Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 154

C g : Bulk gas phase
concentration

ATMOSPHERE

Gas film

C sg

C sl C sg = H C sl

Liquid film

WATER

C l : Bulk liquid phase concentration

FIGURE 4.7 Two-film theory of gas-liquid transfer.

through both films; therefore, the two resistances combine in series, and the total
conductivity (transfer rate constant) is the reciprocal of the total resistances. 78 In
addition to the liquid and gas film resistance, there is actually another resistance
involved, the transport resistance between the two interfaces, which is assumed to
be negligible.
The value of the conductivity (transfer rate constant) depends on the intensity
of turbulence in the water body and in the overlying atmosphere. As the compound's
Henry's law constant increases, the conductivity tends to be increasingly influenced
by the intensity of turbulence in water. Conversely, as the Henry's law constant
decreases, the value of the conductivity tends to be increasingly influenced by the
intensity of atmospheric turbulence. Volatilization, as described by the two-film
theory, is a function of the compound's Henry's law constant, the gas film resistance,
and the liquid film resistance. As described previously, film resistances depend on
diffusion and mixing. The Henry's law constant is the ratio of a chemical's vapor
pressure to its solubility. It is also thermodynamically the ratio of the fugacity of
the chemical (escaping tendency from air and water) in the air to that in water.

H = p g / C sl (4.37)

where
H

= Henry's law constant for the air-water partitioning of the chemical

[atm m 3 mole −1 ]

p g = the partial vapor pressure of the chemical of interest in the gas phase (air)

C sl = the chemical's saturation solubility in water

Henry's law constant (H) serves as a measure of a chemical's volatility caused by
water; the larger a chemical's H value, the more volatile it is, and the more easily it will
transfer from the aqueous phase to the gas phase. 138 The H value of a compound can
be used to develop simplifying assumptions for modeling volatilization. If the resistance
of either the liquid film or the gas film control is significantly greater than the other, the
lesser resistance can be neglected. The threshold of H for gas or liquid film control is