Widespread detection of chlorine oxyacids in the Arctic atmosphere

Chlorine radicals are strong atmospheric oxidants known to play an important role in the depletion of surface ozone and the degradation of methane in the Arctic troposphere. Initial oxidation processes of chlorine produce chlorine oxides, and it has been speculated that the final oxidation steps lead to the formation of chloric (HClO₃) and perchloric (HClO₄) acids, although these two species have not been detected in the atmosphere. Here, we present atmospheric observations of gas-phase HClO₃ and HClO₄. Significant levels of HClO₃ were observed during springtime at Greenland (Villum Research Station), Ny-Ålesund research station and over the central Arctic Ocean, on-board research vessel Polarstern during the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) campaign, with estimated concentrations up to 7 × 10⁶ molecule cm^-3. The increase in HClO₃, concomitantly with that in HClO₄, was linked to the increase in bromine levels. These observations indicated that bromine chemistry enhances the formation of OClO, which is subsequently oxidized into HClO₃ and HClO₄ by hydroxyl radicals. HClO₃ and HClO₄ are not photoactive and therefore their loss through heterogeneous uptake on aerosol and snow surfaces can function as a previously missing atmospheric sink for reactive chlorine, thereby reducing the chlorine-driven oxidation capacity in the Arctic boundary layer. Our study reveals additional chlorine species in the atmosphere, providing further insights into atmospheric chlorine cycling in the polar environment.