Spatially-resolved high-resolution spectroscopy of Venus 2. Variations of HDO, OCS, and SO2 at the cloud tops

While CO, HCl, and HF, that were considered in the first part of this work, have distinct absorption lines in high-resolution spectra and were detected four decades ago, the lines of HDO, OCS, and SO2 are either very weak or blended by the telluric lines and have not been observed previously by ground-based infrared spectroscopy at the Venus cloud tops. The H2O abundance above the Venus clouds is typically below the detection limit of ground-based IR spectroscopy. However, the large D/H ratio on Venus facilitates observations of HDO. Converted to H2O with D/H ≈ 200, our observations at 2722 cm−1 in the Venus afternoon show a H2O mixing ratio of ∼1.2 ppm at latitudes between ±40° increasing to ±60° by a factor of 2. The observations in the early morning reveal the H2O mixing ratio that is almost constant at 2.9 ppm within latitudes of ±75°. The measured H2O mixing ratios refer to 74 km. The observed increase in H2O is explained by the lack of photochemical production of sulfuric acid in the night time. The recent observations at the P-branch of OCS at 4094 cm−1 confirm our detection of OCS. Four distributions of OCS along the disk of Venus at various latitudes and local times have been retrieved. Both regular and irregular components are present in the variations of OCS. The observed OCS mixing ratio at 65 km varies from ∼0.3 to 9 ppb with the mean value of ∼3 ppb. The OCS scale height is retrieved from the observed limb darkening and varies from 1 to 4 km with a mean value of half the atmospheric scale height. SO2 at the cloud tops has been detected for the first time by means of ground-based infrared spectroscopy. The SO2 lines look irregular in the observed spectra at 2476 cm−1. The SO2 abundances are retrieved by fitting by synthetic spectra, and two methods have been applied to determine uncertainties and detection limits in this fitting. The retrieved mean SO2 mixing ratio of 350 ± 50 ppb at 72 km favors a significant increase in SO2 above the clouds since the period of 1980–1995 that was observed by the SOIR occultations at Venus Express. Scale heights of OCS and SO2 may be similar, and the SO2/OCS ratio is ∼500 and may be rather stable at 65–70 km under varying conditions on Venus.