McKay, Donald. "Front matter" Multimedia Environmental Models ...

e coated with organic matter probably of bacterial origin. They thus may be shielded
from the solute by a layer of highly sorbing organic material. It is thus a fair (and
very convenient) assumption that the sorptive capacity of clays and other mineral
surfaces can be ignored. Notable exceptions to this are subsurface environments in
which there may be extremely low organic carbon contents and when the solute
ionizes. In such cases, the inherent sorptive capacity of the mineral matter may be
controlling.
5.5.5 Aerosol-Air Partition Coefficient
One of the most difficult, and to some extent puzzling, sorption partition coefficients
is that between air and aerosol particles. These particles have very high
specific areas, i.e., area per unit volume. They also appear to be very effective
sorbents. The partition coefficient is normally measured experimentally by passing
a volume of air through a filter then measuring the concentrations before and after
filtration, and also the concentrations of the trapped particles. Relationships can then
be established between the ratio of gaseous to aerosol material and the concentration
of total suspended particulates (TSP).
There has been a profound change over the years in our appreciation of this
partitioning phenomenon. The pioneering work was done by Junge and later by
Pankow resulting in the Junge-Pankow equation, which takes the form
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f = C q/(P S L + C q)
where f is the fraction on the aerosol, q is the area of the aerosol per unit volume
of air, C is a constant, and P S L is the liquid vapor pressure. This is a Langmuir type
of equation, which implies that sorption is to a surface, and the maximum extent of
sorption is controlled by the available area.
Experimental data were better correlated by calculating K P. It can be shown that
from which
f = K P TSP/(1 + K P TSP)
K P = f/[TSP(1 – f)]
The units of TSP are usually mg/m 3 , thus it is convenient to express K P in units
of m 3 /mg. K P is usually correlated against P S L for a series of structurally similar
chemicals using the relationship
log K P = m log P S L + b
where m and b are fitted constants, and m is usually close to –1 in value. Bidleman
and Harner (2000) list 21 such correlations and present a more detailed account of
this theory.