Sulfuric acid vapor and sulfur dioxide in the atmosphere of Venus as observed by the Venus Express radio science experiment VeRa

Abstract

Abstract The Venus Express radio science experiment VeRa provided more than 900 neutral atmospheric profiles between the years 2006 and 2014. About 800 of these could be used for an analysis of the radio signal absorption at X-Band (wavelength: 3.6\u202fcm), which is mainly caused by sulfuric acid vapor within the Venus atmosphere. The absorptivity profiles were converted into sulfuric acid vapor profiles. The combined measurements from the entire Venus Express mission reveal a distinct latitudinal H2SO4(g) variation. A latitudinal gradient can be observed at the topside of the H2SO4(g) layer, which is located approx. 4\u202fkm higher at equatorial latitudes compared to polar latitudes. Regions of enhanced sulfuric acid vapor abundance were found at equatorial and polar latitudes. The highest H2SO4(g) values at equatorial latitudes show mean maximal values of more than 12\u202fppm at around 47\u202fkm altitude. At polar latitudes mean maximal values were found at around 43\u202fkm altitude and ranged from 9 to 12\u202fppm. Both latitudinal regions of increased sulfuric acid vapor abundance are clearly separated by a low abundance region located at mid-latitudes with values of 5 to 7\u202fppm. A simplified two-dimensional transport model was developed to study the formation processes of sulfuric acid vapor accumulation at equatorial and polar latitudes. It turned out that the H2SO4(g) accumulation observed at high latitudes can be explained by precipitation of H2SO4(l) droplets that evaporate into gaseous sulfuric acid upon entering lower (warmer) altitudes. The influence of wind transport on this formation process was minor. In contrast, the H2SO4(g) accumulation observed at equatorial latitudes could be reproduced in the model by oppositely directed mass transport (upward winds and sedimentation) as well as by simplified evaporation and condensation processes. The low H2SO4(g) abundance observed at mid-latitudes was reproduced by downward winds in the model calculations. The VeRa observations were additionally used to estimate the abundance of SO2 above the cloud bottom. A latitudinal dependence was found with highest values of 90\u202f±\u202f60\u202fppm at equatorial latitudes, compared to 150\u202f±\u202f50\u202fppm and 160\u202f±\u202f50\u202fppm at southern and northern polar latitudes, respectively. Both the equatorial and polar regions displayed show large variability of the H2SO4(g) and SO2 abundances from observation to observation. A weak tidal influence is also visible in the sulfuric acid vapor abundance in the equatorial region. The northern polar H2SO4(g) abundance, as well as the southern and northern SO2 abundances, exhibit distinct long-term variations.