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

Abstract

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°N, 139–142°E), on 1 September 1989. We conducted 14 C 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 14 C concentration of the CO 2 was measured by a Tandetron accelerator mass spectrometer, using Fe-graphite targets prepared by reducing CO 2 on Fe-powder with hydrogen in a Vycor tube at 650°. The 14 C concentrations, expressed as Δ 14 C, of CO 2 were 267–309‰ at altitudes of 21–30 km, and 134‰ at 19–20 km. The Δ 14 C values at 21–30 km were higher than those of the current tropospheric CO 2 , of around 80–200‰. The observed 14 C concentrations, higher in the stratosphere than the troposphere, seem to be explained by large bomb-produced 14 C inventories and/or high 14 C production by cosmic rays, as well as weak vertical mixing of air masses in the stratosphere.