AIR–SEA TRANSFER: DIMETHYL SULFIDE, COS,
CS 2 , NH 4 , NON-METHANE HYDROCARBONS,
ORGANO-HALOGENS
J. W. Dacey, Woods Hole Oceanographic Institution,
Woods Hole, MA, USA
H. J. Zemmelink, University of Groningen, Haren,
The Netherlands
Copyright & 2001 Elsevier Ltd.
The oceans, which cover 70% of Earth’s surface to an
average depth of 4000 m, have an immense impact on
the atmosphere’s dynamics. Exchanges of heat and
momentum, water and gases across the sea surface
play major roles in global climate and biogeochemical
cycling. The ocean can be thought of as a vast
biological soup with myriad processes influencing
the concentrations of gases dissolved in the surface
waters. The quantities of mass flux across the surface
interface, though perhaps small on a unit area basis,
can be very important because of the extent of the
ocean surface and the properties of the gases or their
decomposition products in the atmosphere.
Gas exchange across the sea–air surface depends, in
part, on differences in partial pressures of the gases
between the ocean surface and the atmosphere. The
partial pressure of a gas in the gas phase can be
understood in terms of its contribution to the pressure
in the gas mixture. So the partial pressure of O 2 , for
example, at 0.21 atm means that at 1 atmosphere
total pressure, O 2 is present as 21% of the gas, or
mixing, volume. Trace gases are present in the atmosphere at much lower levels, usually expressed as
parts per million (10
À6 atm), parts per billion (10
À9
atm) or parts per trillion (10
À12 atm, pptv). Dimethylsulfide (DMS), when present at 100 pptv, accounts for about 100 molecules per 10
12 molecules of
mixed gas phase, or about 10
À10 of the gas volume.
In solution, a dissolved trace gas in equilibrium
with the atmosphere would have the same partial
pressure as the gas in the air. Its absolute concentration in terms of molecules or mass per unit volume
of water depends on its solubility. Gas solubility
varies over many orders of magnitude depending on
the affinity of water for the gas molecules and the
volatility of the gas. Gases range widely in their
solubility in sea water, from the permanent gases like
nitrogen (N 2 ), oxygen (O 2 ), nitrous oxide (N 2 O) and
methane (CH 4 ) that have a low solubility in sea
water to the moderately soluble carbon dioxide
(CO 2 ) and dimethylsulfide (CH 3 ) 2 S, to highly soluble
ammonia (NH 3 and its ionized form NH 4
þ ) and
sulfur dioxide (SO 2 ). Sulfur dioxide is more than 10
6
times more soluble than O 2 or CH 4 . Using the example above of an atmospheric DMS concentration
of 100 pptv, the equilibrium concentration of DMS
in surface water would be about 0.07 nmol l
À1
.
Generally the solubility of any individual gas increases at cooler water temperatures, and solubility
of gases in sea water is somewhat less than for fresh
water because of the so-called ‘salting out’ effect of
dissolved species in sea water.
At any moment the partial pressure difference
between surface water and the atmosphere depends
on an array of variables. The gases in this article are
biogenic, meaning that their mode of formation is
the result of one or more immediate or proximate
biological processes. These dissolved gases may also
be consumed biologically, or removed by chemical
processes in sea water, or they may flux across the sea
surface to the atmosphere. The rates at which the
source and sink processes occur determines the
concentration of the dissolved gas in solution as well
as the turnover, or residence time, of each compound. Similarly, there can be several source and sink
processes for the gases in the atmosphere. Long-lived
compounds in the atmosphere will tend to integrate
more global processes, whereas short-lived compounds are concentrated near their source and reflect
relatively short-term influences of source and sink. In
this sense, carbonyl sulfide is a global gas. At it has a
residence time of several years in the atmosphere, its
concentration does not vary in the troposphere to
any appreciable degree. On the other hand, the
concentration of DMS varies on a diel basis and with
elevation, with higher concentrations at night when
atmospheric oxidants (most notable hydroxyl) are
relatively depleted.
