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Discussion papers
https://doi.org/10.5194/acp-2019-1019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 21 Nov 2019

Submitted as: research article | 21 Nov 2019

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

The Role of Contact Angle and Pore Width on Pore Condensation and Freezing

Robert O. David1,a, Jonas Fahrni2, Claudia Marcolli1, Fabian Mahrt1, Dominik Brühwiler2, and Zamin A. Kanji1 Robert O. David et al.
  • 1Institute for Atmospheric and Climate Science, ETH Zürich, 8092 Zürich, Switzerland
  • 2Institute of Chemistry and Biotechnology, Zürich University of Applied Sciences (ZHAW), 8820 Wädenswil, Switzerland
  • anow at: Department of Geosciences, University of Oslo, Oslo, 0315, Norway

Abstract. It has recently been shown that pore condensation and freezing (PCF) is a mechanism responsible for ice formation under cirrus cloud conditions. PCF is defined as the condensation of liquid water in narrow capillaries below water saturation due to the Kelvin effect, followed by either heterogeneous or homogeneous nucleation depending on the temperature regime and presence of an ice nucleating active site. By using sol-gel synthesized silica with well-defined pore diameters, morphology and distinct chemical surface-functionalization, the role of the water-silica contact angle and pore width on PCF is investigated. We find that contact angle and pore width play an important role in determining the relative humidity required for capillary condensation as predicted by the Kelvin effect and subsequent ice nucleation at cirrus temperatures. For the pore diameters and contact angles covered in this study, 2.2–9.2 nm and 15–78°, respectively, our results reveal that the contact angle plays an important role in predicting the humidity required for pore filling while the pore diameter determines the ability of pore water to freeze. For T > 235 K and below water saturation, pore diameters and contact angles were not able to predict the freezing ability of the particles suggesting an absence of active sites, thus ice nucleation did not proceed via a PCF mechanism. Rather, the ice nucleating ability of the particles depended solely on chemical functionalization. Therefore, parameterizations for the ice nucleating abilities of particles at cirrus conditions should differ from parameterizations at mixed-phase clouds conditions. Our results support PCF as the atmospherically relevant ice nucleation mechanism below water saturation when porous surfaces are encountered in the troposphere.

Robert O. David et al.
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Latest update: 07 Dec 2019
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Short summary
Ice crystal formation plays an important role in controlling the Earth's climate. However, the mechanisms responsible for ice formation in the atmosphere are still uncertain. Here we use surrogates for atmospherically relevant porous particles to determine the role of pore diameter and wettability on the ability of porous particles to nucleate ice in the atmosphere. Additionally, our results are consistent with the pore condensation and freeing mechanism.
Ice crystal formation plays an important role in controlling the Earth's climate. However, the...
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