This study reveals the presence of gas-phase chloric and perchloric acids in the Arctic atmosphere.

Chlorine radicals are strong atmospheric oxidants. Initial oxidation processes of chlorine produce chlorine oxides, and it has long been speculated that the final oxidation steps lead to the formation of HClO3 and HClO4, although these two species were not detected in the atmosphere until now. The observations indicate that the formation of these two acids is linked to the increase in bromine levels in the Arctic spring. We proposed a formation mechanism by which bromine mediates the formation of OClO, which is subsequently oxidised into HClO3 and HClO4 by hydroxyl radicals. HClO3 and HClO4 are not photoactive and therefor their loss via uptake on aerosol and snow surfaces can function as a sink for reactive chlorine, thereby reducing the chlorine-driven oxidation capacity in the Arctic atmosphere. Yee Jun Tham, Nina Sarnela, Siddharth Iyer, Qinyi Li, Hélène Angot, Lauriane L. J. Quéléver, Ivo Beck, Tiia Laurila, Lisa J. Beck, Matthew Boyer, Javier Carmona-García, Ana Borrego-Sánchez, Daniel Roca-Sanjuán, Otso Peräkylä, Roseline C. Thakur, Xu-Cheng He, Qiaozhi Zha, Dean Howard, Byron Blomquist, Stephen Archer, Ludovic Bariteau, Kevin Posman, Jacques Hueber, Detlev Helmig, Hans-Werner Jacobi, Heikki Junninen, Markku Kulmala, Anoop S. Mahajan, Andreas Massling, Henrik Skov, Mikko Sipilä, Joseph S. Francisco, Julia Schmale, Tuija Jokinen and Alfonso Saiz-Lopez. 2023. Widespread detection of chlorine oxyacids in the Arctic atmosphere. Nature Communications, https://doi.org/10.1038/s41467-023-37387-y