30
R. Ebinghaus et al.
5.3
Reemissions from Marine Systems
The distribution, transport, and fate of mercury at the Earth's surface is critically
dependent on the biogeochemical cycling and atmospheric exchange of mercury
in the marine environment since, by its shear size, the ocean is both a significant
source and sink for atmospheric mercury (Mason et al. 1994a; Mason and
Fitzgerald 1996). A number of studies have measured volatile Hg species well
above saturation levels in ocean waters primarily as DGM. Hence, the emission of
mercury from the oceans to the atmosphere is certainly occurring. However,
since deposition is also occurring at the same time, a major question is whether
or not emissions exceed deposition. As even the direction of the net Hg flux
between oceans and atmosphere has not been determined conclusively, it is only
natural that the size of that flux is even more doubtful. As a matter of fact, to date
no field experiments have been attempted to actually measure air-surface
exchange of mercury over oceanic waters, despite the existence of proven
methods for smaller water bodies. All we know is that elemental Hg seems to
occur commonly in surficial ocean waters in supersaturated concentrations.
From this observation, it is frequently derived that there is a net mercury
emission (Vandal et al. 1991; Fitzgerald et al. 1991, 1994), but we have to keep in
mind that this assumption is based on physical chemical theories whose
applicabilities to the water-air system in oceans have not been proven for
mercury beyond doubt. All available flux data were obtained by this model (see
sect. 5.2) and must therefore be seen as rough estimates rather than exact values.
Estimated net Hg emissions range from average hourly fluxes of 0.4 to
16 ng m -2 h -I. The concentrations of elemental mercury and its modeled
evasional flux to the atmosphere in various oceanic regions are given in
Table 16. From these data, the global mercury emission from oceanic sources has
been estimated at 2000 t year -1 (Mason et al. 1994a). Another important factor is
that all these flux calculations are based on calm sea and moderate wind
conditions (under which also all of the sampling campaigns were probably
performed). It has been modeled, though, that fluxes may increase by a factor of
up to 25 during storms and rough sea conditions (Baeyens et al. 1991), so this
estimate may be systematically low. However, such fluxes would be very shortTable 16. Concentrations of elemental mercury measured in various ocean regions and the estimated
associated evasional flux to the atmosphere. (After Mason and Fitzgerald 1996)
Region
EPO· 1990
EPO 1984
150° W, 10° N-12° S
N Pacific
N Atlantic
Mediteranean
• Equatorial Pacific Ocean.
Conc. range (pgll)
10-72
6-46
8-18
6
50-250
<26
% Hg(O)/react. Hg
14 ± IO
5 ± 3
15-100
<20
Potential flux
(ngm 2 h l )
1.3-9
0.5-9.2
0.8-2.8
0.2
15.2 ± lOA
R. Ebinghaus et al.
5.3
Reemissions from Marine Systems
The distribution, transport, and fate of mercury at the Earth's surface is critically
dependent on the biogeochemical cycling and atmospheric exchange of mercury
in the marine environment since, by its shear size, the ocean is both a significant
source and sink for atmospheric mercury (Mason et al. 1994a; Mason and
Fitzgerald 1996). A number of studies have measured volatile Hg species well
above saturation levels in ocean waters primarily as DGM. Hence, the emission of
mercury from the oceans to the atmosphere is certainly occurring. However,
since deposition is also occurring at the same time, a major question is whether
or not emissions exceed deposition. As even the direction of the net Hg flux
between oceans and atmosphere has not been determined conclusively, it is only
natural that the size of that flux is even more doubtful. As a matter of fact, to date
no field experiments have been attempted to actually measure air-surface
exchange of mercury over oceanic waters, despite the existence of proven
methods for smaller water bodies. All we know is that elemental Hg seems to
occur commonly in surficial ocean waters in supersaturated concentrations.
From this observation, it is frequently derived that there is a net mercury
emission (Vandal et al. 1991; Fitzgerald et al. 1991, 1994), but we have to keep in
mind that this assumption is based on physical chemical theories whose
applicabilities to the water-air system in oceans have not been proven for
mercury beyond doubt. All available flux data were obtained by this model (see
sect. 5.2) and must therefore be seen as rough estimates rather than exact values.
Estimated net Hg emissions range from average hourly fluxes of 0.4 to
16 ng m -2 h -I. The concentrations of elemental mercury and its modeled
evasional flux to the atmosphere in various oceanic regions are given in
Table 16. From these data, the global mercury emission from oceanic sources has
been estimated at 2000 t year -1 (Mason et al. 1994a). Another important factor is
that all these flux calculations are based on calm sea and moderate wind
conditions (under which also all of the sampling campaigns were probably
performed). It has been modeled, though, that fluxes may increase by a factor of
up to 25 during storms and rough sea conditions (Baeyens et al. 1991), so this
estimate may be systematically low. However, such fluxes would be very shortTable 16. Concentrations of elemental mercury measured in various ocean regions and the estimated
associated evasional flux to the atmosphere. (After Mason and Fitzgerald 1996)
Region
EPO· 1990
EPO 1984
150° W, 10° N-12° S
N Pacific
N Atlantic
Mediteranean
• Equatorial Pacific Ocean.
Conc. range (pgll)
10-72
6-46
8-18
6
50-250
<26
% Hg(O)/react. Hg
14 ± IO
5 ± 3
15-100
<20
Potential flux
(ngm 2 h l )
1.3-9
0.5-9.2
0.8-2.8
0.2
15.2 ± lOA