Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 138

Atmospheric pollutants

Gaseous Particle-bound

Direct deposition
to surfaces

Washout with
hydrometeors

Direct particle
deposition to surfaces
J dry(P) J dry(G) J wet(G+P)

FIGURE 4.5 Mechanisms of deposition of pollutants from the atmosphere.

air. This is called the washout ratio , W g , from the gas phase. It is easy to recognize
that for equilibrium scavenging W g = 1 /K aw .

For closed systems the total concentration of a species is fixed. If C i T represents
the total concentration of species i per unit volume of air, it is distributed between the
air and water content

θ L (g/m 3 ) , and hence

C i T = C i a + θ L · C i w . (4.37)

Applying Henry's law to the aqueous concentration, C i w = C i a /K aw , we obtain the
following equation for the fraction of species i in the aqueous phase:

θ L
K aw + θ L

φ i w = . (4.38)

For compounds with low Henry's constant, K aw , a significant fraction of species i will
exist in the aqueous phase. With increasing θ L , a greater amount of water becomes

availabletosolubilizespecies i andhencethevalueof

w

φ

i increases.Theconcentration

of species i in the aqueous phase is then given by

C i T
θ L +

C i w = . (4.39)

K aw

4.2.2 W ET D EPOSITION OF A EROSOL -B OUND F RACTION
Compounds that exist in the gaseous form in air are in equilibrium with those that are
loosely bound to atmospheric aerosols; this is called the exchangeable fraction. The
remaining fraction is incorporated within the particle matrix and is tightly bound or
nonexchangeable and does not participate in the equilibrium with the gas phase. The
extent to which the exchangeable molecules are associated with the aerosol is directly
related to the partial pressure of the compound. Junge (1977) provided an equation
relating the adsorbed fraction (

φ i P ) to the total surface area of the aerosol ( S T ) and
the vapor pressure of the compound ( P i ) .