11:00 〜 13:00
[MIS12-P02] 中間圏における核生成による氷粒子の形成
キーワード：核生成、雲、中間圏、氷、夜光雲
The summer polar mesopause region, located at altitudes of 80–90 km, is the coldest part of the Earth’s atmosphere. Clouds of ice particles can form at such heights, some of which are visible from the ground and are referred to as noctilucent clouds [1]. Although the mean temperature is between 128 K at the mesopause and 150 K at 82km, the minimum observed temperatures are close to 100 K [2]. Noctilucent clouds are related to the presence of water ice particles. The ice particles observed in noctilucent clouds comprise particles that are typically tens of nanometers in size, which are large enough to scatter light effectively and therefore, can be detected using a variety of optical remote sensing methods [3].
To explain the formation of clouds in the mesosphere, there are two possibilities: homogeneous and heterogeneous nucleations. We tested these two mechanisms theoretically. For homogeneous nucleation, we used the SP (semi phenomenological ) model, which agreed with the experiments and molecular dynamics simulations [4-6]. The different nucleation models produce large differences in the nucleation process, mainly regarding the nucleation temperature. Using the nucleation rate obtained from the SP model, we calculated the time evolution of the number of water molecules and ice particle growth. Compared to the CNT model, the nucleation temperature was very low. At an initial temperature of 135 K, the ice nucleation temperature was very low, ranging from 50 to 80 K. The nucleation temperature for homogeneous nucleation is far below 100 K, and therefore, below typically observed temperatures. If the cooling rate was slower than 10-5 K s-1, then the nucleation temperature was above 100 K. However, the cooling time in the mesosphere is a few days at most; thus, the cooling rate will not be that slow. Therefore, the potential for homogeneous nucleation in the mesosphere is considered to be very small, although previous studies have suggested that homogeneous nucleation can occur [7].
We also determined the conditions at which heterogeneous nucleation occurs and compared them with observational data. Our results indicate that heterogeneous nucleation occurs effectively in the mesosphere. Because dust from micrometeorites is present at this altitude, heterogeneous nucleation using fragments of micrometeorites as nuclei is considered to occur significantly.
Our study also shows that during the deposition process, the ice can form directly in crystalline state rather than amorphous state. The phase of ice particles in polar mesospheric clouds (PMCs) was determined using observations of the infrared extinction of the mesosphere from the Solar Occultation for Ice Experiment (SOFIE) on the AIM satellite [8]. The observations could be explained using refractive indices of crystalline ice as opposed to amorphous ice; hence suggesting that not amorphous ice particles but rather particles of crystalline ice existed near the mesopause. This observational result is consistent with our theoretical results that the nucleation leads to the formation of crystalline ice.
Acknowledgements: This work was supported in part by JSPS KAKENHI Grants No. 20H05657, 19K03941, and 18K03689 and by Re-355search Council of Norway Grant No. 275503.
[1] Vaste, O.: Noctilucent clouds, Journal of Atmospheric and Terrestrial Physics, 55, 133–143, 1993.
[2] Lübken, F.-J.: Thermal structure of the Arctic summer mesosphere, Geophys. Res. Let., 104, 9135–9149,1999.
[3] Thomas, G. E. and McKay, C. P.: On the mean particle size and water content of polar mesospheric clouds, Planet. Space Sci., 33, 1209–1224, 1985.
[4] Dillmann, A. and Meier, G. E. A.: A refined droplet approach to the problem of homogeneous nucleation from the vapor phase, J. Chem. Phys., 94, 3872–3884, 1991.
[5] Tanaka, K. K., Kawano, A., and Tanaka, H.: Molecular dynamics simulations of the nucleation of water: Determining the sticking probability and formation energy of a cluster, J. Chem. Phys., 140, 114 302(1–10), 2014.
[6] Angelil, R., Diemand, J., and Tanaka, K. K.and Tanaka, H.: Homogeneous SPC/E water nucleation in large molecular dynamics simulations, J. Chem. Phys., 143, 064 507, 2015.
