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 environmentNational Natural Science Foundation of China (42175118)European Research Council Executive Agency under the European Union’s Horizon 2020 Research and Innovation Program (Project ERC-2016-COG726349 CLIMAHAL, ERC-StG GASPARCON - grant agreement no. 714621The EMME-CARE project – grant agreement no. 856612, and grant no. 101002728, ATM-GTP Contract No. 742206)Climate Relevant interactions and feedbacks: the key role of sea ice and Snow in the polar and global climate system(CRiceS, grant number 101003826)Horizon Europe project Non-CO2 Forcers and their Climate, Weather, Air Quality and Health Impacts, (FOCI, project number 101056783)ACCC Flagship funded by the Academy of Finland grant no. 337549Academy professorship funded by the Academy of Finland (grant no. 302958Academy of Finland (project nos. 296628, 328290, 311932, 316114, 325647, 325681, 334792, 337549, 329274, 333397, 328616, 1325656, 347782, 349659, and 334514)Academy of Finland (project nos. 296628, 328290, 311932, 316114, 325647, 325681, 334792, 337549, 329274, 333397, 328616, 1325656, 347782, 349659, and 334514)United States National Science Foundation (grant nos. 1807496 and 1807163),Swiss National Science Foundation (grant 200021_188478)Swiss Polar Institute (grant no. DIRCR-2018-004The Ingvar Kamprad Chair for Extreme Environments Research, sponsored by Ferring PharmaceuticalsLa Caixa” Foundation (ID 100010434) for the fellowship LCF/BQ/DR20/11790027Spanish “Ministerio de Ciencia e Innovación (MICINN)” for funding (Project ref. CTQ2017-87054-C2-2-P,Unit of Excellence María de Maeztu CEX2019-000919-M and “Ramón y Cajal” grant RYC- 2015-19234International Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition with the MOSAiC20192020 tag, with activities supported by Polarstern76 expedition AWI_PS122_00National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory (ARL) for the provision of the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) transport and dispersion model and/or Real-time Environmental Applications and Display sYstem (READY