Geoscience Reference
In-Depth Information
place through eddy diffusion, and the corresponding vapor flux can be described by
Eq.
7.2
, where K
z
is then the eddy diffusion coefficient (Taylor and Spencer
1990
).
The height of the turbulent zone, within the atmospheric boundary layer, is orders
of magnitude greater than that of the laminar flow layer, and dispersion of con-
taminant vapors in the turbulent zone is relatively rapid.
The concept of separate regions of dilution by molecular diffusion and turbulent
mixing is of major importance in understanding the exchange of gases at land
surface and for the identification of physical factors that impede the dispersion
process under various microclimates.
7.1 Gas-Liquid Relationships
Gas-liquid relationships, in the geochemical sense, should be considered liquid-
solid-gas interactions in the subsurface. The subsurface gas phase is composed of
a mixture of gases with various properties, usually found in the free pore spaces of
the solid phase. Processes involved in the gas-liquid and gas-solid interface
interactions are controlled by factors such as vapor pressure-volatilization,
adsorption, solubility, pressure, and temperature. The solubility of a ''pure'' gas in
a closed system containing water reaches an equilibrium concentration at a con-
stant pressure and temperature. A gas-liquid equilibrium may be described by a
partition coefficient, relative volatilization, and Henry's law.
The partition or distribution coefficient between a gas and a liquid is constant at
a given temperature and pressure. The relative volatility is used in defining the
equilibrium between a volatile liquid mixture and the atmosphere. The partition
coefficient expresses the relative volatility of a species A distributed between a
vapor phase (A1) and a liquid phase (A2). Henry's law applies to the distribution of
dilute solutions of chemicals in a gas, liquid, or solid at a specific ambient con-
dition. Equilibrium is defined by
P
A1
¼ H
A
x
A
;
ð
7
:
3
Þ
where P
A1
is the partial pressure of the chemical A1 in the gas phase, H
A
is Henry's
law constant for species A, and x
A
is the mole fraction of chemical A in solution.
Under these conditions, H
A
has dimensions of pressure.
The Henry's law constant, as a function of the activity coefficient of A2in
water, c
A2
,is
H
A
¼
c
A2
P
A1
P
ð
7
:
4
Þ
where P and P
A
0
denote the total pressure and partial pressure of A1, respectively.
Henry's law constants and relative volatility values for the most common gases, as
well as their solubility in pure water, can be found in any chemical handbook.
