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

Measuring PM2.5 and its components
62. In the laboratory, a 1.5 cm 2 punch is taken from each filter and analysed for
elemental and organic carbon in a thermo-optical procedure in which the
metrics are method-defined. It involves heating the sample to remove the PM
from the filter, and conversion of the carbon to methane, followed by detection
by flame ionisation in a Sunset Laboratories instrument. In a helium atmosphere,
the sample is gradually heated to 870°C to remove organic carbon on the filter.
During this first phase there are usually some organic compounds that are
pyrolytically converted to elemental carbon. Measuring the transmission and
reflection of a laser beam through the filter continuously monitors this pyrolytic
conversion and allows a correction to be made for it. Elemental carbon is
detected in the same way after heating to 870°C in the presence of oxygen
and helium. The protocol used is termed Quartz, a close variant of the NIOSH
(US National Institute for Occupational Safety and Health) protocol. Other
protocols, based on different temperatures and timings, are used by other
groups in the UK, Europe and elsewhere, as described below. The optical
correction can be made using either the transmitted or reflected signal.
63. In essence the method provides a measurement of total carbon (TC), the
accumulation of all carbon atoms released from the filter during all phases of
heating, which is divided into EC and OC (TC = EC + OC) according to the
time at which the carbon was released. The laser signal is used to determine
when the time division is made, based on the assumption that when the laser
signal returns to its initial value, any pyrolytically converted material has been
removed. In practice the TC value can be calibrated accurately using standard
solutions of organic material on clean filters, but the EC/OC split can vary
significantly depending on the laser signal used (transmittance or reflectance),
as the underlying assumption is not strictly valid. The chemical significance of
the EC/OC split will also depend on the broader composition of the particulate
matter and the temperature protocols used, and is complicated further by the
consideration of carbonates, which can be significant components of the TC
that do not rightly belong in either the EC or OC categories.
64. Broadly speaking, the use of a reflectance signal for the pyrolysis correction
leads to significantly higher EC values than when a transmittance signal is used
(by up to around a factor of two), with correspondingly less OC. Protocols that
have a lower maximum temperature during the inert gas heating phase, such as
the EUSAAR 2 protocol with a maximum of 650°C, tend to record significantly
higher EC values than protocols such as NIOSH and Quartz (again by up to
around a factor of two), with correspondingly less OC (see for example Watson
et al., 2005, and Cavalli et al., 2010).
65. Comparisons of EC and OC data therefore need to take into account the
method used for their analysis. Standardisation of EC/OC methods for European
regulation is at an early stage. A Technical Report TR 16243:2011 Ambient air
quality – Guide for the measurement of elemental carbon (EC) and organic
carbon (OC) deposited on filters has been produced by CEN, but this is aimed
at reducing variations within several prescribed methods, each of which will
produce different results, rather than defining a single method.
66. The Birmingham EC/OC data used in Chapters 3 and 4 were obtained using
a protocol similar to EUSAAR II, as described in the references given in those
chapters.
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