First measurements of ClO in the Venus atmosphere - Altitude dependence and temporal variation

The 352.88 GHz submm absorption spectrum of 35ClO in the Venus atmosphere was observed on Oct 23 and Nov 11, 2010, using the James Clerk Maxwell Telescope (JCMT) on Mauna Kea, HI. This is the first detection of Venus ClO, which is found to exhibit much larger abundances in the upper (85-100 km) versus lower (70-85 km) mesosphere, based upon retrieval of the altitude distribution from the shape of the pressure broadened absorption. The 2-day average of all collected data indicates 2.6 ±0.5 ppb above 85 ±2 km, and undetectable below. Temporal variation is evident between the Oct 23 and Nov 11 observations, which indicate upper mesospheric mixing ratios of 3.7 ±0.9 and 1.5 ±0.5 ppb, respectively. At these abundances catalytic destruction of ozone by highly reactive chlorine compounds (ClOx) must occur, reinforcing the (Montmessin et al., 2011) conclusion that their measured ozone profiles were consistent with chlorine-catalyzed ozone destruction above 90 km. Detection of ClO in the nightside atmosphere reveals that ClOx abundances exceed those of highly reactive nitrogen (NOx). Recent observations of HCl in both the night- and day-side atmosphere (Sandor and Clancy, 2012; 2017) demonstrate that upper mesospheric HCl represents less than half the total chlorine budget. This required a non-HCl reservoir consisting of highly reactive gas-phase chlorine compounds (ClOx), HCl aerosol particles, or both. The measured 1.5-3.7 ppb ClO directly indicates hundreds of ppb of Cl, demonstrating that ClOx (rather than aerosol) is a primary upper mesospheric chlorine reservoir. Finally, this first detection of ClO constitutes the first empirical corroboration that the (Yung and DeMore, 1982) chlorine-catalysis mechanism is responsible for stabilizing the Venus atmosphere as 97% CO2, 3% N2 (rather than comparable (within a factor of 2) CO, O2 and CO2 abundances).