• Measurement of 14C Concentrations of Stratospheric CO2 by Accelerator Mass Spectrometry

      Nakamura, Toshio; Nakazawa, Takakiyo; Nakai, Nobuyuki; Kitagawa, Hiroyuki; Honda, Hideyuki; Itoh, Tomozio; Machida, Toshinobu; Matsumoto, Eiji (Department of Geosciences, The University of Arizona, 1992-01-01)
      In order to measure the concentrations of anthropogenically influenced gases in the stratosphere, we have collected air samples from the lower stratosphere since 1985, by a balloon-borne cryogenic sampling method, developed at the Institute of Space and Astronautical Science (ISAS). Air samples of ~16 liters at STP were collected in the stratosphere at altitudes from 18.6 to 30.4 km, over the northeastern part of Japan (39.5 degrees N, 139-142 degrees E), on 1 September 1989. We conducted 14C analyses to study the vertical and horizontal air-mass movement in the stratosphere, and to investigate the air transport mechanism between troposphere and stratosphere. Carbon dioxide (containing a few mg carbon) was separated cryogenically from the air samples, and the 14C concentration of the CO2 was measured by a Tandetron accelerator mass spectrometer, using Fe-graphite targets prepared by reducing CO2 on Fe-powder with hydrogen in a Vycor tube at 650 degrees C. The 14C concentrations, expressed as Delta-14C, of CO2 were 267-309 per mil at altitudes of 21-30 km, and 134 per mil at 19-20 km. The Delta-14C values at 21-30 km were higher than those of the current tropospheric CO2, of around 80-200 per mil. The observed 14C concentrations, higher in the stratosphere than the troposphere, seem to be explained by large bomb-produced 14C inventories and/or high 14C production by cosmic rays, as well as weak vertical mixing of air masses in the stratosphere.