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

PM2.5 in the UK
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average of 93.6%, with 13 out of 81 sites falling below 90% for the relatively
simple ozone measurement, and 89.6%, with 34 out of 115 sites falling below
90% for the more complicated NO2 measurements. In 2010, the corresponding
numbers were 82.6% for PM2.5 (40/78), 92.7% for ozone (15/80) and 90.5%
for NO2 (26/117). The robustness of the data therefore falls short of the Directive
requirements, and of that achieved by other pollutants. It must be appreciated,
however, that technical discussions on how best to operate automated PM2.5
monitoring networks and assess their uncertainties are currently active at a
European level, although they are at a much less advanced stage than for
gaseous pollutants. UK representatives are prominent in these discussions.
78. Second, we need to consider whether conclusions about changes smaller than
the ±25% uncertainty required by the Directive can be drawn from UK data.
Much of the measurement uncertainty is associated with differences from the
reference method, which is itself more intrinsically uncertain than those used
for gaseous pollutants. Data obtained using the same type of instrument and
the same QA/QC procedures (such as FDMS data from individual sites in the
AURN) are expected to be comparable with each other such that variations are
significantly less than 25%. Variations will be less when longer term averages
are taken, removing random variations. However, relevant practical issues
in the operation of such relatively complicated instruments are still being
discovered and evaluated, and it is difficult to put a precise figure on the relative
uncertainties.
79. This aspect is directly relevant to checking compliance with the exposure
reduction target in the 2008 Air Quality Directive, based on assessing the
national PM2.5 average exposure indicator (AEI) over periods ten years apart. For
the United Kingdom, this is expected to mean complying with a 15% reduction
target. If there is any significant change in the monitoring instrumentation
during the ten-year period, the measured change in the AEI is likely to have a
large relative uncertainty, even if attempts are made to correct for the effects
of the instrument change. If the same FDMS instruments are used throughout
the period, the operational issues may still mean difficulties in quantifying the
change in the AEI with sufficient accuracy. Of course, all measurements taken
to determine compliance with a limit value or target value have an associated
measurement uncertainty, so there is nothing new in the fact that uncertainties
can obscure a clear-cut result; however, the nature of the exposure reduction
target and the difficulties of PM2.5 measurement conspire to mean that the
available data may not be fit for purpose.
80. A further aspect to consider is whether the measurements are robust enough
to improve our understanding of the sources of primary PM2.5 and PM2.5
precursors, together with the chemical and other processes involved in
PM2.5 formation and evolution, or in other words whether we can use PM
measurements effectively to evaluate PM2.5 models and emissions inventories.
As discussed in Chapter 5, the uncertainties in PM2.5 measurement data make
them far from ideal for this purpose. This situation can be seen as the result of
a combination of factors, namely that the PM2.5 metric is defined operationally
(making it difficult to model a priori), the relatively large uncertainties that arise
in the practical measurement of the metric (as described in this chapter), and
the inherent complexities in the formation and evolution of airborne particles.