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

PM2.5 in the UK
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12. These difficulties apply even when only the reference method is being used.
In practice, monitoring networks mostly use automatic methods, differing
significantly from the reference method but designed to give results that
are equivalent, introducing other uncertainties that make the situation more
complicated. This chapter will outline:
• the principles of the reference method for measuring PM2.5 and the process
used to determine whether other methods are equivalent to the reference
method (Section 2.2);
• the principles and operation of the methods used to produce the PM2.5 data
used within the report (Section 2.3); and
• the methods used to produce data for related metrics such as anion
concentration and elemental and organic carbon (Section 2.4).
2.2 The reference method for measuring PM2.5 and
the determination of equivalence
2.2.1 The reference method
13. As explained above, the reference method for measuring PM2.5 described by
CEN within EN 14907 effectively defines what is meant by PM2.5 for regulatory
monitoring purposes within the European Union. The method is a manual
gravimetric method for daily concentrations. Samples of PM are taken by
pumping ambient air through a size-selective inlet followed by a filter; the
concentration of PM is determined by measuring the change in mass of the
filter in a specialised weighing laboratory at prescribed temperature and
humidity conditions, and combining the result with the volume of air sampled.
14. The desired cut-off curve for airborne particles close to 2.5 µm is defined by
the design of the standard inlets and the control of the flow through them.
The size-selective inlets are impactors. They force the airstream along a
convoluted path so that a combination of inertia and aerodynamic drag forces
the larger particles to collide with a greased plate and be removed. Cyclones
are a different design of size-selective inlet, but are based on the same general
principle. The retention of semi-volatile material is constrained by restrictions
on the temperature near the filter during and after sampling. The quantity
of water in the collected PM is constrained by limits on the temperature and
humidity during a conditioning period before the filters are weighed: currently
19-21ºC and 45-55% relative humidity. In an experiment involving the weighing
of 20 UK PM filter samples at different humidities, the allowed range of relative
humidity led to variations in measured PM2.5 of 4-9% (Butterfield and Quincey,
2009). Examples of other relevant experimental work can be found in Brown
et al. (2006) and Rasmussen et al. (2010).
15. Two versions of the method are described in EN 14907, a low volume method with
a sampling flow of 2.3 m 3 hr -1 and filters of 47 mm diameter, and a high volume
method with a sampling flow of 30 m 3 hr -1 and filters of 150 mm diameter.
16. The filter material used for sampling can influence the result, primarily because
the filter mass can vary over time. This mass change can be in response to