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

36. The behaviour of semi-volatile constituents is a major problem both for the
measurement and modelling of PM2.5. In the case of measurement, the degree
of partitioning of semi-volatile compounds into the measured particles is
determined by factors such as atmospheric temperature and relative humidity.
Consequently, if these change during the sampling interval (as they typically
do from day to night), sampling artefacts can occur through the evaporation
or condensation of such materials from/on the air filter used in sampling.
Additionally, the reduced pressure used to draw air through a sampling filter
can encourage the vaporisation of semi-volatile components. The difficulties
for modelling start with the prediction of the formation of secondary semivolatile
components and continue with the problem of describing their partition
into the particles. This is affected by many factors, most notably temperature,
relative humidity and the existing composition of the particles, which affect
the thermodynamic activities of the soluble components and the uptake into
organic films for the hydrophobic components. A further difficulty for modellers
is caused by the fact that air sampling methods for PM2.5 are not artefact
free and therefore measure concentrations that may not reflect well the true
airborne suspended particle mass.
37. Since semi-volatile organic matter and ammonium salts comprise a substantial
part of PM2.5 mass, these issues have major implications for both measurement
and modelling of concentrations.
5.4.4 Other components
38. In addition to primary PM2.5 from sources represented in the emission inventory
and secondary inorganic aerosol, there is a large contribution from other
components, many of which are more uncertain and include natural or land
use related contributions not subject to control. These include rural dust from
wind-suspended soil particles varying with soil characteristics, and urban dust
including material resuspended by traffic. Sea salt is another component of
natural origin, the influence of which decreases with distance from the coast.
39. Finally, particle bound water makes an additional contribution to particle mass.
It might seem that this is another natural component, but it has been argued
in legislation concerning PM10 that it is partly associated with man-made SIA
contributions. However, estimating any reduction in particle bound water in
response to reductions in SO2, NOx and NH3 emissions is extremely uncertain,
involving both direct and indirect effects on cloud processes. Hence, in Table 5.5
for example, where water content in UKIAM is based on modelling provided
by EMEP, water content is kept constant in future projections (which may be
pessimistic). By contrast the PCM model associates the water entirely with the
SIA components and scales it accordingly.
5.5 Model evaluation for PM2.5
Modelling PM2.5 and the future
40. Evaluating models that predict PM2.5 is complex due to the multiple components
that make up PM2.5 mass. At one level it is possible to compare absolute mass
predictions directly with PM2.5 measurements, as illustrated in Figure 5.10.
The increased number of sites measuring PM2.5 in recent years will aid model
evaluation in this respect. However, while useful, such a comparison is limited
because it will not be known why model predictions depart from measured
values, which is a critical issue for effective model evaluation. For this reason,
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