Isotopic ratios of water vapor and methane in the stratosphere: Comparison between ATMOS measurements and a one‐dimensional model

Abstract

A one‐dimensional model simulating the transport and chemistry of methane and water vapor including their isotopic ratios in the tropical stratosphere is compared to measurements by the Atmospheric Trace Molecule Spectroscopy experiment (ATMOS) instrument. The model and measurements show good agreement in the isotopic ratio profiles. The δ D depletion for water vapor is −600‰ to −500‰ at the tropopause with a small increase up to ∼10 hPa. Above this altitude the modeled isotopic ratio shows a strong increase due to methane oxidation. The measured profiles are rather noisy above 10 hPa but give an indication of a stronger increase in the isotopic ratio than modeled. If the isotopic ratio of water vapor is allowed to vary at the tropopause simulating an annual cycle in the input values, a wave pattern that is transported upwards arises on the vertical profile. This is a similar effect as the “tape recorder” for water vapor. A wave pattern can also be detected in the tropical δ D profiles from ATMOS. The methane isotopic ratio shows behavior similar to that of water vapor but without the wave pattern. The increase in methane δ D above 10 hPa is very strong. The measured profiles are again rather noisy above this altitude, but measurements from inside the polar vortex show that the methane isotopic ratio in the upper stratosphere is very high. The δ D values are in the range of +300‰ to +500‰ at altitudes as low as 40 hPa (∼25 km) in the polar vortex.