Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities
Heterogeneous ice nucleation is an important mechanism for the glaciation of mixed phase clouds and may also be relevant for cloud formation and dehydration at the cirrus cloud level. It is thought to proceed through different mechanisms, namely contact, condensation, immersion and deposition nucleation. Conceptually, deposition nucleation is the only pathway that does not involve liquid water, but occurs by direct water vapor deposition onto a surface. This study challenges this classical view by putting forward the hypothesis that what is called deposition nucleation is in fact pore condensation and freezing (PCF) occurring in voids and cavities that may form between aggregated primary particles and host water at relative humidity RH w < 100% because of the inverse Kelvin effect. Homogeneous ice nucleation is expected to occur below 235 K when at least one pore is filled with water. Ice nucleation in pores may also happen in immersion mode but with a lower probability because it requires at least one active site in a water filled pore. Therefore a significant enhancement in ice nucleation efficiency is expected when temperature falls below 235 K. For a deposition nucleation process from water vapor no discontinuous change in ice nucleation efficiency should occur at T = 235 K because no liquid water is involved in this process. Studies on freezing in confinement carried out on mesoporous silica materials such as SBA-15, SBA-16, MCM-41, zeolites and KIT have shown that homogeneous ice nucleation occurs abruptly at T = 230–235 K in pores with diameters ( D) of 3.5–4 nm or larger but only gradually at T = 210–230 K in pores with D = 2.5–3.5 nm. Pore analysis of clay minerals shows that kaolinites exhibit pore structures with pore diameters ( Dp) of 20–50 nm. The mesoporosity of illites and montmorillonites is characterized by pores with Dp = 2–5 nm. The number and size of pores is distinctly increased in acid treated montmorillonites like K10. Water adsorption isotherms of MCM-41 show that pores with Dp = 3.5–4 nm fill with water at RH w = 56–60% in accordance with an inverse Kelvin effect. Water in such pores should freeze homogeneously for T < 235 K even before relative humidity with respect to ice (RH i) reaches ice saturation. Ice crystal growth by water vapor deposition from the gas phase is therefore expected to set in as soon as RH i > 100%. Pores with D > 7.5 nm fill with water at RH i > 100% for T < 235 K and are likely to freeze homogeneously as soon as they are filled with water. Given the pore structure of clay minerals, PCF should be highly efficient for T < 235 K and may occur at T > 235 K in particles that exhibit active sites for immersion freezing within pores. Most ice nucleation studies on clay minerals and mineral dusts indeed show a strong increase in ice nucleation efficiency when temperature is decreased below 235 K in accordance with PCF and are not explicable by the classical view of deposition nucleation. PCF is probably also the prevailing ice nucleation mechanism below water saturation for glassy, soot, and volcanic ash aerosols. No case could be identified that gives clear evidence of ice nucleation by water vapor deposition onto a solid surface.