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

PM2.5 emissions and receptor modelling
86. In order to obtain greater insight into the sources responsible for organic
compounds in the atmosphere, Yin et al. (2010) carried out a chemical mass
balance (CMB) modelling study of the organic matter combined with major
component chemical composition information to allow a more sophisticated
reconstruction of organic matter mass. In order to employ a CMB model, they
analysed a large number of organic molecular marker compounds within the
particulate matter, including 13 n-alkanes, nine hopanes, 13 PAHs, 14 organic
acids and two sterols. These were in addition to major chemical components
and a range of trace element species (Harrison and Yin, 2010). This allowed
disaggregation of the organic carbon (Yin et al., 2010) into the following source
components:
• vegetative detritus, i.e. fragments from leaves and other components of
plants
• wood smoke
• particles from natural gas combustion
• particles from coal combustion
• suspended dust and soil
• particles from diesel engines
• particles from gasoline engines
• particles from smoking engines.
87. The chemical profiles of the sources used in the CMB model are derived from
North American studies. Smoking engines are made up of poorly-tuned diesel
engines (black smokers) and older petrol engines burning significant oil (white
smokers). These are thought to parallel most closely engine emissions from offroad
machinery such as tractors, diggers, etc., and two-stroke engines in the UK.
88. The CMB method is able to apportion those components for which source
chemical profiles are input. It is not possible to approach secondary organic
carbon directly as there is no unique profile, but the method assigns mass to
“other organic carbon”, i.e. from sources for which profiles have not been
input. In the case of the samples from urban background and rural sites in
the West Midlands, the “other organic carbon” represented a substantial
proportion of the total carbonaceous mass and was found to correlate highly
(R 2 = 0.92 for the rural site) with secondary organic carbon estimated using
the elemental carbon tracer method (Yin et al., 2010). Once again, in this
dataset the secondary organic carbon showed a close relationship to nitrate
concentrations.
89. Measurements were also made of other major ionic components of the particles
allowing estimation of concentrations of ammonium sulphate, ammonium
nitrate and sea salt. The results for an urban background site in Birmingham
(EROS) and a rural site (CPSS) located approximately 20 km west of Birmingham
appear in Figure 4.10.
111