Abstract
An earlier study of the concentration and isotopic abundances of C 13 and O 18 of carbon dioxide in rural air ( Keeling, 1958) has been extended by reporting the analysis of 106 additional samples of rural air and thirteen samples of air collected over tropical waters of the eastern Pacific Ocean. At locations far removed from terrestrial plants and the influence of cities, the concentration and C 13 abundance of carbon dioxide in the air are found to be nearly constant; but the O 18 abundance of carbon dioxide, under various circumstances, appears to show a systematic variation with air temperature, ocean water temperature, or season. Extreme values are, concentration: from 0.0303 to 0.0320 volume per cent of original air; C 13 C 12 ratio: from −6.7 to −7.4 per mil; O 18 O 16 ratio: −0.8 to −0.6 per mil. A correlation between C 13 abundance and concentration of carbon dioxide previously observed for forest air is again observed. On the basis of this correlation, the C 13 C 12 ratio of the carbon dioxide released by the forest plants has been computed and is found to vary for different stations between −21 and −26 per mil. The O 18 abundance of carbon dioxide in forest air is observed to be variable but shows no simple relationship with other measured quantities.
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