where SCa, the Schmidt number is equal to
Sc = va
a Da
M.R. Preston
where Va is the air viscosity and Da is the diffusivity of the substance in air. Conveniently, SCa is related to the square root of the relative molecular mass of a trace gas
giving
7.3.7
Determination of the Water Phase Transfer Velocity (kw)
The fundamental work relating to measurement of kw was performed by Liss and
Merlivat (1986). Subsequent confirmation of this work (e.g. Watson et al. 1991) using
purposeful trace studies have provided the following equations for lew:
kw= o.17u
kw = 5·9u - 49·3
Values of some gas transfer velocity values (Kw) calculated by Duce et al. (1991) are
given in Table 7.3.
Table 7.3. Calculated overall gas transfer velocities (Kw) for some chlorinated contaminants in various
oceans. (Duce et al.1991); Kw (cm h- 1 )
Atlantic
Pacific
Indian Ocean
Compound
North
South
North
South
u-HCH
0.32
0.31
0.47
0.35
0.46
r-HCH
0.17
0.16
0.25
0.18
0.24
HCB
4.1
4.0
4.8
4.3
4.8
PCB (1254)
3.3
3.2
4.0
3.5
4.0
PCB (1242)
3.5
3.4
4.2
3.6
4.2
Chlordane
2.4
2.3
3.1
2.5
3.0
Dieldrin
1.1
1.1
1.5
1.2
1.5
DDE
2.5
2.4
3.1
2.6
3.1
DDT
1.0
0.98
1.4
1.1
1.4