The millimeter-wavelength absorption of sulfuric acid vapor measured under simulated Venus conditions

Laboratory measurements of the 2–4 millimeter-wavelength rotational absorption spectrum of H2SO4 vapor pressure-broadened by CO2 have been made under simulated Venus conditions using a semi-confocal microwave open resonator. Results of the measurements are used to derive an absorption model based on the JPL Millimeter Spectral Line Catalog using a Van Vleck-Weisskopf lineshape. The millimeter-wavelength H2SO4 vapor absorption predicted by this derived model contrasts with extrapolation of the absorption model of Kolodner and Steffes (1998), which is based on the results of centimeter-wavelength measurements made under similar conditions. The results of the measurements presented here and other laboratory measurements under Venus conditions have been used to develop a Venus microwave radiative transfer model. This model can be used to predict the wavelength-dependent brightness temperature observed by remote sensing platforms and Earth-based radio telescopes. Based on the results of the new model for H2SO4 vapor absorption, previously measured brightness temperature variations >10 K in the 3 millimeter wavelength range are unlikely directly due to variations in the abundance of gaseous H2SO4.