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

PM2.5 emissions and receptor modelling
brake wear) and nickel and vanadium (fuel oil combustion). Considerable care is
required in their use as other sources may contribute in some localities.
69. One of the few sources which is typically quantified from a single component
is biomass burning, which in the UK would typically refer to wood burning and
bonfires, although occasionally woodland and forest fires would also contribute.
The carbohydrate compound levoglucosan is typically used as a single marker
of biomass burning as this is by far its major atmospheric source. Consequently,
there is little risk of contributions from other sources but there remains the
problem of converting the mass of levoglucosan into a mass of wood smoke
particles. While many measurements exist of the ratio of wood smoke particles
to levoglucosan mass, the ratio is highly variable depending on combustion
conditions. Consequently, when using levoglucosan as an atmospheric tracer,
there are large uncertainties in the subsequent conversion to a wood smoke
mass. Other tracers of wood smoke include fine particle potassium (after
correction for a contribution from wind-blown soil and sea salt), but a similar
problem remains, namely that the wood smoke to fine potassium mass ratio is
highly dependent upon combustion conditions and there is no unique factor
for the conversion as it relates to the atmosphere. It is also possible to use an
aethalometer to estimate wood smoke mass but the method (Sandradewi
et al., 2008) was developed in a Swiss valley where there are only two sources
of carbonaceous particles, road traffic and wood smoke. In situations such as
the UK where there may well be other sources of carbonaceous particles, the
two component model on which the calculation is based is unreliable; as yet
there is no agreed way of using the aethalometer to calculate wood smoke
mass in the UK. Perhaps the most reliable way of estimating wood smoke
mass is from the analysis of radiocarbon ( 14 C). Radiocarbon is associated with
contemporary sources of carbon and not with fossil sources. Consequently,
if contemporary elemental carbon is found in the atmosphere, it most likely
arises from the combustion of biomass. Consequently, it is a fairly reliable tracer
of wood smoke but there again remains a question over conversion of the
elemental carbon mass to the mass of wood smoke particles, once again this is
heavily dependent upon combustion conditions.
70. One of the other problem areas in source attribution relates to cooking aerosol.
Early work from the United States used cholesterol as a marker of meat cooking
and this was used by Yin et al. (2010) in their UK study, but the airborne
concentrations were extremely low and no mass concentration was assigned to
particles from meat cooking. However, Allan et al. (2010) used a variant on the
multivariate statistical receptor modelling techniques to identify a contribution
from cooking particles in the atmosphere of London. They applied PMF to
mass spectral information obtained from non-refractory atmospheric particles
using an aerosol mass spectrometer (AMS). The AMS volatilises particles before
measuring their mass spectrum, and PMF is able to decompose the overall mass
spectrum into the individual mass spectra of specific contributory particle types.
Allan et al. (2010) found a particle type whose mass spectrum did not fit that of
the conventional sources (road traffic and coal burning) but was similar to that
of particles generated from hot corn oil. As a result, they assigned 34% of the
primary organic particles in their sample to cooking, but this finding has yet to
be replicated by other techniques.
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