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

PM2.5 in the UK
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77. Early efforts towards understanding secondary organic aerosol formation were
based on the assumption that each SOA precursor formed a SOA surrogate
species by atmospheric oxidation. When the concentrations of the SOA
surrogates reached saturation in the gas phase, then any additional material
formed was transferred to the particulate phase and equilibrium was restored
(Pandis et al., 1992). Odum et al. (1996) built upon the absorptive partitioning
approach of Pankow (1994) by assuming that each SOA precursor formed
several SOA surrogates, typically two products. A major improvement to this
approach, referred to as the Volatility Basis Set (VBS), was proposed by Donahue
et al. (2006). This assigns compounds to volatility categories which partition into
the particles according to empirically-derived factors. These approaches based
on absorptive partitioning are the basis for the methods of SOA modelling
that have been implemented in the CMAQ and CAMx models in the USA.
When Utembe et al. (2005) tried to use absorptive partitioning to describe the
incorporation of secondary organic matter into airborne particles, they found
that they needed to increase the partition coefficients by a factor of several
hundred in order to simulate the measured mass of organic matter. This has
led to a recognition that most secondary organic compounds are appreciably
oxidised and are therefore rather polar molecules which partition far more
effectively into aqueous droplets than organic liquids. This is leading to the
development of models of the partitioning into the aqueous phase of particles,
but even that is likely to prove an inadequate descriptor of the partition process
as there is now accumulating evidence for chemical reactions of secondary
organic compounds within the aqueous phase, hence displacing the equilibrium
further in favour of the condensed (particle) phase.
78. Another important process affecting semi-volatile materials is connected with
the fact that the vapour pressure above a highly curved surface exceeds that
above a less curved surface (the so-called Kelvin effect). The implication is that
semi-volatile materials have a tendency to evaporate from smaller particles
and condense into larger particles, hence affecting the PM size distribution.
However, this process is not generally sufficiently rapid or important enough
to influence the distribution of material into the coarse particle fraction. One
process, however, which is important in shifting material from the fine to
the coarse particle mode involves chemical reaction. In particular, nitric acid
vapour, either arising directly from the oxidation of nitrogen dioxide or from
the dissociation of ammonium nitrate (reaction (1) above), can react with
the surface of coarse particles, as exemplified in equations (3) and (4) below
for sodium chloride and calcium carbonate respectively. As a result, one acid
displaces another and nitrate is incorporated into coarse particles with the
displacement of hydrogen chloride or carbon dioxide respectively into the gas
phase.
NaCl + HNO3 → NaNO3 + HCl (3)
CaCO3 + 2HNO3 → Ca(NO3)2 + H2O + CO2 (4)