Observations of HCl altitude dependence and temporal variation in the 70-100Â km mesosphere of Venus

Spectroscopically resolved observations of the 625.9 GHz H35Cl absorption lines were obtained with the James Clerk Maxwell Telescope on October 23, November 7, and December 5, 2010. The nightside atmosphere was observed on all three dates, and the dayside morning crescent was also observed on December 5. Shape of the pressure-broadened absorption constrains altitude distribution of HCl over 70-100 km. Retrieved abundances at 70-80 km are consistent with 74 km values reported by Krasnopolsky (Krasnopolsky, V.A. [2010]. Icarus 208, 539-547), and do not agree with smaller abundances measured at the North polar terminator with Venus Express (Vandaele, A.C. [2008]. J. Geophys. Res. 113(E00B23), 1-16). Observed HCl mixing ratios decrease rapidly with altitude above 80 km, with 90 km abundances decreased by a factor of two or more relative to those in the lower mesosphere. This behavior is distinctly different from photochemical model profiles, which predict 90 and 110 km mixing ratios only 10% and 25% smaller, respectively, than cloud top values. While model photochemistry indicates HCl should be the dominant chlorine reservoir in the mesosphere and lower thermosphere, observations show HCl abundance is less than half that of total chlorine above 90 km. It directly follows that at least one other chlorine containing molecule must be present in the upper mesosphere at abundances far higher than predicted, which in turn suggests chlorine chemistry in this region differs significantly from the current theoretical understanding. Above 85 km, the equatorial HCl abundances reported here are consistent with polar vortex values from Venus Express, suggesting that the unknown mechanism depleting HCl above 80 km at low latitudes may be important throughout the mesosphere at high latitude. The polar vortex (Venus Express) and equatorial nightside (this study) of Venus are regions of dynamical downwelling, such that downward transport of air from a thermospheric region of HCl destruction is consistent with both data sets as a mechanism for upper mesospheric HCl depletion. Temporal variation on a 1 month timescale is clearly determined among the three observation dates for HCl at 80-90 km, with far less HCl present on December 5 than on October 23 or November 7. Local time variation among measurements on October 23, November 7, and one of the two spatial locations observed on December 5 is very small, such that the temporal variation is not associated with the diurnal cycle. Abundances measured in the dayside and nightside atmosphere on December 5, 2010 are identical within measurement uncertainty, indicating no diurnal variation on this date. While the number of measurements, and range of local times sampled are too small to rule out a diurnal influence on HCl abundance, the data clearly show a temporal change which is not diurnally driven.