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

PM2.5 in the UK
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2.4.4 PM2.5 elemental carbon/organic carbon
67. Daily EC/OC measurements of the PM2.5 size fraction, using the same analysis
methods as for the PM10 fraction described above, started at Harwell and
Auchencorth Moss in late 2011. However, no ratified results are yet available to
include in this report.
2.4.5 PM2.5 anion and cation measurements using the MARGA
68. Two MARGA instruments (manufactured by Metrohm Applikon) are operated
as part of the UK Eutrophying and Acidifying Atmospheric Pollutants Network
at the rural sites Auchencorth Moss and Harwell. These provide hourly
measurements of ionic components in PM2.5 (NH4 + , Na + , K + , Ca 2+ , Mg 2+ , Cl - ,
NO3 - , and SO4 2- ), together with similar analysis of PM10, and measurement of
the water-soluble gases NH3, HCl, HNO3, HNO2 and SO2.
69. The instrument contains an automated continuous-flow wet rotating denuder
and steam-jet aerosol samplers. The denuder removes gases from the sampled
airstream before the residual particles (which pass through the denuder)
are converted by steam into droplets which are subsequently captured
and analysed. The solutions of dissolved gases and dissolved particles are
analysed online by ion chromatography, using parallel systems for cations
and anions. Dual samplers are installed at the UK EMEP Supersites (Harwell
and Auchencorth Moss), with separate analysis of PM10 and PM2.5 aerosols.
Internal standards of Li + (lithium cations) and Br - (bromide anions) are used for
calibration checks.
70. The steam-jet aerosol collector measures only water-soluble aerosol, with
parallel and simultaneous ion chromatography for anions and cations. Some
discrepancies have been noted at Auchencorth Moss, with PM2.5 occasionally
giving higher readings than PM10. At least part of this is attributed to lack
of flow control leading to uncertainty in the volume of air sampled. Data
submitted to EMEP are flagged as uncertain. This problem has been rectified on
later models (such as the one installed in 2009 at Harwell) and the Auchencorth
instrument was upgraded to include mass flow control during autumn 2011.
The MARGA has been operating at Auchencorth since June 2006 with varying
reliability in terms of data capture. Recent upgrades to software have improved
data capture and the latest planned upgrade should improve matters further.
71. As described in Chapter 3 later, concerns about the quality of the MARGA data
have meant that their analysis has not been included in the report.
2.4.6 Acid Gases and Aerosols Monitoring Network (AGANet)
measurements using the DELTA system
72. The DELTA system uses a long-time integrated measurement (four weeks) of
trace gases and particles. Gases are removed on a series of acid- and alkalicoated
cylindrical denuders prior to trapping particles on an alkali-impregnated
filter. The alkali decomposes any trapped ammonium nitrate (NH4NO3) to
release ammonia (NH3) which is subsequently trapped on an acid-washed filter.
Sampling is done with no size-selective inlet to avoid losses of gases such as
nitric acid (HNO3). Measurements at CEH have shown that the effective