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

PM2.5 in the UK
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4. The delivery of air quality policy objectives relies upon the ability to measure
pollutants in an accurate, reproducible and reliable way. Currently there
are significant challenges associated with the reliable and reproducible
measurement of PM2.5.
5. Long-term self-consistent records are essential for the fulfilment of policy
requirements and to understand the impact of policy actions on the
concentration of any given pollutant. At this time, it is not clear that we have
measurements sufficient to meet this requirement for PM2.5 owing to the nature
of the measurement systems used. The answer to the question ‘Do we have a
robust measure of PM2.5?’ remains substantially uncertain. A major difficulty
for assessment of compliance is that PM2.5 measurement methods are still
evolving and the reference method is currently being revised. Consequently,
measurements made in 2020 may not be directly comparable to those made
in the period 2009-2011 (the base period for the EU Air Quality Directive
requirement). This sheds serious doubt on our ability to provide evidence that
the EU exposure reduction target is being met for PM2.5. These measurement
difficulties, and also the interpretation of PM2.5 data, provide significant
challenges for the modelling community. The measurement uncertainty is
currently at the limit of being meaningful for interpretation by models and
vice versa.
6. AQEG strongly recommends that a focused working group is put together to
make a short-term assessment of the risks of, and solutions and opportunities
to mitigate, measurement uncertainties and of the way that uncertainties
impact on our ability to deliver air quality policy in the UK.
7. The second critical requirement concerns the availability and quality of
chemically-speciated PM2.5 measurements. It is clear these are essential to
the development of effective mitigation policy via source apportionment, as
they allow the delineation of source–receptor relationships, as well as being a
better comparator for models that deal with individual components. Speciated
measurements could also be used in epidemiological studies to strengthen the
knowledge of the influences of particle size and composition upon toxicity and
allow the development of more refined metrics for the prediction of effects
on health.
8. Modelling data and some limited measurement data suggest that a large
proportion of the regional background is comprised of the secondary inorganic
aerosols (SIA) nitrate and sulphate, as well as secondary organic aerosols (SOA).
In cities this is enhanced by an urban increment (Chapter 5, Figure 5.6). What
do the measurements and models tell us about mitigation options?
(a) Reducing urban increment alone may not meet exposure reduction
targets but there are sources that could be mitigated.
(b) There is a significant transboundary import and export of SIA into and
out of the UK, and therefore there is a need for regional-based action.
Modelling estimates indicate that per tonne of sulphur dioxide (SO2),
nitrogen oxides (NOx) and ammonia (NH3) emitted by the UK, about
50%, 55% and 35% respectively of the overall resulting population
exposure to PM2.5 is incurred outside the UK, i.e. results in exposure in