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

[1] 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 deltaD depletion for water vapor is -600parts per thousand to -500parts per thousand at the tropopause with a small increase up to similar to10 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 deltaD profiles from ATMOS. The methane isotopic ratio shows behavior similar to that of water vapor but without the wave pattern. The increase in methane deltaD 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 deltaD values are in the range of +300parts per thousand to +500parts per thousand at altitudes as low as 40 hPa (similar to25 km) in the polar vortex.