Upper limits for H 2SO 4 in the mesosphere of Venus

Abstract

Rapid temporal variability of SO 2 and SO in the Venus 85–100 km mesosphere ( Sandor, B.J., Clancy, R.T., Moriarty-Schieven G.H. [2007]. Bull. Am. Astron. Soc. 39, 503; Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60) requires in situ sources and sinks for these molecules. While many loss mechanisms are recognized, no process for in situ production is known. Observational investigations to find, or constrain other potential sulfur reservoirs offer one method toward understanding the applicable photochemistry. Here, we report upper limits for gas-phase H 2SO 4 (sulfuric acid) abundances in Venus’ 85–100 km upper mesosphere, derived from 16 ground-based sub-mm spectroscopic observations in the period 2004–2008. Unlike the ubiquitous sulfuric acid solid/liquid aerosol, the gas phase would be photochemically active, potentially both source and sink for SO and SO 2. H 2SO 4 is retrieved from sub-mm lines located in the same bandpass as the SO 2 and SO lines described by Sandor et al. (Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60). H 2SO 4 upper limits reported here are thus simultaneous and spatially coincident with measurements of SO 2 and SO, providing for analysis of the three sulfur species collectively. The average H 2SO 4 abundance over 16 observations is 1 ± 2 ppb (i.e. <3 ppb). Upper limits for individual observations range from 3 to 44 ppb, where quality of the observing weather is the dominant constraint on measurement precision. The sum of H 2SO 4, SO 2 and SO varies widely. In one comparison, the sum [H 2SO 4 + SO 2 + SO] measured on one date differs by 10- σ from the sum measured 2 months later. We conclude that upper mesospheric sulfur atoms are not conserved among the three molecules, that H 2SO 4 is not a significant sulfur reservoir for balancing the observed variations of [SO 2 + SO], and is not relevant to the (still unknown) photochemistry responsible for observed behavior of SO 2 and SO. Having ruled out H 2SO 4, we infer that elemental sulfur is the most probable candidate for the needed third reservoir.