4 COMPOSITION OF THE ATMOSPHERE 
An oxygen deficiency in antarctic air, claimed by 
Lockhart and Court [c. 12], cannot yet be regarded as 
well established, since no check analyses with normal 
air were carried out. 
In spite of Paneth’s conservative estimate [16] that 
the second decimal is still not exactly known, the author 
feels inclined, in view of the agreement of the two best 
surveys with a recent redetermination by M. Shepherd 
[c. 16] which gave 20.945 per cent, to recommend an 
absolute value of 20.946 + 0.002 per cent as the most 
likely figure for the O2 content of uncontaminated air, 
in combination with an average CO, value of 0.033 per 
cent. 
Apart from possible minor changes resulting from 
the greater solubility of O2 than of No in ocean water, 
all major variations of the O: content must result 
from the combustion of fuel, from the respiratory ex- 
change of organisms, or from the assimilation of CO» in 
plants. The first process does not result in more than 
local changes of the O2 content, while the latter two 
processes, though locally altering the CO:/O: ratio, 
leave their sum unchanged. 
Carbon Dioxide (CO,). Though the extensive in- 
vestigations of Benedict and of Krogh suggest that the 
CO; content of atmospheric air over land does not vary 
except within very narrow limits, significant variations 
of the CO: content have been observed by workers 
both before and after them. 
In particular, Callendar [5] has drawn attention to 
an increase in the CO. content during the last fifty 
years which is best demonstrated in Fig. 1. This in- 
330 
COp CONTENT (PPM) 
1860 1950 
1925 
Fig. 1—Increase of CO. in air. (After Callendar [5].) 
1900 
crease in the total atmosphere of about 30 ppm (parts 
per million) represents a quantity of CO. (2 xX 10" 
tons) which is approximately equal to the amount re- 
sulting from the combustion of fuels produced during 
this period. This implies that not much of this excess 
CO: has been lost to the ocean water, an assumption 
which is justified in view of the fact that, apart from a 
thin agitated surface layer, the transport of CO» inside 
the water proceeds by diffusion and is very slow. 
Variations of the CO: Content over the Sea. The vari- 
ations of CO. over the sea are now well understood 
(Buch, Wattenberg [c. 5]). Because of an excess of 
strongly basic cations over strongly acid anions in sea 
water, CO: is soluble in sea water not only as dis- 
solved CO2, but also in the form of carbonate and bi- 
carbonate ions, the quantities being roughly of the 
order 1:8:150. The result of this is that one litre of sea 
water contains about 150 times as much CO, as the 
same volume of air. 
For a given content of total COs, the equilibrium 
pressure varies considerably with the water tempera- 
ture. To give an example: For water with a chloride 
content of 1.95 per cent and a total CO» of 2.07 X 10-3 
mol 11, the CO. pressures? in air at equilibrium at 
OC, 10C, 20C, and 30C are 1.6, 2.5, 3.6, and 5.1 X 
10~* atm, respectively. Thus, far from having an equal- 
izing effect on the CO, content of the air, as was be- 
lieved during the last century and the earlier part of 
this century, changes in the surface temperature of the 
sea upset the otherwise comparatively constant CO» 
content of air. This explains the low values of the CO, 
content found near the polar regions. The lowest value 
(1.52 X 10-4 atm) was observed near Spitsbergen by 
Buch [e. 5]. This value corresponds roughly to the 
equilibrium value at OC. 
In the most northerly regions, particularly over polar 
ice, the CO. content is again normal, a feature which 
was explained by Buch on the basis of Bjerknes’ scheme 
of the air circulation over the Atlantic. According to 
the latter, air in the Arctic is more or less constantly 
descending, and since it has by-passed at great height 
the cold-water regions on its way from the south, its 
CO, content would be expected to be near that of the 
temperate zones. Buch found 2.57 and 2.91 xX: 1074 
atm. 
Equally complicated is the situation in regions where 
masses of water rise to the surface from greater depths. 
The CO, content of sea water, after falling slightly in 
the first 50 m below the surface (due to CO: assimila- 
tion), rises to a maximum at about 500-m depth where 
the CO: pressure may be as much as 11 X 1074 atm 
(obviously due to decay of organic matter). If these 
CO.-rich water masses rise to the surface (as observed 
near the west coast of Africa), the CO. content of air 
may locally rise to 7 X 10-4 atm. Similarly high values 
have been observed by Krogh [14] in the vicinity of 
West Greenland, and by Moss [e. 14] at latitude 
82°27’N, though in these two cases the origin of the 
CO, was not traced, and the effect may possibly, but 
not necessarily, be spurious. 
These phenomena apparently do not affect the air 
masses to a very great depth. Near Petsamo, Finland, 
arctic air (range 297 to 313 ppm) differs unmistakably 
from continental and tropical air (range 319 to 335 
ppm), so that in this region the CO: content can serve 
as an indicator for the origin of the air masses. How- 
ever, these differences become smaller as we go farther 
south. Thus the difference is still appreciable at Kew, 
England, where, on the average, subtropical air con- 
tains 19 ppm more COQ, than polar and maritime air; 
but the mean difference is only 8 ppm near Dieppe, 
France, and Gembloux, Belgium [e. 5]. 
2. The CO, pressure in atmospheres is very nearly, but not 
quite, identical with the “parts per volume”’ unit, 7.e., 1074 
atm ~ 100 ppm. 