[7] Murray, B. J. and Jensen, E.: Homogeneous nucleation of amorphous solid water particles in the upper mesosphere, J. of Atmos. and Solar-Terrestrial Phys., 72, 51–61,2010.
[8] Hervig, M. E. and Gordley, L. L.: Temperature, shape, and phase of mesospheric ice from Solar Occultation for Ice Experiment observations, J. Geophys. Res., 115, D15 208(1–9), 2010.
To explain the formation of clouds in the mesosphere, there are two possibilities: homogeneous and heterogeneous nucleations. We tested these two mechanisms theoretically. For homogeneous nucleation, we used the SP (semi phenomenological ) model, which agreed with the experiments and molecular dynamics simulations [4-6]. The different nucleation models produce large differences in the nucleation process, mainly regarding the nucleation temperature. Using the nucleation rate obtained from the SP model, we calculated the time evolution of the number of water molecules and ice particle growth. Compared to the CNT model, the nucleation temperature was very low. At an initial temperature of 135 K, the ice nucleation temperature was very low, ranging from 50 to 80 K. The nucleation temperature for homogeneous nucleation is far below 100 K, and therefore, below typically observed temperatures. If the cooling rate was slower than 10-5 K s-1, then the nucleation temperature was above 100 K. However, the cooling time in the mesosphere is a few days at most; thus, the cooling rate will not be that slow. Therefore, the potential for homogeneous nucleation in the mesosphere is considered to be very small, although previous studies have suggested that homogeneous nucleation can occur [7].
We also determined the conditions at which heterogeneous nucleation occurs and compared them with observational data. Our results indicate that heterogeneous nucleation occurs effectively in the mesosphere. Because dust from micrometeorites is present at this altitude, heterogeneous nucleation using fragments of micrometeorites as nuclei is considered to occur significantly.
Our study also shows that during the deposition process, the ice can form directly in crystalline state rather than amorphous state. The phase of ice particles in polar mesospheric clouds (PMCs) was determined using observations of the infrared extinction of the mesosphere from the Solar Occultation for Ice Experiment (SOFIE) on the AIM satellite [8]. The observations could be explained using refractive indices of crystalline ice as opposed to amorphous ice; hence suggesting that not amorphous ice particles but rather particles of crystalline ice existed near the mesopause. This observational result is consistent with our theoretical results that the nucleation leads to the formation of crystalline ice.
Acknowledgements: This work was supported in part by JSPS KAKENHI Grants No. 20H05657, 19K03941, and 18K03689 and by Re-355search Council of Norway Grant No. 275503.
[1] Vaste, O.: Noctilucent clouds, Journal of Atmospheric and Terrestrial Physics, 55, 133–143, 1993.
[2] Lübken, F.-J.: Thermal structure of the Arctic summer mesosphere, Geophys. Res. Let., 104, 9135–9149,1999.
[3] Thomas, G. E. and McKay, C. P.: On the mean particle size and water content of polar mesospheric clouds, Planet. Space Sci., 33, 1209–1224, 1985.
[4] Dillmann, A. and Meier, G. E. A.: A refined droplet approach to the problem of homogeneous nucleation from the vapor phase, J. Chem. Phys., 94, 3872–3884, 1991.
[5] Tanaka, K. K., Kawano, A., and Tanaka, H.: Molecular dynamics simulations of the nucleation of water: Determining the sticking probability and formation energy of a cluster, J. Chem. Phys., 140, 114 302(1–10), 2014.
[6] Angelil, R., Diemand, J., and Tanaka, K. K.and Tanaka, H.: Homogeneous SPC/E water nucleation in large molecular dynamics simulations, J. Chem. Phys., 143, 064 507, 2015.
[7] Murray, B. J. and Jensen, E.: Homogeneous nucleation of amorphous solid water particles in the upper mesosphere, J. of Atmos. and Solar-Terrestrial Phys., 72, 51–61,2010.
[8] Hervig, M. E. and Gordley, L. L.: Temperature, shape, and phase of mesospheric ice from Solar Occultation for Ice Experiment observations, J. Geophys. Res., 115, D15 208(1–9), 2010.