Carbon and oxygen isotopic ratios of carbon dioxide of a stratospheric profile over Japan

Abstract

Four stratospheric air samples from 19 to 25 km altitudes over Japan were collected by using a balloon-borne cryogenic sampling system to measure the vertical profiles of carbon and oxygen stable isotopic ratios of the lower stratospheric CO₂.

The δ¹³C value of the stratospheric CO₂ increased with increasing altitude, while the CO₂ mixing ratios decreased, in accordance with anthropogenic input of the isotopically light, fuel CO₂ into the atmosphere. However, the relationship between δ¹³C and CO₂ mixing ratio (-0.07% ppmv^-1) was somehow similar to that for the seasonal variation in the troposphere rather than that for the secular variation so far observed in the troposphere (-0.02 ∼ -0.03% ppmv^-1).

The δ¹⁸O of the stratospheric CO₂ was found to be about 2% greater than that of the tropospheric CO₂ at an altitude of 19 km and to increase further with increasing altitude. Two possibilities for explaining this phenomenon are presented and discussed. One possibility is that the tropical CO₂ with high δ¹⁸O might upwell and advect horizontally to the midlatitudinal stratosphere, although the δ¹⁸O enhancement by this process is insufficient to raise the δ¹⁸O value up to the observed level. The other is that the δ18^O might be enriched by an insitu oxygen isotope exchange between CO₂ and O₂, which is possibly accelerated through photochemical dissociations of O₂ and CO_2. A relationship between δ¹⁸O of CO₂ and ¹⁸O enrichment of ozone in the stratosphere is also suggested.