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

Annex 2: PM modelling in the UK
46. The response of PM2.5 to 30% reductions in SO2 emissions is necessarily the
result of a number of factors and interactions involving each of the PM2.5
components. Sulphate levels decrease but the increased availability of NH3 leads
to an increase in ammonium nitrate formation and an increase in the extent
of neutralisation of sulphuric acid (H2SO4). As a consequence, ammonium
decreases by much less than 30%, fine nitrate increases and coarse nitrate
decreases. Hence, PM2.5 responds less than linearly to reductions in SO2
emissions, with a sensitivity coefficient of 0.21.
47. The corresponding response of PM2.5 to 30% reductions in NOx emissions is
much less than linear. Fine nitrate levels decrease in response to NOx emission
reductions but the attendant increase in hydroxyl (OH - ) levels reduces the
fine and coarse nitrate responses and also leads to an increase in sulphate.
Ammonium levels fall somewhat, as a result of decreasing levels of ammonium
nitrate and increasing levels of ammonium sulphates. The sensitivity coefficient
of PM2.5 to NOx emissions was accordingly 0.13.
48. Because the chemical environment of the southern UK was found to be
‘ammonia-limited’, the response of PM2.5 to 30% reductions in NH3 emissions
was not straightforward. Fine nitrate declined exactly linearly with the decline
in NH3 emissions because gaseous nitric acid formation was independent of
ammonia and hence there was a linear decline in ammonium nitrate formation.
This decline in ammonium nitrate formation drives an increase in coarse nitrate
formation. Sulphate levels were left unchanged by reductions in ammonia
emissions but significant changes were observed in the extent of neutralisation
of H2SO4. Overall, the sensitivity of PM2.5 to NH3 emissions was found to be 0.30
and was the largest for the SO2, NOx and NH3 PM precursors.
49. The linearity of the chemical production pathways forming secondary PM
components was examined by sensitivity studies to 30% reductions in SO2,
NOx, NH3, VOC and CO emissions. The chemical environment revealed by
these sensitivity studies appeared to be ‘ammonia-limited’. Consequently, PM
mass concentrations appeared to be markedly non-linear with PM precursor
emissions. Policy strategies for PM2.5 therefore need to take into account
emission reductions for a wide range of primary PM components and secondary
PM precursors and to focus primarily on the abatement of NH3. This complex
interlinking may help to explain why PM levels have remained constant despite
falling primary PM emissions.
50. In summary, Figure A2.6.2 presents the fractional reduction in annual mean
PM2.5 concentrations at Harwell, Oxfordshire, for a given reduction in precursor
emissions. That is to say, for an x% reduction in precursor emissions, the
reduction in ammonia emissions would give the greatest reduction in PM2.5
levels out of all the precursor species considered (i.e. NH3, NOx, SO2, VOCs, CO,
EC and OC).
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