Seasonal variations in 14C concentrations of stratospheric CO2 measured with accelerator mass spectrometry

Stratospheric air samples of about 20-30 liters at STP were collected for up to 12 altitude intervals from 16 to 35 km, by a cryogenic sampler, on 1 September 1989 and 4 June 1990, over Iwate Prefecture (39-40 N, 139-142 E), north-eastern Japan. CO2 was separated cryogenically from aliquots of the air samples, and its 14C concentration was measured with a Tandetron accelerator mass spectrometer at Nagoya University. The stratospheric CO214C concentrations, Δ14C, changed for both years, from 130-170 permille , almost equal to the values of tropospheric CO2, to 250-310 permille as the altitude increased. However, the vertical variation of Δ14C differed for the two years: high Δ14C values are observed above 21 km for 1989 samples, while they are observed only above 28 km for the 1990 samples. Smooth temporal variations of Δ14C for stratospheric air imply that the high 14C concentration in the stratosphere can be attributed to bomb-produced 14C inventories and weak vertical mixing of stratospheric air masses. By using the temporal changes of Δ14C for stratospheric and tropospheric air, the CO2 exchange rate between the two spheres was estimated to be 0.110±0.002 yr-1 (mean residence time: 9.1±0.2 yr) using a box model. ((orig.))