Vertical profiling of SO2 and SO above Venus’ clouds by SPICAV/SOIR solar occultations

New measurements of sulfur dioxide (SO2) and monoxide (SO) in the atmosphere of Venus by SPICAV/SOIR instrument onboard Venus Express orbiter provide ample statistics to study the behavior of these gases above Venus' clouds. The instrument (a set of three spectrometers) is capable to sound atmospheric structure above the clouds in several observation modes (nadir, solar and stellar occultations) either in the UV or in the near IR spectral ranges. We present the results from solar occultations in the absorption ranges of SO2(190-230nm, and at 4 mu m) and SO (190-230nm). The dioxide was detected by the SOIR spectrometer at the altitudes of 65-80km in the IR and by the SPICAV spectrometer at 85-105km in the UV. The monoxide's absorption was measured only by SPICAV at 85-105km. We analyzed 39 sessions of solar occultation, where boresights of both spectrometers are oriented identically, to provide complete vertical profiling of SO2of the Venus' mesosphere (65-105km). Here we report the first firm detection and measurements of two SO2layers. In the lower layer SO2mixing ratio is within 0.02-0.5ppmv. The upper layer, also conceivable from microwave measurements by Sandor et al. (Sandor, B.J., Todd Clancy, R., Moriarty-Schieven, G., Mills, F.P. [2010]. Icarus 208, 49-60) is characterized by SO2increasing with the altitude from 0.05 to 2ppmv, and the [SO2]/[SO] ratio varying from 1 to 5. The presence of the high-altitude SO|>x species could be explained by H2SO4 photodissociation under somewhat warmer temperature conditions in Venus mesosphere. At 90-100km the content of the sulfur dioxide correlates with temperature increasing from 0.1ppmv at 165-170K to 0.5-1ppmv at 190-192K. It supports the hypothesis of SO2 production by the evaporation of H2SO4 from droplets and its subsequent photolysis at around 100km.