234
Chemical Oceanography, 4th Edition
where f i D i / τ is called the exit coefficient or the transfer velocity. The laminar model
equates f i to D i / τ. If one assumes that the rate- determining factor is the time θ, a volume
element is at the air–sea interface:
f i 2(D i / πθ) 1/2
(6.23)
The existence of a laminar layer at the air–sea interface has been supported by observations.
Fick’s first law can also be expressed as
F = dC/ dt = kΔC
(6.24)
where F is the flux (mol cm –2 s –1 ), ΔC is the change in concentration across the air–sea
interface (mol cm –3 ), and k is called the transfer velocity (cm s –1 ), permeability coefficient,
mass transfer coefficient, absorption coefficient, exit coefficient, or piston velocity. If the
thin- film layer model is used, k is proportional to D/ τ, where D is the diffusion coefficient
and τ is the boundary thickness. If the surface renewal model is used, k is proportional
to D 1/2 , and if a boundary layer model is used, k is proportional to D 2/3 . The most popular
model presently used in oceanography to describe the flux across the air–sea interface is
the model of Liss (1975). This model is a stagnant film model with a two- layer boundary
between the interfaces (see Figure 6.3).
As with the thin- film model, the air above and the liquid below the gas and liquid films
are assumed to be well mixed. Gas transfer occurs by molecular diffusion, and the resistance
to gas transport is diffusion across the gas–liquid interfacial layer. The value of k depends
on the turbulence in the gas and liquid phases and the chemical reactivity of the gas. The
reciprocal of the exchange coefficient is a measure of the resistance to transfer across the
interface. This resistance is divided into the resistance of both the air and water phases:
1/k = R = R g = R l
(6.25)
Wind Velocity (ms
–1 )
4
6
8
10
12
14
τ (microns)
10
20
30
40
50
60
70
80
90
Wind tunnel
Natural
14 C
Radon
Trade Winds
15°N
GEOSECS
Global Mean
Summer
Antarctic
Winter
50°N
Figure 6.2
The thickness of the laminar layer as a function of the wind speed.
Chemical Oceanography, 4th Edition
where f i D i / τ is called the exit coefficient or the transfer velocity. The laminar model
equates f i to D i / τ. If one assumes that the rate- determining factor is the time θ, a volume
element is at the air–sea interface:
f i 2(D i / πθ) 1/2
(6.23)
The existence of a laminar layer at the air–sea interface has been supported by observations.
Fick’s first law can also be expressed as
F = dC/ dt = kΔC
(6.24)
where F is the flux (mol cm –2 s –1 ), ΔC is the change in concentration across the air–sea
interface (mol cm –3 ), and k is called the transfer velocity (cm s –1 ), permeability coefficient,
mass transfer coefficient, absorption coefficient, exit coefficient, or piston velocity. If the
thin- film layer model is used, k is proportional to D/ τ, where D is the diffusion coefficient
and τ is the boundary thickness. If the surface renewal model is used, k is proportional
to D 1/2 , and if a boundary layer model is used, k is proportional to D 2/3 . The most popular
model presently used in oceanography to describe the flux across the air–sea interface is
the model of Liss (1975). This model is a stagnant film model with a two- layer boundary
between the interfaces (see Figure 6.3).
As with the thin- film model, the air above and the liquid below the gas and liquid films
are assumed to be well mixed. Gas transfer occurs by molecular diffusion, and the resistance
to gas transport is diffusion across the gas–liquid interfacial layer. The value of k depends
on the turbulence in the gas and liquid phases and the chemical reactivity of the gas. The
reciprocal of the exchange coefficient is a measure of the resistance to transfer across the
interface. This resistance is divided into the resistance of both the air and water phases:
1/k = R = R g = R l
(6.25)
Wind Velocity (ms
–1 )
4
6
8
10
12
14
τ (microns)
10
20
30
40
50
60
70
80
90
Wind tunnel
Natural
14 C
Radon
Trade Winds
15°N
GEOSECS
Global Mean
Summer
Antarctic
Winter
50°N
Figure 6.2
The thickness of the laminar layer as a function of the wind speed.