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

Measuring PM2.5 and its components
• There are long periods of negative concentrations both for the volatile and
non-volatile components of the PM2.5 at this location, which cannot be
exactly correct. However, this feature is particularly apparent because of the
low concentrations, and the measurements still fall within the uncertainties
of the method and based on current reporting rules would pass as ratified.
• In some cases FDMS instruments are prone to reporting negative data
after a filter change but in this case it is clearly a more general problem.
Instrument filter changes are however recommended to be undertaken only
when essential, and the AURN QA/QC Unit has worked closely with the
FDMS instrument supplier to develop upgraded filter changing procedures.
Concentration / µg m -3
40
35
30
25
20
15
10
5
0
-5
-10
Jan-11
Feb-11
Mar-11
Auchencorth Moss PM 2.5 FDMS Measurements
Apr-11
May-11
Jun-11
Jul-11
Aug-11
FDMS Non-Volatile
(Base)
Figure 2.4: Example FDMS data. The y-axis shows the volatile and non-volatile
components of PM2.5 in µg m -3 .
41. For correct operation, FDMS instruments must operate within certain
temperature and humidity tolerances. Hence, the ratification process also
closely inspects these data. As a general rule there must always be at least 4°C
separation between the internal and external dew points to prevent moisture
condensation on the filter.
42. Provisional non-automatic (Partisol) PM2.5 daily mean measurements are
calculated by the CMCU from the change in mass of weighed filters following
exposure to ambient air. The relevant instrument flow rates, filter exposure
period, instrument alarms and diagnostics are all required for data ratification by
the QA/QC Unit.
Sep-11
Oct-11
Nov-11
Dec-11
31