| Summary: | The surfaces of ice crystals near the melting point are covered with thin liquid water layers, called quasi-liquid layers (QLLs), which play crucial roles in various chemical reactions in nature. So far, there have been many spectroscopic studies of such chemical reactions on ice surfaces, however, revealing the effects of atmospheric gases on ice surfaces remains an experimental challenge. In this study, we chose HNO<sub>3</sub> as a model atmospheric gas, and directly observed the ice basal faces by advanced optical microscopy under partial pressure of HNO<sub>3</sub> (~10<sup>−4</sup> Pa), relevant to those found in the atmosphere. We found that droplets (HNO<sub>3</sub>-QLLs) appeared on ice surfaces at temperatures ranging from −0.9 to −0.2 °C with an increase in temperature, and that they disappeared at temperatures ranging from −0.6 to −1.3 °C with decreasing temperature. We also found that the size of the HNO<sub>3</sub>-QLLs decreased immediately after we started reducing the temperature. From the changes in size and the liquid−solid phase diagram of the HNO<sub>3</sub>-H<sub>2</sub>O binary system, we concluded that the HNO<sub>3</sub>-QLLs did not consist of pure water, but rather aqueous HNO<sub>3</sub> solutions, and that the temperature and HNO<sub>3</sub> concentration of the HNO<sub>3</sub>-QLLs also coincided with those along a liquidus line.
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