Seasonal, latitudinal, and secular variations in the abundance and isotopic ratios of atmospheric carbon dioxide: 1. Results from land stations

Between March 1977 and February 1982, 517 samples of air were collected in 51 glass flasks at four stations in the northern hemisphere near the Pacific ocean and at the South Pole. First, the CO 2 concentration of each sample was determined by nondispersive infrared gas analysis, and then the 13 C/ 12 C and 18 O/ 16 O ratios of the cryogenic extracted CO 2 were determined with a triple collector mass spectrometer. For each station the secular trend and seasonal variation in 13 C/ 12 C ratio have been established as a function of CO 2 concentration. The seasonally adjusted 13 C/ 12 C ratio is found to have decreased at a rate of about 0.02‰ per ppm increase in the seasonally adjusted CO 2 concentration and to vary with latitude as expected if CO 2 is being released by fossil fuel combustion in the northern hemisphere and from ocean water near the equator. At the three northernmost stations (La Jolla, California, at 33°N and two Hawaiian stations near 19°N) the 13 C/ 12 C ratio of the CO 2 added to and withdrawn from the atmosphere during one annual cycle is circa −29‰ with respect to standard PDB, as expected from the exchange of atmospheric CO 2 with the carbon of terrestrial plants. Near the equator at Fanning Island (4°N) the similarly computed 13 C/ 12 C ratio is −21‰, while at the South Pole it is about −13‰. This suggests that the seasonal variation in the southern hemisphere and tropics is partially a result of oceanic CO 2 exchange.