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Recent observations of sulfur containing species (SO 2 , SO, OCS, and H 2 SO 4 ) in Venus' mesosphere have generated controversy and great interest in the scientific community. These observations revealed unexpected spatial patterns and spatial/temporal variability that have not been satisfactorily explained by models. Sulfur oxide chemistry on Venus is closely linked to the global-scale cloud and haze layers, which are composed primarily of concentrated sulfuric acid. Sulfur oxide observations provide therefore important insight into the on-going chemical evolution of Venus' atmosphere, atmospheric dynamics, and possible volcanism. This paper is the first of a series of two investigating the SO 2 and SO variability in the Venus atmosphere. This first part of the study will focus on the vertical distribution of SO 2 , considering mostly observations performed by instruments and techniques providing accurate vertical information. This comprises instruments in space (SPICAV/SOIR suite on board Venus Express) and Earth-based instruments (JCMT). The most noticeable feature of the vertical profile of the SO 2 abundance in the Venus atmosphere is the presence of an inversion layer located at about 70–75 km, with VMRs increasing above. The observations presented in this compilation indicate that at least one other significant sulfur reservoir (in addition to SO 2 and SO) must be present throughout the 70–100 km altitude region to explain the inversion in the SO 2 vertical profile. No photochemical model has an explanation for this behaviour. GCM modelling indicates that dynamics may play an important role in generating an inflection point at 75 km altitude but does not provide a definitive explanation of the source of the inflection at all local times or latitudes The current study has been carried out within the frame of the International Space Science Institute (ISSI) International Team entitled 'SO 2 variability in the Venus atmosphere'. ; Investigator Sandor was supported by the U.S. National Science Foundation under Grant no. AST-1312985, and by NASA under Grant nos. NNX10AB33G, NNX12AI32G and NNX14AK05G. F.P. Mills also acknowledges partial support under NASA Grant NNX12AI32G to Space Science Institute. The research program was supported in Belgium by the Belgian Federal Science Policy Office and the European Space Agency (ESA, PRODEX program, contracts C 90268, 90113, and 17645). Some authors also recognize the support from the FP7 EuroVenus project (G.A. 606798). We acknowledge the support of the "Interuniversity Attraction Poles" program financed by the Belgian government (Planet TOPERS). This research was also supported by a BRAIN research grant BR/143/A2/SCOOP of the Belgian Federal Science Policy Office. A. Mahieux thanks the FNRS for the position of "chargé de recherche". O. Korablev, D. Belyaev acknowledge support from Roscosmos and the Russian Academy of Science (FANO). E.Marcq, F. Montmessin, F. Lefèvre and A. Stolzenbach acknowledge support from CNES and from the Programme National de Planétologie (PNP) of CNRS/INSU. C. D. Parkinson also acknowledges support with funding in part by NASA Grant #NNX11AD81G to the University of Michigan. Limaye acknowledges support for NASA Participating Scientist for Venus Express Grant # NNX09AE85G. The HST observations were obtained through NASA/HST program 12433. Support for this program was provided through a grant from Space Science Telescope Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NAS5-26555. Additional funding for the analysis of the HST observations was provided through funding from the NASA Early Careers Program, NASA Grant NNX11AN81G and the NASA Planetary Atmospheres Program, Grant NNX12AG55G. The authors would additionally like to acknowledge Adriana Ocampo, NASA Headquarters, John Grunsfield, NASA Headquarters, Alan Stern, SwRI, Claus Leither, Space Telescope Science Institute, and Håkan Svedhem, Venus Express Project Scientist for their support in the acquisition of the joint HST-Venus Express Venus Observing Campaign.

The vertical distribution of sulfur species in the Venus atmosphere has been investigated and discussed in Part I of this series of papers dealing with the variability of SO 2 on Venus. In this second part, we focus our attention on the spatial (horizontal) and temporal variability exhibited by SO 2 . Appropriate data sets – SPICAV/UV nadir observations from Venus Express, ground-based ALMA and TEXES, as well as UV observation on the Hubble Space Telescope – have been considered for this analysis. High variability both on short-term and short-scale are observed. The long-term trend observed by these instruments shows a succession of rapid increases followed by slow decreases in the SO 2 abundance at the cloud top level, implying that the transport of air from lower altitudes plays an important role. The origins of the larger amplitude short-scale, short-term variability observed at the cloud tops are not yet known but are likely also connected to variations in vertical transport of SO 2 and possibly to variations in the abundance and production and loss of H 2 O, H 2 SO 4 , and S x . ; Investigator Sandor was supported by the U.S. National Science Foundation under Grant no. AST-1312985, and by NASA under Grant nos NNX10AB33G, NNX12AI32G and NNX14AK05G. F.P. Mills also acknowledges partial support under NASA Grant NNX12AI32G to Space Science Institute. The research program was supported in Belgium by the Belgian Federal Science Policy Office and the European Space Agency (ESA, PRODEX program, contracts C 90268, 90113, and 17645). Some authors also recognize the support from the FP7 EuroVenus project (G.A. 606798). We acknowledge the support of the "Interuniversity Attraction Poles" program financed by the Belgian government (Planet TOPERS). This research was also supported by a BRAIN research grant BR/143/A2/SCOOP of the Belgian Federal Science Policy Office. A. Mahieux thanks the FNRS for the position of "chargé de recherche". O. Korablev, D. Belyaev acknowledge support from Roscosmos and the Russian Academy of Science (FANO). E. Marcq, F. Montmessin, F. Lefèvre and A. Stolzenbach acknowledge support from CNES and from the Programme National de Planétologie (PNP) of CNRS/INSU. Co-authors affiliated at IKI and LATMOS/CNRS acknowledge support from the FRRI #10-52-16011 in frames of Russian-French GDRI cooperation. S. Limaye acknowledges support for NASA Participating Scientist for Venus Express Grant # NNX09AE85G. C. D. Parkinson also acknowledges support with funding in part by NASA Grant #NNX11AD81G to the University of Michigan. Limaye acknowledges support for NASA Participating Scientist for Venus Express Grant # NNX09AE85G. The HST observations were obtained through NASA/HST program 12433. Support for this program was provided through a grant from Space Science Telescope Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NAS5-26555. Additional funding for the analysis of the HST observations was provided through funding from the NASA Early Careers Program, NASA Grant NNX11AN81G and the NASA Planetary Atmospheres Program, Grant NNX12AG55G. The authors would additionally like to acknowledge Adriana Ocampo, NASA Headquarters, John Grunsfield, NASA Headquarters, Alan Stern, SwRI, Claus Leither, Space Telescope Science Institute, and Håkan Svedhem, Venus Express Project Scientist for their support in the acquisition of the joint HST-Venus Express Venus Observing Campaign.