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Abstract. Heterogeneous ice nucleation is thought to be the primary pathway for the formation of ice in mixed-phase clouds, with the number of active ice-nucleating particles (INPs) increasing rapidly with decreasing temperature. Here, molecular-dynamics simulations of heterogeneous ice nucleation demonstrate that the ice nucleation rate is also sensitive to pressure and that negative pressure within supercooled water shifts freezing temperatures to higher temperatures. Negative pressure, or tension, occurs naturally in water capillary bridges and pores and can also result from water agitation. Capillary bridge simulations presented in this study confirm that negative Laplace pressure within the water increases heterogeneous-freezing temperatures. The increase in freezing temperatures with negative pressure is approximately linear within the atmospherically relevant range of 1 to −1000 atm. An equation describing the slope depends on the latent heat of freezing and the molar volume difference between liquid water and ice. Results indicate that negative pressures of −500 atm, which correspond to nanometer-scale water surface curvatures, lead to a roughly 4 K increase in heterogeneous-freezing temperatures. In mixed-phase clouds, this would result in an increase of approximately 1 order of magnitude in active INP concentrations. The findings presented here indicate that any process leading to negative pressure in supercooled water may play a role in ice formation, consistent with experimental evidence of enhanced ice nucleation due to surface geometry or mechanical agitation of water droplets. This points towards the potential for dynamic processes such as contact nucleation and droplet collision or breakup to increase ice nucleation rates through pressure perturbations.
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Cleaning up our water: reducing interferences from nonhomogeneous freezing of “pure” water in droplet freezing assays of ice-nucleating particles
Abstract. Droplet freezing techniques (DFTs) have been used for half a century tomeasure the concentration of ice-nucleating particles (INPs) in the atmosphereand determine their freezing properties to understand the effects of INPs onmixed-phase clouds. The ice nucleation community has recently adopted dropletfreezing assays as a commonplace experimental approach. These dropletfreezing experiments are often limited by contamination that causesnonhomogeneous freezing of the “pure” water used to generate the dropletsin the heterogeneous freezing temperature regime that is being measured.Interference from the early freezing of water is often overlooked and notfully reported, or measurements are restricted to analyzing the moreice-active INPs that freeze well above the temperature of the backgroundwater. However, this avoidance is not viable for analyzing the freezingbehavior of less active INPs in the atmosphere that still have potentiallyimportant effects on cold-cloud microphysics. In this work we review a numberof recent droplet freezing techniques that show great promise in reducing theseinterferences, and we report our own extensive series of measurements usingsimilar methodologies. By characterizing the performance of differentsubstrates on which the droplets are placed and of different pure watergeneration techniques, we recommend best practices to reduce theseinterferences. We tested different substrates, water sources, dropletmatrixes, and droplet sizes to provide deeper insight into what methodologiesare best suited for DFTs. Approaches for analyzing droplet freezingtemperature spectra and accounting and correcting for the background “pure”water control spectrum are also presented. Finally, we propose experimentaland data analysis procedures for future homogeneous and heterogeneous icenucleation studies to promote a more uniform and reliable methodology thatfacilitates the ready intercomparison of ice-nucleating particles measured byDFTs.
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- Award ID(s):
- 1554941
- PAR ID:
- 10164336
- Date Published:
- Journal Name:
- Atmospheric Measurement Techniques
- Volume:
- 11
- Issue:
- 9
- ISSN:
- 1867-8548
- Page Range / eLocation ID:
- 5315 to 5334
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/amt-11-5315-2018
