Search for HBr and bromine photochemistry on Venus

Abstract

HBr (1–0) R2 2605.8/6.2 cm−1, the strongest line of the strongest band of HBr, was observed when searching for this species on Venus. The observation was conducted using the NASA IRTF and a high-resolution long-slit spectrograph CSHELL with resolving power of 4 ×104. 101 spectra of Venus were analyzed, and the retrieved HBr abundances varied from -8 to+ 5 ppb. Their mean value is -1.2 ppb, standard deviation is 2.5 ppb, and uncertainty of the mean is 0.25 ppb. The negative value presumes a systematic error, and the estimated upper limit of the HBr mixing ratio at the cloud tops of Venus is ∼1 ppb. From the simultaneously retrieved CO2 abundances, this corresponds to an altitude of 78km for the uniform distribution of HBr. A simplified version of the bromine photochemistry is included into the photochemical model (Krasnopolsky 2012, Icarus 218, 230–246). Photolysis of HBr and its reactions with O and H deplete the HBr mixing ratio at 70–80km relative to that below 60km by a factor of ≈300. Reanalysis of the observational data with the calculated profile of HBr gives an upper limit of 20–70 ppb for HBr below 60km and the aerosol optical depth of 0.7 at 70km and 3.84µm.The bromine chemistry may be effective on Venus even under the observed upper limit. However, if a Cl/Br ratio in the Venus atmosphere is similar to that in the Solar System, then HBr is ≈1 ppb in the lower atmosphere and the bromine chemistry is insignificant. Thermodynamic calculations based on the chemical kinetic model (Krasnopolsky 2013, Icarus 225, 570–580) predict HBr as a major bromine species in the lower atmosphere.