Geology Reference
In-Depth Information
7.2.2 Air-Sea Exchange
The oceans cover some two-thirds of the Earth's surface and conse-
quently provide a massive area for exchange of energy (climatologically
important) and matter (an important component of geochemical cycles).
The seas are a source of aerosol (i.e. small particles), which transfer to
the atmosphere. These will subsequently deposit, possibly after chemical
modification, either back in the sea (the major part) or on land (the
minor part). Marine aerosol comprises largely unfractionated seawater,
but may also contain some abnormally enriched components. One
example of abnormal enrichment occurs on the eastern coast of the
Irish Sea. Liquid effluents from the Sellafield nuclear fuel reprocessing
plant in west Cumbria are discharged into the Irish Sea by pipeline. At
one time, permitted discharges were appreciable and as a result radio-
isotopes such as
137
Cs and several isotopes of plutonium have accumu-
lated in the waters and sediments of the Irish Sea. A small fraction of
these radioisotopes were carried back inland in marine aerosol and
deposited predominantly in the coastal zone.
9
While the abundance of
137
Cs in marine aerosol was reflective only of its abundance in seawater
(an enrichment factor - see Chapter 4 - of close to unity), plutonium was
abnormally enriched due to selective incorporation of small suspended
sediment particles in the aerosol. This has manifested itself in enrich-
ment of plutonium in soils on the west Cumbrian coast,
10
shown as
Pu deposition (pCi cm
2
) to soil in Figure 3.
The seas may also act as a receptor for depositing aerosol. Deposition
velocities of particles to the sea are a function of particle size, density,
and shape, as well as the state of the sea. Experimental determination of
aerosol deposition velocities to the sea is almost impossible and has to
rely upon data derived from wind tunnel studies and theoretical models.
The results from two such models appear in Figure 4, in which particle
size is expressed as aerodynamic diameter, or the diameter of an aero-
dynamically equivalent sphere of unit specific gravity.
11,12
2391240
contours of
If the air-
borne concentration in size fraction of diameter d
i
is c
i
, then
Total Flux
¼
X
i
n
d
ð
d
i
Þ
c
i
where v
d
(d
i
) is the mean value of deposition velocity appropriate to the
size fraction d
i
. Measurements show that while most of the lead, for
example, is associated with small, sub-micrometre particles, the larger
particles compose the major part of the flux.
Airborne concentrations of particulate pollutants are not uniform
over the sea. The spatial distribution of zinc over the North Sea
13
