Hydrogen halides measurements in the Venus mesosphere retrieved from SOIR on board Venus express

The SOIR instrument on board Venus Express regularly sounds the Venus mesosphere using the solar occultation technique. Densities and volume mixing ratios of HCl and HF are measured in the 70-115 km and 75-110 km altitude region respectively, at the Venus terminator. All latitudes from pole to pole are covered. In this work, we study the latitude and long-term variations of the volume mixing ratio (VMR) of HCl, and the long-term time trend of HF, from June 2006 to February 2013. This period of time corresponds to approximately eleven Venusian years. Large variations in the VMR profiles are observed, mostly on the short-term. Both hydrogen halides present unforeseen positive exponential gradients of their VMR with pressure, which shows time and latitude variations. Long-term trends on the whole period of the HCl VMR are also observed at certain pressure levels in the equatorial and polar regions. HF also presents a time dependence of its VMR at certain pressure levels. Results are compared to previous HCl and HF VMR observations. The ability of SOIR to target both (HCl)-Cl-35 and (HCl)-Cl-37 isotopologues has also been investigated. Numerous concomitant density profiles lead to the determination of the Cl-37/Cl-35 isotopic ratio on Venus, found to be equal to 0.34 +/- 0.13, which is compatible with the value found on Earth. (C) 2014 Elsevier Ltd. All rights reserved.