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

PM2.5 in the UK
30
35. For non-automatic PM2.5 monitoring, QA/QC may also include auditing of the
filter weighing facilities to ensure that all documented procedures are being
followed correctly, that the lab is managed and operated effectively, and that all
staff are suitably trained for the tasks required of them.
2.3.6 Particulate data ratification
36. Initial data validation checks carried out by the Central Management and
Control Unit (CMCU) and QA/QC units to allow “provisional” data to be
reported to the public are followed by more thorough checking at three-month
intervals. This latter process is called data “ratification”. These checks ensure
that the final reported data are reliable and consistent.
37. Ratification takes into account all the available information on the operation of
the particulate monitors as outlined in the following paragraphs. The first step
of ratification is to go back to the “raw” unvalidated data from the analyser;
this allows traceability of all operations on the data within the ratification
process to be maintained.
38. In general, the ratification of PM2.5 monitoring data should follow similar
procedures and allow the same full traceability as applies to other particulate
and gaseous monitoring data. General procedures should include:
• ratification of the basic reporting averages (e.g. hourly or 15-minute data)
in order to maximise data quality, not of the composite daily means which
may mask instrument operating problems; and
• taking account of all information about the monitoring site and its
surrounding location during the ratification process.
39. FDMS instruments require ratification of both base (non-volatile) and reference
or purge (volatile) measurement channels. These parameters are used to
calculate the total PM2.5 concentration in ambient air on an hour-by-hour basis.
40. Since for an FDMS instrument, total PM2.5 mass concentration is the sum of
the volatile and non-volatile parts, the data processing and reporting of these
three concentrations are inextricably linked. A total PM2.5 concentration can
only be reported if valid volatile and non-volatile measurements were made.
Similarly, if the total PM2.5 mass concentration is judged to be unreliable (e.g.
excessive noise), then both the volatile and non-volatile parts are deleted. Figure
2.4 presents FDMS concentrations for a problematical AURN monitoring site
where we are trying to measure especially low concentrations. There are several
features to note here:
• The volatile concentrations during this period were low so that the
total mass concentration is only slightly greater than the non-volatile
concentration.
• Unfortunately, all air quality instruments are affected by signal noise. This
noise becomes more apparent as sensitivity is increased to detect the lower
concentrations. The FDMS instrument at this site appears to be producing
noisy data throughout, believed to be related to difficulties in controlling
the enclosure temperature.