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

PM2.5 in the UK
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49. Many of the issues with FDMS data quality have been perceived to be related
to the performance of the instruments’ dryer units. Whilst any specific problem
with the dryer unit is often difficult to identify, the following symptoms have
often been observed:
• PM2.5 higher than PM10 at locations where the two parameters are being
measured in parallel;
• an increasing baseline offset on either PM2.5 or PM10 measurements over an
extended duration of months or even years. This is usually most evident in
the volatile fraction but not exclusively so;
• a sudden (usually downwards) step change in the measured concentrations
following the replacement of a dryer unit. Again this is usually most evident
in the volatile fraction but not exclusively so;
• the volatile measurements at an individual site being consistently higher
than those at other locations within a radius of 130 km; and
• poor comparison with other co-located or nearby reference equivalent
measurements.
50. Whilst the dryer-related issues are still not fully understood, further research is
ongoing to determine whether the baseline offset can be routinely quantified
and therefore corrected for during data ratification. This will enable data of
previously unacceptable quality to be recovered in future and thereby improve
the overall ratified data capture of FDMS instruments.
51. Data capture refers to the amount of data meeting the uncertainty requirement
compared with the largest achievable set of data for the year. In 2009, the
network mean data capture for PM2.5 was 85.8%, with 34 out of 76 sites falling
below 90%. This compares with an average of 93.6%, with 13 out of 81 sites
falling below 90% for the relatively simple ozone measurement, and 89.6%,
with 34 out of 115 sites falling below 90% for the more complicated nitrogen
dioxide (NO2) measurements. In 2010, the corresponding numbers were 82.6%
for PM2.5 (40/78); 92.7% for ozone (15/80); and 90.5% for NO2 (26/117).
The data capture for PM2.5 has therefore fallen short of Air Quality Directive
requirements (90%), and is significantly less than that achieved for other
pollutants. Further details about data capture at individual sites are available at
http://uk-air.defra.gov.uk.
2.4 Methods used to produce data for other metrics in
this report
2.4.1 PM2.5 black carbon (aethalometry)
52. The black carbon data in Chapter 3 of this report were obtained from the UK’s
Black Carbon Network. Black carbon is measured by aethalometers, with size
selection of the sampled aerosol being made by a PM2.5 cyclone placed close to
the inlet of the aethalometer. The Network has only been producing data since
2008, when aethalometers were installed to replace the earlier black smoke
instruments. 2
2 Further details about the Black Carbon Network are available at http://uk-air.defra.gov.uk.