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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Nov 2018

Research article | 05 Nov 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Unravelling the microphysics of polar mesospheric cloud formation

Denis Duft1, Mario Nachbar1, and Thomas Leisner1,2 Denis Duft et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, 76021, Germany
  • 2Institute of Environmental Physics, University of Heidelberg, Heidelberg, 69120, Germany

Abstract. Polar mesospheric clouds are the highest water ice clouds occurring in the terrestrial atmosphere. They form in the polar summer mesopause, the coldest region in the atmosphere. It has long been assumed that these clouds form by heterogeneous nucleation on meteoric smoke particles which are the remnants of material ablated from meteoroids in the upper atmosphere. However, until now little was known about the properties of these nm-size particles and application of the classical theory for heterogeneous ice nucleation was impacted by large uncertainties. In this work, we performed laboratory measurements on the heterogeneous ice formation process at mesopause conditions on small (r=1 to 3nm) iron-silicate nano-particles serving as meteoric smoke analogues. We observe, that ice growth on these particles sets in for saturation ratios with respect to hexagonal ice below Sh=50, a value that is commonly exceeded during NLC season, affirming meteoric smoke particles as likely nuclei for heterogeneous ice formation in mesospheric clouds. We find that ice formation on iron-silicate nano-particles occurs by condensation of amorphous solid water rather than by nucleation of crystalline ice and that amorphous solid water has to be considered as a relevant ice polymorph in polar mesospheric cloud formation. We present a simple ice activation model that takes into account the water affinity of iron-silicates of various compositions. For particles with radius larger than r=1nm no significant effect of the nano-particle charge on the activation threshold was found.

Denis Duft et al.
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Denis Duft et al.
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Short summary
The formation of ice on meteoric smoke particles in polar mesospheric clouds is not well understood. We measured the water adsorption and onset conditions for ice formation on meteoric smoke analogues in the laboratory. We find that the particles are highly water-affine which enhances their ice formation ability substantially. As a result, we propose a new ice-activation model for mesospheric clouds. We also show that the particle charge does not play a significant role in this process.
The formation of ice on meteoric smoke particles in polar mesospheric clouds is not well...