The role of contact angle and pore width on pore condensation and freezing
<p>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 inverse Kelvin effect, followed by eith...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/20/9419/2020/acp-20-9419-2020.pdf |
| Summary: | <p>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 inverse 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 for the
pore diameters (2.2–9.2 nm) and water contact angles (15–78<span class="inline-formula"><sup>∘</sup></span>) covered in this study, our results reveal that the water 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
<span class="inline-formula"><i>T</i>>235</span> K and below water saturation, pore diameters and water
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 in 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.</p> |
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| ISSN: | 1680-7316 1680-7324 |