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

8. Similar problems exist for PM10, which is, in principle, the same measurement
but with a size cut-off at 10 µm instead of 2.5 µm. Unfortunately, the problems
are proportionately greater for PM2.5 both because the absolute PM mass is
smaller, and because in general the particles will contain a larger proportion of
semi-volatile and hygroscopic material.
9. The approach for regulatory purposes in Europe has been to standardise a
reference measurement method through the European standardisation body
CEN. The standard for PM2.5 (EN 14907) was published in 2005 and is a
modified and improved version of the earlier standard for PM10
(EN 12341:1999).
10. These methods are manual gravimetric methods, based on the weighing of
filters. Because of the issues outlined above and variations allowed within the
reference method as described in Section 2.2, such as the use of different filter
materials, it is, for example, possible for two operators to follow the procedures
within the standard and obtain results that differ by 10%. As the metric is
effectively defined by convention to be the result obtained by the standard
method, it is not possible to say that one result is more correct than the other,
and this must be borne in mind when comparing:
• measurements made in different places (especially in different countries), as
the methods used may have systematic differences;
• measurements made at different times, as the methods used may have
changed in a subtle way that has a significant effect on the results; and
• measurements with modelled concentrations, as the semi-volatile loss and
water content in measured samples is not well defined.
11. Undesirable effects can have a positive or negative influence on the weighed
filter mass, and these are briefly summarised in Table 2.1.
Table 2.1: A summary of factors affecting the reliability of filter weighing as the basis for
determining PM.
Effects causing undesirable filter mass increase Effects causing undesirable filter mass decrease
Sorption of water vapour by the filter material over
time (highly dependent on filter material)
Sorption of reactive gases by the filter material or PM
on the filter during sampling
Filter conditioning at the post-sampling weighing
being carried out at a higher end of the allowed
range for temperature or relative humidity*
* the conditioning parameters are given in Section 2.2.1.
Measuring PM2.5 and its components
Physical loss of filter material, especially fibres, or PM
due to poor handling
Excessive loss of semi-volatile PM due to overheating
of the filter during sampling
Filter conditioning at the post-sampling weighing
being carried out at a lower end of the allowed range
for temperature or relative humidity*
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