Kinetic fractionation of noble gases in the stratosphere over Japan

Abstract. Gravitational separation of gas species in the stratosphere is caused mainly by molecular diffusion and is a powerful tool to diagnose stratospheric transport processes. Previous studies have shown that isotopic and elemental ratios of major atmospheric components decrease with increasing altitude in proportion to the differences of their mass numbers. However, there have been no reports of the vertical changes of Kr, Xe, and Ne in the stratosphere. Here we report the results of the first study of the vertical changes of Kr, Xe, and Ne in the stratosphere based on high-precision analyses. Our goal was to reveal the vertical distributions of noble gases and to clarify the mechanisms governing their separations. Noble gases were measured for the stratospheric air collected by balloon-borne cryogenic air samplers over Japan. We found that the isotopic and elemental ratios of all noble gases decreased and increased with increasing altitude for heavy and light noble gases, respectively. Vertical distributions normalized for the mass number differences indicated that the larger the mass number, the smaller the separation of both the isotopic and elemental ratios. The implication was that kinetic fractionation occurred in the stratosphere because of the differences of molecular diffusivities. We performed model simulations and were able to reproduce the kinetic fractionations for heavier noble gases. Results of model simulations suggested that the kinetic fractionations of noble gases were usable as a new tool to diagnose stratospheric transport processes. In the modern atmosphere, it is difficult to detect the long-term change of the stratospheric circulation from noble gases, except for Ar/N2 ratio, in the troposphere. However, it was suggested that changes in the stratospheric circulation during glacial and interglacial cycles may have affected the noble gas elemental ratios in ice core samples.