[M-IS17] 結晶成⻑、溶解における界⾯・ナノ現象

[MIS17-01] Nucleation of water particles in the mesosphere

*田中 今日子^1,2、木村 勇気³、Mann Ingrid⁴ (1.東北大学、2.日本大学、3.低温科学研究所、北海道大学、4.UiT the Arctic University of Norway)

キーワード：水、凝縮、核生成、中間圏、氷

In the summer polar mesosphere at the altitude of 80-90 km, which is the coldest part of the earth’s atmosphere, small ice particles can form. They are observed from satellites, they form radar echoes observed from ground and they can form a dawn cloud from mid latitudes to the North, referred as a nuctilucent cloud (NLC) [1]. Although the condensed ice particles are known to exist in the high altitude mesosphere, the mechanism of condensation remains uncertain. In this study, we investigate a process of non-equilibrium condensation from dilute water vapor in relation to the origin of the NLCs. The previous study showed that amorphous solid water particles can nucleate homogeneously under conditions in the mesosphere using classical nucleation theory (CNT) [2]. However, it is known that rate predictions of water in the CNT disagree with experimental measurements by several order of magnitude [3]. In this study, we focus on the nucleation process and consider two possibilities of the homogeneous and heterogeneous nucleations. We adopt a semi-phenomenological model for nucleation process which corrects the evaluation of the formation energy of a molecular cluster with the use of the second virial coefficient and agrees with the results of both experiments and molecular dynamics simulations [3-5]. The condensation process depends on the atmospheric conditions such as temperature, pressure, number density of dust grains, and cooling rate. We find that the homogeneous nucleation of water is extremely difficult to occur in the mesosphere. The results show that the number density of dust grains necessary for heterogeneous nucleation is much lower than the observed value for the cooling rate smaller than 0.1 K/s. Our results imply that the heterogeneous nucleation occurs effectively in the upper mesosphere and the dust particles from meteoroids would work as nuclei of heterogeneous nucleation because meteoroids ablate at the high upper altitude in the mesosphere.



[1] T. Antonsen , O. Havnes , and I. Mann, Journal of Geophysical Research: Atmospheres, 122, 12,353, 2017

[2] B. J. Murray and E. J. Jensen, Journal of Atmospheric and Solar-Terrestrial Physics 72, 51, 2010

[3] A.Dillmann and G.E.A.Meier, Journal of Chemical Physics, 94, 3872, 1991

[4] K.K. Tanaka, A. Kawano, and H. Tanaka, Journal of Chemical Physics, 140, 114302, 2014

[5] R. Angelil, J. Diemand, K. K. Tanaka, and H. Tanaka, Journal of Chemical Physics, 143, 064507, 2015