally, and hence the very
patchy and intermittent pattern of NH3 concentrations. These NH3 emissions
arise predominantly in rural areas from agricultural activities, but there are also
some urban sources, in particular from some vehicle exhausts (see Chapter 4).
30. The chemical interactions between pollutants result in a complex response
to the reduction of pollutant emissions, for example a change in NOx or SO2
emissions can affect NH4 + concentrations as well as NO3 - and SO4 2- , depending
on the mix of pollutants. This results in a non-linear relationship, whereby
percentage changes in NO3 - , SO4 2- and NH4 + are not directly proportional to
percentage changes in NOx, SO2 and NH3 emissions respectively. However,
source–receptor relationships, derived from more complex models by estimating
the change in SIA concentration at receptor points in response to small
reductions of one pollutant at a time from each emitting source, are often used
as a linear approximation in simpler modelling approaches to analyse different
emission scenarios. This is the approach used in integrated assessment models
such as GAINS and UKIAM, avoiding the need to run a complex model such as
EMEP or CMAQ for each new scenario, but introducing increasing uncertainties
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