EXCHANGE OF MATTER BETWEEN ATMOSPHERE AND SEA 417 



Interface where p„ = 0, i.e., we do not recognize molecules that are 

 leaving the surface. Then we have the exchange rate 



Fa = WPa (8) 



between the sea surface and the level chosen. 



However, we have ignored the effect of differences in the molecu- 

 lar diffusion coefficients in the laminar boundary layer. These dif- 

 ferences are comparatively small, and it can be shown that, in the 

 case of oxygen and in using water vapor evaporation for computing 

 the transport velocity, the error introduced by the assumption 

 Ka = Kb is less than 10%. 



Thus in the case of gaseous exchange we can also use a transport 

 velocity as in the case of aggregate exchange discussed earlier. The 

 concept of a vertical transport velocity for gaseous transports is not 

 new, although it has been named differently in the past. It seems to 

 have been used for the first time by Egner (1932) in connection 

 with a study of the loss of ammonia from manure spread out in a 

 field. From the rate of evaporation of water and the relative hu- 

 midity he estimated the volume of air per day and unit surface that 

 had to get in close contact with the ground to account for the 

 evaporation. He pointed out also that this volume corresponded 

 to a vertical wind. This term is, of course, confusing. Recently 

 Chamberlain and Chadwick (1953) studied the absorption of 

 radioiodine from air by grassland and used the term ''deposition 

 velocity' for the rate at which radioiodine in air was absorbed by 

 the ground. Bolin (1959), who discusses their results from various 

 points of view, also uses this term, which is still not quite satisfac- 

 tory. It really implies a unidirectional transport mechanism. A 

 term like "transport velocity" is more adequate. 



It may be of interest to discuss qualitatively the vertical varia- 

 tion of the transport velocity w when the sea surface is used as a 

 lower reference level, because w defined in this way is in fact the 

 velocity at which air from a certain level is brought in contact with 

 the sea surface. In other words, w is the velocity of air exchange 

 between a point in the atmosphere and the sea surface, but it must 

 be remembered that this velocity of exchange pertains only to the 

 sea surface, not to the sea or any larger portion of it. 



