Aerosol Chemistry and Physics - Aerosols: Measurement, Dosimetry, and Health Effects - page 154

6.3.2.2.4 Sorption/Desorption
While supersaturation is necessary for particle formation entirely from gas molecules, the extent of
gas ad sorption onto existing dry particles or ab sorption into liquid ilms on particles depends not only
on the extent of saturation, but also on the amount of available surface area. Sorption and desorption
inluence the size and mass of particles in both the nucleation and accumulation size modes.
Once gas-phase concentrations of low-volatility compounds (from direct primary emission or
secondary reactions) exceed their saturation vapor pressures, they will distribute between the gas
phase and available surfaces that can be described using thermodynamic concepts. The partition
coeficient K p is the most commonly used parameter for describing gas/particle partitioning at equi-
librium, primarily because of its log-linear relationship to compound vapor pressure, p o . Although a
compound's vapor pressure at the temperature of interest has the greatest inluence on partitioning,
the interaction between compound structure and the sorptive medium plays an important role (e.g.,
compound size and polarity versus ad sorptive afinity or ab sorptive capacity). Plots of log K p versus
log p o can provide information on the nature of the partitioning.
Using Pankow's nomenclature (Pankow, 1999 and Seinfeld and Pankow, 2003), the equilibrium
partitioning of an SVOC to an environmental surface S can be represented most simply by

G + S = P (6.1)

where G and P represent the gas and particulate phases of the sorbing molecule. Since the gas and
particulate phases of the molecule are usually collected on an adsorbent and ilter, respectively,
their concentrations have been conveniently represented by A and F in the literature (ibid.). If
the sorbing surface is total suspended particulate matter, its concentration can be represented by
TSP. (G/P theory takes the same form for size-segregated particles, but TSP is used here to be
consistent with Pankow's development.) At equilibrium, the gas/particle partitioning constant K p
for ads orption of the SVOC compound i onto the solid surface of a particle can be expressed as
in Equation 6.2:
Ad sorption to a solid surface

F
A TSP

i

K = (6.2)

p

i ⋅

Pankow (1987) showed that Langmuir adsorption theory predicts that K p (at constant temperature)
is inversely proportional to the vapor pressure of i. If i is a solid, the subcooled liquid vapor pressure
p o is used.

( Q Q − ) / RT

N a Te

1 v

s tsp

K = adsorption

(6.3)

p

o

1600 p

L

N s and a tsp are terms for the number of adsorption sites per unit area and the surface area of the
particles, respectively. For compounds of the same class, with similar enthalpies of desorption and
vaporization Q among the members, plots of log K p versus log p o will be linear slope of −1, as
shown in Equation 6.4.

( Q Q − ) / RT

N a Te

1 v

o s tsp

log K = − log p + log adsorption

(6.4) p L

1600

Semi-volatile compounds can also ab sorb into liquid particles such as secondhand tobacco smoke or
liquid (organic and/or water) ilms on particles with solid cores, as Pankow (1994) has shown. Gas/
particle partitioning of SVOCs in urban areas is better explained as ab sorption than ad sorption. The
ab sorptive partitioning of SVOC i into a liquid organic layer on a particle is like a gas dissolving