Geoscience Reference
In-Depth Information
coming from sea spray and particulate sulfates coming, for example,
from oxidation reactions of DMS), and indeed influence the
atmosphere's oxidizing power (halogen or bromide compounds, N
2
0,
etc.). The presence of sea ice inhibits or strongly reduces a significant
proportion of the exchanges between the ocean and atmosphere, even
if the deposits on the ice (and the frozen water in the ice as well as the
snow) eventually rejoin the ocean. Gas emissions, in particular, are
strongly reduced by the presence of sea ice.
Let us return to these different modes of ocean-atmosphere
exchange by evaluating their main roles and in some cases identifying
the processes that are responsible for these exchanges.
3.2.4.1.
Exchanges of gas
These exchanges are understood in a context very similar to that of
evaporation, but while noting that the equilibrium occurs between a
dissolved phase of the gas and a vapor phase at the liquid-air interface
(a free air-sea surface or on the walls of bubbles), or during
evaporation from spray. For each gas, there exists a linear solubility
ratio between the dissolved concentrations and partial pressure in
water (equal to that in the air when in equilibrium) of which the
coefficient depends in the first place on the temperature and salinity
(Henry's law). This solubility is weak for certain gases, such as
oxygen, but strong for others, such as carbonic gas. This partial
pressure is linked to the atmospheric pressure, possibly modified for
the behaviors of non-perfect gases, while taking account of humidity
for a saturated atmosphere at the temperature of the air-sea interface.
The specific character of gas exchanges is that the profile in water
of a dissolved gas displays a significant gradient beside the interface,
as a result of molecular diffusion. In practice it is the weakest
diffusion, the diffusion in water, that controls the state at the interface.
This enables the partial pressure in air of the gas concerned to be
estimated using measurements taken at a certain distance from the
surface, without the local exchanges having a noticeable effect on this
atmospheric composition; this is especially the case with oxygen, for
which the measurements are greatly facilitated. We also note that the
molecular layer concerned in the water has a slight thickness, a
