Heterogeneous Production of Perchlorate and Chlorate by Ozone Oxidation of Chloride: Implications on the Source of (Per)Chlorate in the Solar System

The occurrence of chlorate (ClO 3 – ) and perchlorate (ClO 4 – ) in the terrestrial and extraterrestrial environments has been partly attributed to ozone (O 3 )-mediated oxidation of chlorine-bearing compounds. This is based on varying elevated Δ 17 O values in all measured terrestrial natural ClO 4 – as well as the nearly universal co-equal occurrence of ClO 3 – and ClO 4 –, which has only been reported to occur for dry oxidation of Cl –, a process for which little information is available. In this study, we examine possible factors influencing ClO 4 – and ClO 3 – formation by O 3 oxidation of sodium chloride (NaCl) salt and hydrochloric acid (HCl) gas in glass reactor vessels. We show that longer reaction times increase production of ClO 4 – and ClO 3 –, with ClO 3 – production generally being lower than ClO 4 – by 1–2 orders of magnitude. For 1 day oxidation periods, ClO 4 – /ClO 3 – ratios were relatively constant (∼50) for low Cl – masses and decreased over 3 orders of magnitude for higher (∼100×) Cl – masses. Perchlorate mass increased with increasing glass reactor surface areas but not the salt surface area. Increasing the relative humidity (RH) from 2 to 67% increased ClO 3 – production but did not affect the amount of ClO 4 – produced, confirming previous reports that free water will promote additional ClO 3 – but not ClO 4 – production pathways. Additionally, oxidation of HCl (g) produced ClO 4 – at higher yields than oxidation of NaCl but produced less ClO 3 – . Our findings suggest that sufficient O 3 saturation and availability of active sites are essential for heterogeneous formation of ClO 4 – and ClO 3 – . While glass surfaces per se are not relevant to environmental production, catalytic surfaces (silicate or others) abound in terrestrial and extraterrestrial environments. The Cl – form oxidized and amount of water vapor present will also significantly impact the ClO 4 – /ClO 3 – ratio, which could be helpful in evaluating the sources of ClO 4 – and ClO 3 – in extraterrestrial material, with important implications on the availability of water during formation.