Geoscience Reference
In-Depth Information
thickness (and gradient) that is expressed as a function of Schmidt's
number
Sc=
ν/ε
(or
ν
is the cinematic viscosity and
ε
is the molecular
diffusivity of the gas).
The law of exchange, which is written as:
[3.9]
FKA A
=−
([
]
−
[
])
a
w
where [
A
w
] and [
A
a
] are the concentrations in water (beneath the
diffusive layer) and air, has been the subject of numerous studies in
the course of the past 30 years, (see, for example, [LIS 86, WAN 92]),
which resulted in important difference on global estimations of
exchanges of gas, notably for carbonic gas. This coefficient,
K
, is
expressed as a function of Schmidt number (to an appropriate power
that depends on the state of the interface: a stable film for very weak
winds, or by renewal of the interface in a regime with intermediate
wind) and a friction speed in water
u
*w
, linked to the friction speed of
the wind at the surface. The exchanges at the level of bubbles play a
particularly important role for gases that are slightly soluble, such as
O
2
, and can explain a slight oversaturation in oxygen at oceanic
surface layers (from 1 to 2%). Laws of exchange dependant on the
variability of sea surface slopes have also been proposed, which
appear to take good account of the effects of surface waves and
surfactants. The latter are more or less hydrophobic films, leading to
tensio-active modifications of the surface, which influence the
spectrum of short waves as much as the exchanges of gas. These laws
of exchange are less used, however, because data for surface slopes
are less readily available than those for the wind.
3.2.4.1.1. Surface concentrations of dissolved gas
For most gases, in particular inert gases, concentrations in water
are always fairly close to concentrations in the atmosphere, despite
seasonal variations in the saturation solubility in the ocean (change in
temperature, as well as changes in atmospheric pressure). This results
from the rapidity of equilibration in mixed oceanic layers due to
exchanges with the atmosphere (particularly true for very volatile inert
gases, such as helium).
