[AAS07-P28] Nitric acid gas captured in quasi-liquid layers on ice surfaces
Keywords:Ice, quasi-liquid layer, nitric acid gas, optical microscopy
Ice crystal surfaces act as “reaction fields” for heterogeneous chemical reactions that involve atmospheric acidic gases, thereby causing serious environmental issues such as the catalytic ozone depletion by hydrogen chloride gas and the generation of nitrogen oxide gases (NOx) from the photolysis of nitric acid/nitrates. These chemical reactions cannot be explained solely by homogeneous processes. In addition, the surfaces of ice crystals near the melting point are covered with thin liquid water layers, called quasi-liquid layers (QLLs), which may play crucial roles in various chemical reactions. In this study, we chose HNO3 as a model atmospheric gas, and directly observed the QLLs on ice basal faces by advanced optical microscopy [1]. Because PHNO3 in the troposphere shows considerable variations (e.g., ranging from ~10-6 Pa in clean air to ~10-2 Pa in polluted urban air [2,3]), the observations in this study were performed under some PHNO3 conditions (0, 10-4 and 10−2 Pa).
Irrespective of the presence/absence of the HNO3 gas, the pure-QLLs and HNO3-QLLs appeared with increasing temperature and disappeared with decreasing temperature. The shape of pure-/HNO3-QLLs showed spherical dome and the contact angle of them on the ice basal face was ~1°. The appearance temperatures of the pure-/HNO3-QLLs were not so different (-1.9 and -0.5 to -1.8 °C, respectively). Although the disappearance temperature of pure-QLLs (-2.2 °C) was almost same as the appearance temperature, the disappearance temperature of the HNO3-QLLs (-6.4 °C) was significantly lower than the appearance temperature of them under high-PHNO3 condition (10-2 Pa). The large thermal hysteresis between the appearance and disappearance temperatures suggests that the disappearance mechanisms of the pure-/HNO3-QLLs were different. We found that the HNO3-QLLs are not composed of pure water, but rather of aqueous HNO3 solutions, and also that the HNO3-QLL and the ice crystal were in equilibrium. The evidence and the disappearance mechanism are showed as follows.
The size of the HNO3-QLLs decreased immediately after we started reducing the temperature. We could not observe such changes in the sizes of pure-QLL with temperature in the absence of the HNO3 gas. When we assumed that the mass of HNO3 in the HNO3-QLL was constant during the relatively short observation time period, the volume reduction of the HNO3-QLLs with decreasing temperature meant increasing of HNO3 concentration of the HNO3-QLL. We calculated the volume reductions as a function of temperature by using the HNO3-H2O phase diagram. The calculated volume reductions were in good agreement with the volume reductions determined experimentally.
One of the plausible causes for the disappearance of HNO3-QLLs could be the evaporation of HNO3 from the HNO3-QLLs. We calculated the equilibrium HNO3 partial vapor pressure, Pe(HNO3), of the HNO3-QLLs. As temperature decreases, Pe(HNO3) increases. It is very reasonable to expect that when Pe(HNO3) exceeds PHNO3 in the observation chamber with decreasing temperature, HNO3 evaporates from the HNO3-QLLs, resulting in the disappearance of the HNO3-QLLs.
Recently, we studied the effects of hydrogen chloride gas on the behavior of QLLs (HCl-QLLs) on ice basal faces [4,5]. We found that the HCl-QLLs were also aqueous hydrochloric acid solution, and that the temperature and HCl concentration of the HCl-QLLs were also very close to those of a liquidus line: these results were similar to those found in this study. Therefore, ice crystal surfaces would capture large amount of acidic gas components in the acidic-QLLs.
[1] Nagashima et al. (2020) Crystals 10, 72.
[2] Goldan et al. (1983) Atmos. Environ. 17, 1355.
[3] Hanke et al. (2003) Atmos. Chem. Phys. 3, 417.
[4] Nagashima et al. (2016) Cryst. Growth Des. 16, 2225.
[5] Nagashima et al. (2018) Cryst. Growth Des. 18, 4117.
The details of this study are shown in our paper [1]. We will present this study from other viewpoints in “A-CC39 Glaciology” and "M-IS23: Growth and dissolution of crystal" sessions.
Irrespective of the presence/absence of the HNO3 gas, the pure-QLLs and HNO3-QLLs appeared with increasing temperature and disappeared with decreasing temperature. The shape of pure-/HNO3-QLLs showed spherical dome and the contact angle of them on the ice basal face was ~1°. The appearance temperatures of the pure-/HNO3-QLLs were not so different (-1.9 and -0.5 to -1.8 °C, respectively). Although the disappearance temperature of pure-QLLs (-2.2 °C) was almost same as the appearance temperature, the disappearance temperature of the HNO3-QLLs (-6.4 °C) was significantly lower than the appearance temperature of them under high-PHNO3 condition (10-2 Pa). The large thermal hysteresis between the appearance and disappearance temperatures suggests that the disappearance mechanisms of the pure-/HNO3-QLLs were different. We found that the HNO3-QLLs are not composed of pure water, but rather of aqueous HNO3 solutions, and also that the HNO3-QLL and the ice crystal were in equilibrium. The evidence and the disappearance mechanism are showed as follows.
The size of the HNO3-QLLs decreased immediately after we started reducing the temperature. We could not observe such changes in the sizes of pure-QLL with temperature in the absence of the HNO3 gas. When we assumed that the mass of HNO3 in the HNO3-QLL was constant during the relatively short observation time period, the volume reduction of the HNO3-QLLs with decreasing temperature meant increasing of HNO3 concentration of the HNO3-QLL. We calculated the volume reductions as a function of temperature by using the HNO3-H2O phase diagram. The calculated volume reductions were in good agreement with the volume reductions determined experimentally.
One of the plausible causes for the disappearance of HNO3-QLLs could be the evaporation of HNO3 from the HNO3-QLLs. We calculated the equilibrium HNO3 partial vapor pressure, Pe(HNO3), of the HNO3-QLLs. As temperature decreases, Pe(HNO3) increases. It is very reasonable to expect that when Pe(HNO3) exceeds PHNO3 in the observation chamber with decreasing temperature, HNO3 evaporates from the HNO3-QLLs, resulting in the disappearance of the HNO3-QLLs.
Recently, we studied the effects of hydrogen chloride gas on the behavior of QLLs (HCl-QLLs) on ice basal faces [4,5]. We found that the HCl-QLLs were also aqueous hydrochloric acid solution, and that the temperature and HCl concentration of the HCl-QLLs were also very close to those of a liquidus line: these results were similar to those found in this study. Therefore, ice crystal surfaces would capture large amount of acidic gas components in the acidic-QLLs.
[1] Nagashima et al. (2020) Crystals 10, 72.
[2] Goldan et al. (1983) Atmos. Environ. 17, 1355.
[3] Hanke et al. (2003) Atmos. Chem. Phys. 3, 417.
[4] Nagashima et al. (2016) Cryst. Growth Des. 16, 2225.
[5] Nagashima et al. (2018) Cryst. Growth Des. 18, 4117.
The details of this study are shown in our paper [1]. We will present this study from other viewpoints in “A-CC39 Glaciology” and "M-IS23: Growth and dissolution of crystal" sessions.