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

4.5.4 Aerosol dynamics
PM2.5 emissions and receptor modelling
74. A number of important constituents of PM2.5 are semi-volatile, i.e. they exist
simultaneously in the condensed and vapour phases and are able to transfer
between the two. The most important semi-volatile inorganic components
are ammonium nitrate and ammonium chloride. Both constituents will tend
to equilibrate between the condensed phase and vapours as in the reactions
below:
NH3(g) + HNO3(g) NH4NO3 (s or aq) (1)
NH3(g) + HCl(g) NH4Cl (s or aq) (2)
These equilibria are sensitive to changes in the airborne concentrations of
ammonia, nitric acid and hydrochloric acid vapours. In the vicinity of a source
of ammonia, the chemical reactions can operate quite rapidly to re-establish
equilibrium in the event of a change in the atmospheric concentration of any of
the species. The implication is that nitrate concentrations may vary on a spatial
scale of hundreds of metres if local ammonia concentrations vary due to source
characteristics on the same spatial scale. However, a study in central England
(Marner and Harrison, 2004) reported far less local variability in nitrate than in
ammonia.
75. The partition between particles and vapour depends primarily upon the
atmospheric temperature and relative humidity, which determine whether
the particles are solid or present as deliquescent aqueous solutions. Generally,
lower temperature and higher humidities favour incorporation into particles,
whereas high temperatures and low humidities favour the vapours. A further
complication is that the particle phase is typically made up of a mixture of
substances which affects the thermodynamic relationship between the two
phases. However, the thermodynamic properties of the condensed phase are
reasonably well understood and programmes such as ISORROPIA II (Fountoukis
and Nenes, 2007) are available to predict partitioning between the phases.
76. The other main semi-volatile component of PM2.5 is organic matter, particularly
secondary organic compounds. The partitioning of these compounds between
the phases is far less well understood than for the inorganic components. It
was for a long time assumed that the partitioning was based upon vapour
equilibrating with a solution of the compound in organic liquids of low volatility
present in the particles and that the partitioning could be described by the
octanol–air partition coefficient. This concept was used for many years for
polycyclic aromatic hydrocarbons (PAHs) but it was subsequently recognised
that, in addition to the absorption process, polycyclic aromatic hydrocarbons are
also likely to be subject to adsorption onto substrates such as elemental carbon.
It was found that by including adsorption processes, numerical models could
better describe the partition of PAHs between vapour and particles (Dachs and
Eisenreich, 2000). Robinson et al. (2007) have argued that as vehicle-emitted
particles advect away from their source, the adsorbed organic compounds
vaporise and are oxidised to less volatile compounds which condense into
secondary organic aerosol (SOA).
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