The extent of disequilibrium between the partial
pressures of a gas in the surface water and in the
atmosphere determines the thermodynamic gradient
which drives gas flux. The kinetics of flux ultimately
depend on molecular diffusion and larger-scale
mixing processes. Molecular diffusivity is generally
captured in a dimensionless parameter, the Schmidt
number (ratio of viscosity of water to molecular
583
CS 2 , NH 4 , NON-METHANE HYDROCARBONS,
ORGANO-HALOGENS
J. W. Dacey, Woods Hole Oceanographic Institution,
Woods Hole, MA, USA
H. J. Zemmelink, University of Groningen, Haren,
The Netherlands
Copyright & 2001 Elsevier Ltd.
The oceans, which cover 70% of Earth’s surface to an
average depth of 4000 m, have an immense impact on
the atmosphere’s dynamics. Exchanges of heat and
momentum, water and gases across the sea surface
play major roles in global climate and biogeochemical
cycling. The ocean can be thought of as a vast
biological soup with myriad processes influencing
the concentrations of gases dissolved in the surface
waters. The quantities of mass flux across the surface
interface, though perhaps small on a unit area basis,
can be very important because of the extent of the
ocean surface and the properties of the gases or their
decomposition products in the atmosphere.
Gas exchange across the sea–air surface depends, in
part, on differences in partial pressures of the gases
between the ocean surface and the atmosphere. The
partial pressure of a gas in the gas phase can be
understood in terms of its contribution to the pressure
in the gas mixture. So the partial pressure of O 2 , for
example, at 0.21 atm means that at 1 atmosphere
total pressure, O 2 is present as 21% of the gas, or
mixing, volume. Trace gases are present in the atmosphere at much lower levels, usually expressed as
parts per million (10
À6 atm), parts per billion (10
À9
atm) or parts per trillion (10
À12 atm, pptv). Dimethylsulfide (DMS), when present at 100 pptv, accounts for about 100 molecules per 10
12 molecules of
mixed gas phase, or about 10
À10 of the gas volume.
In solution, a dissolved trace gas in equilibrium
with the atmosphere would have the same partial
pressure as the gas in the air. Its absolute concentration in terms of molecules or mass per unit volume
of water depends on its solubility. Gas solubility
varies over many orders of magnitude depending on
the affinity of water for the gas molecules and the
volatility of the gas. Gases range widely in their
solubility in sea water, from the permanent gases like
nitrogen (N 2 ), oxygen (O 2 ), nitrous oxide (N 2 O) and
methane (CH 4 ) that have a low solubility in sea
water to the moderately soluble carbon dioxide
(CO 2 ) and dimethylsulfide (CH 3 ) 2 S, to highly soluble
ammonia (NH 3 and its ionized form NH 4
þ ) and
sulfur dioxide (SO 2 ). Sulfur dioxide is more than 10
6
times more soluble than O 2 or CH 4 . Using the example above of an atmospheric DMS concentration
of 100 pptv, the equilibrium concentration of DMS
in surface water would be about 0.07 nmol l
À1
.
Generally the solubility of any individual gas increases at cooler water temperatures, and solubility
of gases in sea water is somewhat less than for fresh
water because of the so-called ‘salting out’ effect of
dissolved species in sea water.
At any moment the partial pressure difference
between surface water and the atmosphere depends
on an array of variables. The gases in this article are
biogenic, meaning that their mode of formation is
the result of one or more immediate or proximate
biological processes. These dissolved gases may also
be consumed biologically, or removed by chemical
processes in sea water, or they may flux across the sea
surface to the atmosphere. The rates at which the
source and sink processes occur determines the
concentration of the dissolved gas in solution as well
as the turnover, or residence time, of each compound. Similarly, there can be several source and sink
processes for the gases in the atmosphere. Long-lived
compounds in the atmosphere will tend to integrate
more global processes, whereas short-lived compounds are concentrated near their source and reflect
relatively short-term influences of source and sink. In
this sense, carbonyl sulfide is a global gas. At it has a
residence time of several years in the atmosphere, its
concentration does not vary in the troposphere to
any appreciable degree. On the other hand, the
concentration of DMS varies on a diel basis and with
elevation, with higher concentrations at night when
atmospheric oxidants (most notable hydroxyl) are
relatively depleted.
The extent of disequilibrium between the partial
pressures of a gas in the surface water and in the
atmosphere determines the thermodynamic gradient
which drives gas flux. The kinetics of flux ultimately
depend on molecular diffusion and larger-scale
mixing processes. Molecular diffusivity is generally
captured in a dimensionless parameter, the Schmidt
number (ratio of viscosity of water to molecular
583