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

PM2.5 in the UK
Figure 5.9: Comparison of total SOA concentrations from HARM/ELMO model (left) and the
NAME model (right).
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5.4.3 Semi-volatile components
35. A number of the major components of PM2.5 are semi-volatile. This term implies
that they actively partition between vapour and the condensed phase of the
aerosol and, in doing so, can significantly influence the measured mass of PM2.5.
There are at least three main mechanisms by which this partitioning occurs:
(a) Some chemical compounds have vapour pressures which are high enough
so that an appreciable proportion of their mass is present as vapour, but
low enough that not all of their mass vaporises from particles; consequently
some remains to be weighed as PM2.5. Such compounds typically adsorb to
the surface of other particles, an example being higher molecular weight
hydrocarbons present in engine exhaust.
(b) Water-soluble compounds of intermediate vapour pressure will tend to
partition into the aqueous component of the particles. Their equilibrium
with the particles is determined by Henry’s Law but may also be influenced
by chemical reactions within the aqueous phase which serve to reduce
the dissolved component concentration. Many components of secondary
organic aerosol are highly oxygenated organic compounds which partition
in this way.
(c) Some compounds such as ammonium nitrate are able to dissociate
into vapour phase components (nitric acid and ammonia in the case of
ammonium nitrate) establishing a partitioning between the vapour phase
components and the original compound within the solid or liquid particle.
Ammonium nitrate and ammonium chloride are prime examples of
compounds behaving in this way.