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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.

NOMAD is a suite of three spectrometers that will be launched in 2016 as part of the joint ESA-Roscosmos ExoMars Trace Gas Orbiter mission. The instrument contains three channels that cover the IR and UV spectral ranges and can perform solar occultation, nadir and limb observations, to detect and map a wide variety of Martian atmospheric gases and trace species. Part I of this work described the models of the UVIS channel; in this second part, we present the optical models representing the two IR channels, SO (Solar Occultation) and LNO (Limb, Nadir and Occultation), and use them to determine signal to noise ratios (SNRs) for many expected observational cases. In solar occultation mode, both the SO and LNO channel exhibit very high SNRs >5000. SNRs of around 100 were found for the LNO channel in nadir mode, depending on the atmospheric conditions, Martian surface properties, and observation geometry. ; NOMAD has been made possible thanks to funding by the Belgian Science Policy Office (BELSPO) and financial and contractual coordination by the ESA Prodex Office. The research was performed as part of the "Inter university Attraction Poles" programme financed by the Belgian government (Planet TOPERS). UK funding is acknowledged under the UK Space Agency grant ST/I003061/1. ; https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-4-3790

NOMAD is a suite of three spectrometers that will be launched in 2016 as part of the joint ESA-Roscosmos ExoMars Trace Gas Orbiter mission. The instrument contains three channels that cover the IR and UV spectral ranges and can perform solar occultation, nadir and limb observations, to detect and map a wide variety of Martian atmospheric gases and trace species. Part I of this work described the models of the UVIS channel; in this second part, we present the optical models representing the two IR channels, SO (Solar Occultation) and LNO (Limb, Nadir and Occultation), and use them to determine signal to noise ratios (SNRs) for many expected observational cases. In solar occultation mode, both the SO and LNO channel exhibit very high SNRs >5000. SNRs of around 100 were found for the LNO channel in nadir mode, depending on the atmospheric conditions, Martian surface properties, and observation geometry. ; NOMAD has been made possible thanks to funding by the Belgian Science Policy Office (BELSPO) and financial and contractual coordination by the ESA Prodex Office. The research was performed as part of the "Interuniversity Attraction Poles" programme financed by the Belgian government (Planet TOPERS). UK funding is acknowledged under the UK Space Agency grant ST/I003061/1. ; Peer Reviewed

A publisher correction to this article was published on 17 April 2019 ; Global dust storms on Mars are rare1,2 but can affect the Martian atmosphere for several months. They can cause changes in atmospheric dynamics and inflation of the atmosphere3, primarily owing to solar heating of the dust3. In turn, changes in atmospheric dynamics can affect the distribution of atmospheric water vapour, with potential implications for the atmospheric photochemistry and climate on Mars4. Recent observations of the water vapour abundance in the Martian atmosphere during dust storm conditions revealed a high-altitude increase in atmospheric water vapour that was more pronounced at high northern latitudes5,6, as well as a decrease in the water column at low latitudes7,8. Here we present concurrent, high-resolution measurements of dust, water and semiheavy water (HDO) at the onset of a global dust storm, obtained by the NOMAD and ACS instruments onboard the ExoMars Trace Gas Orbiter. We report the vertical distribution of the HDO/H2O ratio (D/H) from the planetary boundary layer up to an altitude of 80 kilometres. Our findings suggest that before the onset of the dust storm, HDO abundances were reduced to levels below detectability at altitudes above 40 kilometres. This decrease in HDO coincided with the presence of water-ice clouds. During the storm, an increase in the abundance of H2O and HDO was observed at altitudes between 40 and 80 kilometres. We propose that these increased abundances may be the result of warmer temperatures during the dust storm causing stronger atmospheric circulation and preventing ice cloud formation, which may confine water vapour to lower altitudes through gravitational fall and subsequent sublimation of ice crystals3. The observed changes in H2O and HDO abundance occurred within a few days during the development of the dust storm, suggesting a fast impact of dust storms on the Martian atmosphere. © 2019, The Author(s), under exclusive licence to Springer Nature Limited. ; This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493); by the Spanish MICINN through its Plan Nacional and by European funds under grants ESP2015-65064-C2-1-P and ESP2017-87143-R (MINECO/FEDER); by the UK Space Agency through grants ST/R005761/1, ST/P001262/1, ST/R001405/1, ST/S00145X/1, ST/R001367/1, ST/P001572/1 and ST/R001502/1; and the Italian Space Agency through grant 2018-2-HH.0. The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). This work was supported by the Belgian Fonds de la Recherche Scientifique - FNRS under grant number 30442502 (ET_HOME). The ACS experiment is led by IKI, Space Research Institute in Moscow, assisted by LATMOS in France. The project acknowledges funding by Roscosmos and CNES. The science operations of ACS are funded by Roscosmos and ESA. IKI affiliates acknowledge funding under grant number 14.W03.31.0017 and contract number 0120.0 602993 (0028-2014-0004) of the Russian government. ; Peer Reviewed

NOMAD (Nadir and Occultation for MArs Discovery) is one of the four instruments on board the ExoMars Trace Gas Orbiter, scheduled for launch in March 2016. It consists of a suite of three high-resolution spectrometers - SO (Solar Occultation), LNO (Limb, Nadir and Occultation) and UVIS (Ultraviolet and Visible Spectrometer). Based upon the characteristics of the channels and the values of Signal-to-Noise Ratio obtained from radiometric models discussed in (Vandaele et al., 2015a, 2015b; Thomas et al., 2016), the expected performances of the instrument in terms of sensitivity to detection have been investigated. The analysis led to the determination of detection limits for 18 molecules, namely CO, HO, HDO, CH, CH, CH, HCO, CH, SO, HS, HCl, HCN, HO, NH, NO, NO, OCS, O. NOMAD should have the ability to measure methane concentrations Inter-university Attraction Poles> programme financed by the Belgian Government (Planet TOPERS no P7-15) and a BRAIN Research Grant BR/143/A2/SCOOP. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement no. 607177 CrossDrive. UK funding is acknowledged under the UK Space Agency Grant ST/I003061/1. ; Peer Reviewed