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

Abstract

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 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°C. 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.

On the use of 14CO2 as a tracer for fossil fuel CO2: Quantifying uncertainties using an atmospheric transport model

“…C agrees fairly well with the very few stratospheric D 14 CO 2 observations made in recent years [Nakamura et al, 1994[Nakamura et al, , 1992. Although this vertical distribution weights the 14 CO 2 production to somewhat higher altitudes than theory suggests [Masarik and Beer, 1999;Lal, 1988], when we applied a vertical distribution with more production close to the surface, our model did not build up enough 14 C in the stratosphere (Figure 2).…”

“…In all cases, the selected model level is above the model surface, but below the true sampling site altitude, consistent with expectations. The , and each is compiled from stratospheric observations collected over Sanriku, Japan [Nakamura et al, 1992[Nakamura et al, , 1994Turnbull, 2006], and an associated monthly mean value from Jungfraujoch, Switzerland [Levin and Kromer, 2004]. Diamonds and error bars indicate the observed values and their measurement uncertainties.…”

Radiocarbon in the Atmosphere

“…This stratosphere-troposphere 14 CO 2 gradient continues to be exploited in studies of cross-tropopause transport of air and stratospheric residence times during the post-bomb period. Observed stratosphere-troposphere gradients of 14 CO 2 in 1989-1990 were used to estimate an average turnover time of the stratosphere of *9 years (Nakamura et al 1992(Nakamura et al , 1994. In addition, very strong vertical gradients observed within the stratosphere indicate vertical mixing of stratospheric air is slow.…”

Radiocarbon analysis of stratospheric CO<sub>2</sub> retrieved from AirCore sampling

“…Aircraft sampling of atmospheric CO 2 is regularly performed at various altitudes, but unfortunately air samples are only collected up to the upper troposphere/lower stratosphere (Sweeney et al, 2015;Machida et al, 2008;Brenninkmeijer et al, 2007Brenninkmeijer et al, , 1995. Although balloon-based sampling has been demonstrated as a method for collecting stratospheric air for measurements of radiocarbon in stratospheric CO 2 (Kanu et al, 2016;Ashenfelter et al, 1972;Nakamura et al, 1992Nakamura et al, , 1994Hagemann et al, 1959), this method of sampling is extremely expensive and difficult to sustain for longer periods. Here we describe the use of the AirCore sampling method (Karion et al, 2010) as a viable and affordable alternative for sampling stratospheric air for the measurements of radiocarbon in stratospheric CO 2 .…”