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

PM2.5 in the UK
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2.3.3 The Partisol 2025 instrument
26. The Partisol 2025 is a gravimetric sampler which employs a sequential sampling
system of uniquely identified filters that enables up to 16 fixed 24-hour period
particulate samples (00:00-00:00 hours GMT) to be undertaken between
routine site visits. For the purposes of the UK AURN operations site attendance
is required every 14 days to exchange filter cassette magazines from the storage
and supply positions within the unit. The results from this method have not
been shown to be equivalent to the reference method.
27. The provision of gravimetric sampling, where PM is pulled through a filter via
a vacuum pump, requires that pre- and post-exposed filters are weighed in
standardised environmental conditions. The increase in mass between the two
weighings provides for a measurement of the mass in PM captured on the
filter during the course of the specified sampling period. The calculation of PM
concentration (mass per unit volume of air) is determined by the division of the
mass (in µg) by the total volume of air sampled (in m 3 at ambient temperature
and pressure conditions). The volumetric flow rate is controlled internally using
a mass flow meter linked to ambient temperature and pressure measurements,
and this flow is checked periodically using an external, calibrated flow meter.
28. Pre- and post-conditioning of filters is carried out in accordance with the
requirements of EN14907, the CEN reference method for PM2.5 measurements.
Conditioned filters are provided in pre-loaded filter cassettes via the filter
cassette magazine. The magazine is loaded into the supply position within the
Partisol 2025 unit and unique filter identifiers are loaded into the software
programme to enable identification of each filter exposed during each sampling
period. Sampling of air takes place via an initial PM10 size-selective impactor
combined with a PM2.5 Sharp Cut Cyclone (SCC).
2.3.4 Quality assurance/quality control procedures
29. Data from specific types of instrument must be evaluated in the wider context
of the checks and calibrations employed during their use and in the production
of published data, a process usually known as quality assurance and quality
control (QA/QC). In the case of the AURN, this is the role of the QA/QC Unit. All
PM2.5 measurements, whether from automatic monitors or manual samplers,
are subject to a rigorous procedure of validation and ratification before they are
published on the UK-AIR website 1 , in part to meet the legal obligations of the
Air Quality Directive (2008/50/EC).
30. QA/QC checks are employed to specifically ensure that AURN data are:
• genuinely representative of ambient concentrations existing in the various
areas under investigation;
• sufficiently accurate and precise to meet monitoring objectives;
• comparable and reproducible. Results must be internally consistent and
comparable with international or other accepted standards, if these exist;
• consistent over time. This is particularly important if long-term trend
analysis of the data is to be undertaken; and
1 http://uk-air.defra.gov.uk