Observation of HClO3 and HClO4 in the Arctic atmosphere

Atmospheric halogens chemistry like the catalytic reaction of bromine and chlorine radicals with ozone (O₃) has been known to cause the springtime surface-ozone destruction in the polar region. Although the initial atmospheric reactions of chlorine with ozone are well understood, the final oxidation steps leading to the formation of chlorate (ClO₃^-) and perchlorate (ClO₄^-) remain unclear due to the lack of direct evidence of their presence and fate in the atmosphere. In this study, we present the first high-resolution ambient data set of gas-phase HClO₃ (chloric acid) and HClO₄ (perchlorate acid) obtained from the field measurement at the Villum Research Station, Station Nord, in high arctic North Greenland (81°36’ N, 16°40’ W) during the spring of 2015. A state-of-the-art chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-TOF) was used in negative ion mode with nitrate ion as the reagent ion to detect the gas-phase HClO₃ and HClO₄. We measured significant level of HClO₃ and HClO₄ only during the springtime ozone depletion events in the Greenland, with concentration up to 9x10⁵ molecule cm^-3. Air mass trajectory analysis shows that the air during the ozone depletion event was confined to near-surface, indicating that the O₃ and surface of sea-ice/snowpack may play important roles in the formation of HClO₃ and HClO₄. We used high-level quantum-chemical methods to calculate the ultraviolet-visible absorption spectra and cross-section of HClO₃ and HClO₄ in the gas-phase to assess their fates in the atmosphere. Overall, our results reveal the presence of HClO₃ and HClO₄ during ozone depletion events, which could affect the chlorine chemistry in the Arctic atmosphere.