Abstract
Abstract. Pore condensation and freezing (PCF) is an ice nucleation mechanism that explains ice formation at low ice supersaturation. It assumes that liquid water condenses in pores of solid aerosol particles below water saturation, as described by the Kelvin equation, followed by homogeneous ice nucleation when temperatures are below about 235 K or immersion freezing at higher temperatures, in case the pores contain active sites that induce ice nucleation. Porewater is under tension (negative pressure) below water saturation as described by the Young–Laplace equation. This negative pressure affects the ice nucleation rates and the stability of the pore ice. Here, pressure-dependent parameterizations of classical nucleation theory are developed to quantify the increase in homogeneous ice nucleation rates as a function of tension and to assess the critical diameter of pores that is required to accommodate ice at negative pressures. Growth of ice out of the pore into a macroscopic ice crystal requires ice supersaturation. This supersaturation as a function of the pore opening width is derived, assuming that the ice phase first grows as a spherical cap on top of the pore opening before it starts to expand laterally on the particle surface into a macroscopic ice crystal.
Highlights
Cirrus are high-altitude ice clouds that influence the Earth’s climate by reflecting incoming solar short-wave radiation and regulating long-wave emissions to space, resulting in a net warming effect (Stephens et al, 1990; Lohmann et al, 2008; Kärcher, 2017; Matus and l’Ecuyer, 2017)
The homogeneous ice nucleation threshold (HNT) at about 235 K, they can form through homogeneous ice nucleation (IN) in diluting liquid aerosol particles at relatively high ice supersaturation along the homogeneous freezing line of solution droplets (Koop et al, 2000) or heterogeneously at lower ice supersaturation aided by ice nucleating particles (INPs), which may induce freezing through immersion nucleation when coated with watersoluble material (Kärcher and Lohmann, 2003; Kuebbeler et al, 2014)
The conditions derived for ice nucleation within pores and growth of ice out of pores show that porous particles are able www.atmos-chem-phys.net/20/3209/2020/
Summary
Cirrus are high-altitude ice clouds that influence the Earth’s climate by reflecting incoming solar short-wave radiation and regulating long-wave emissions to space, resulting in a net warming effect (Stephens et al, 1990; Lohmann et al, 2008; Kärcher, 2017; Matus and l’Ecuyer, 2017). They vary in optical thickness and vertical extent depending on the atmospheric conditions and their formation mechanism (Kärcher, 2017; Kienast-Sjögren et al, 2016).
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