412 BOUNDARIES OF THE SEA 



Transport Mechanisms at the Sea Surface 



Matter is transported to and from the air-sea interface by two 

 processes. One is statistical in its nature and is caused by landom 

 motion of molecules, as in molecular diffusion, or by random motion 

 of parcels of air or water, as in turbulent difTusion. The other type 

 is due to certain forces acting upon aggregates of molecules, as in 

 the fall of particulate matter, or formation of strong jets of water 

 from bursting bubbles. 



In the vicinity of the air-sea interface one can formulate the 

 vertical flux (i.e., flow rate) of any substance by 



F^ -K^^+ Vp (1) 



dz 



where F is the flux per unit surface area, A' a difi^usion coefficient, 

 molecular or turbulent, p the density of the substance, z the vertical 

 coordinate, and V the vertical velocity of the substance due to 

 gravity in the case of falling particulate matter or to the energy 

 released in bursting bubbles in the case of jets of water. 



The application of equation (1) in the immediate neighborhood 

 of the air-sea interface requires some consideration. Under normal 

 wind conditions, the turbulent region in the atmosphere is believed 

 to terminate close to the interface where, at least in the air, a lami- 

 nar boundary layer is generally supposed to exist. With current 

 theories for computing it (e.g., Sverdrup, 1951), the thickness of 

 this layer is of the order of 1 mm. Of course, nothing is known of 

 conditions in the liquid phase, but it is not impossible that, on an 

 average, such a laminar boundary layer exists there also. In this 

 laminar boundary layer only molecular phenomena occur, so that 

 the statistical part of the transport takes place only through molecu- 

 lar difi^usion. For the transport of gaseous compounds, therefore, 

 molecular diffusion is the only transport means close to the inter- 

 face. As for aggregate constituents, i.e., particulate matter, molecu- 

 lar random motion of these will be extremely small, at least in the 

 size range considered here. Hence, fall by gra\'ity or rise by ejection 

 from bursting bubbles are dominant in the transport of particulate 

 matter close to the air-sea interface. Furthermore, it is rather un- 

 likely that over the sea the impingement of particulate matter on 



