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

• representative over the period of measurement; for most purposes, a
yearly data capture rate of not less than 90% is required for determining
compliance with EU limit values. Data capture rates actually achieved are
described later in this chapter.
The main aspects of QA/QC are site inter-calibrations and data ratification.
2.3.5 Network inter-calibration and site audits
Measuring PM2.5 and its components
31. Within the UK, a system of routine site audit and network inter-calibration has
been in place for many years. The primary aims and objectives of the site audit
and network inter-comparison exercise can be summarised as follows:
• to check and evaluate a wide range of analyser key functions via a
comprehensive set of tests and calibrations; and
• to carry out on-site instrument calibrations using standards that are directly
traceable to the QA/QC Unit Calibration Laboratory and national metrology
standards.
32. This network inter-calibration is undertaken by the QA/QC Unit at six-monthly
intervals and includes every site and every analyser in the network. The intercalibration
therefore provides detailed and quantified information on overall
network performance. The aim of each audit exercise is to provide the following
information:
• leak and flow checks to ensure that ambient air reaches the analysers
without being compromised in any way;
• TEOM ko evaluation (the stiffness of the glass tapered element). The
analyser uses this factor (ko) to calculate mass concentrations, so the value
is calculated with test masses to determine its accuracy;
• particulate analyser flow rate checks to ensure that the flow rates through
critical parts of the analyser are within specified limits; and
• site infrastructure and environment checks to ensure that metadata on the
station remains accurate and up to date, and that any site safety issues are
noted for action.
33. In principle these tests are performed to meet the requirements for QA/QC set
out in CEN Standard Methods so as to comply with the Air Quality Directive.
In the case of PM2.5, the standard is only available in draft form, but UK
participation in the CEN Working Group ensures that UK procedures are very
similar to draft procedures. The QA/QC Unit is also accredited by the United
Kingdom Accreditation Service under BS EN17025 to perform these tests;
accreditation involves annual assessment by an external expert.
34. The site audit and inter-calibration results provide vital information for the data
ratification process. For example, if the TEOM k0 or flow rate checks are outside
acceptable limits (2.5% and 10% respectively) then data may need to be
re-scaled or rejected depending on both the reason for the outlier and severity
of the discrepancy.
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