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

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 delta C-13 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 delta C-13 and CO2 mixing ratio (-0.07 parts per thousand 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 similar to -0.03 parts per thousand ppmv(-1)). The delta O-18 of the stratospheric CO2 was found to be about 2 parts per thousand 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 delta O-18 might upwell and advect horizontally to the midlatitudinal stratosphere, although the delta O-18 enhancement by this process is insufficient to raise the delta O-18 value up to the observed level. The other is that the O-18 might be enriched by an in-situ oxygen isotope exchange between CO2 and O-2, which is possibly accelerated through photochemical dissociations of O-2 and CO2. A relationship between delta O-18 of CO2 and O-18 enrichment of ozone in the stratosphere is also suggested.