A one-dimensional simulation of the water vapor isotope HDO in the tropical stratosphere

An existing one-dimensional chemical model has been extended with dynamics and isotopic chemistry to simulate chemical production, vertical ascent, and diffusion of (H2O)-O-16 and HDO in the tropical stratosphere. Less abundant isotopes of water vapor and methane have been added to the model's original chemical scheme. This has led to 11 additional compounds being considered., and the methane oxidation chain has been extended by 47 new reactions. The dynamical model includes vertical diffusion and a vertical ascent rate that varies throughout the year. The results of the model show values of the isotopic ratio that are expected from theoretical calculations. The deltaD values range from around -550parts per thousand at the tropopause to about -300parts per thousand at 1 hPa. The model simulations are also in agreement with the few existing measurements of deltaD in the stratosphere. An annual variation of the isotopic ratio at the tropopause will cause a wave pattern in the vertical profile similar to the "tape recorder effect" for water vapor. The size and shape of this annual variation is not clear, but simulations show that the effect it has on the resulting deltaD profile is significant.