EXCHANGE OF MATTER BETWEEN ATMOSPHERE AND SEA 419 



momentum transport amounts to 0.9-1.2 X 10~^-600 g cm^^- 

 sec~^ = 0.6 dyne -cm"-. This produces the so-called wind stress 

 on the surface. 



We have found that the vertical transport velocity at the top 

 of the troposphere is perhaps about 0.2 cmsec~^ By converting 

 this into residence time, this would be 40 days at most, but probably 

 less on an average. Because w varies so strongly, the concept of 

 residence time loses much of its significance. It can, however, be 

 compared to the residence time of carbon dioxide estimated by 

 Craig (1957). He found this exchange with the sea to be about 7 

 years or anywhere between 5 and 10 years. It is apparent from this 

 that the carbon dioxide must be relatively well mixed in the tropo- 

 sphere. It is also apparent that some conditions in the sea must be 

 limiting the rate of exchange of carbon dioxide with the sea. We 

 shall examine several possibilities, but first a short description of 

 the carbon dioxide system in the sea may be helpful. 



The solubility of carbon dioxide in water is relatively high com- 

 pared with that of other gases. One volume of water absorbs about 

 one volume of carbon dioxide at equilibrium. Solubility depends 

 on temperature, but this need not be considered in the present 

 discussion. Part of the carbon dioxide reacts with water to form 

 carbonic acid. The equilibrium, however, is such that dissolved 

 carbon dioxide is the dominant molecular species. For all practical 

 reasons, therefore, we can regard dissolved carbon dioxide as made 

 up entirely of carbon dioxide molecules. 



The rate of the hydration process forming carbonic acid is known 

 to be comparatively slow, although no rate data exist. This process 

 is, however, a very important step because carbonic acid dissociates 

 to form bicarbonate and carbonate ions, which are dominant ionic 

 species of carbon dioxide in sea water. The proportion of carbon 

 dioxide to bicarbonate and carbonate ions in sea water is about 1 

 to 150. In the exchange of carbon dioxide with the sea it is apparent 

 that the rate of hydration to carbonic acid may be of importance. 



We may now investigate several possibilities to account for the 

 observed residence time of carbon dioxide in the atmosphere. First 

 we assume that the rate 6i mixing of the sea water near the surface 

 does not limit the exchange of carbon dioxide into the upper layer 



