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

PM2.5 in the UK
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4.6.1 Receptor modelling of particulate matter in the UK
81. Early approaches to source apportionment of particles in the UK atmosphere
(e.g. Clarke et al., 1984) used major component chemical composition to
“reconstruct” the measured mass of particles, but this left a significant fraction
of mass unassigned. This approach was developed further by Harrison et al.
(2003) based upon the simplified premise that the mass of airborne particles
could be accounted for by the following chemical components/sources:
• ammonium sulphate derived from the oxidation of sulphur dioxide and
neutralisation by ammonia. Earlier work had shown very low levels of
acid sulphate in the UK and consequently it is a fair assumption that the
sulphate was wholly neutralised as ammonium sulphate;
• ammonium nitrate derived from the neutralisation by ammonia of nitric
acid, itself formed from the oxidation of nitrogen dioxide. The compound
is formed by gas-to-particle conversion and exists mainly in the fine particle
fraction;
• sodium nitrate derived mainly from the reaction of sea salt with nitric acid
vapour leading to formation of nitrate in the sea salt particles which are
predominantly in the coarse size range;
• sodium chloride derived from sea salt;
• soil minerals represented by gypsum (CaSO4.2H2O);
• road dust, mainly coarse particles, generated by traffic for which iron is a
valuable tracer;
• elemental carbon derived from combustion sources. In UK cities the vast
majority arises from diesel vehicle emissions;
• organic compounds, both primary and secondary in nature. The primary
component is derived from a wide variety of primary sources, and the
secondary component from many different VOC precursors; and
• bound water. Under the conditions of weighing the air filters (specified
by EN12341 as 45-55% relative humidity and 20 ± 2ºC), the hygroscopic
particles retain a significant amount of strongly bound water which is not
taken account of directly in the chemical analysis.
82. Airborne particles are sampled onto a filter which is weighed before and
after sampling to determine the mass of particles. It is subsequently analysed
for sulphate, nitrate, chloride, calcium, iron, elemental carbon and organic
carbon, whose masses are converted to chemical compounds or source-related
constituents using the numerical factors in Table 4.5. Some of the numerical
factors (for example, the one that converts sulphate to ammonium sulphate) are
determined directly from molecular weights, whilst others, such as the factor
converting iron to a mass of road dust, are based upon regression analyses, the
aim being ultimately to account through the model for entire “mass closure”
such that the reconstructed particle mass equals the gravimetrically-determined
mass. By applying this method to particles from adjacent roadside and urban