Abstract
Accommodation of vapor-phase water molecules into ice crystal surfaces is a fundamental process controlling atmospheric ice crystal growth. Experimental studies investigating the accommodation process with various techniques report widely spread values of the water accommodation coefficient on ice, αice, and the results on its potential temperature dependence are inconclusive. We run molecular dynamics simulations of molecules condensing onto the basal plane of ice Ih using the TIP4P/Ice empirical force field and characterize the accommodated state from this molecular perspective, utilizing the interaction energy, the tetrahedrality order parameter, and the distance below the instantaneous interface as criteria. Changes of the order parameter turn out to be a suitable measure to distinguish between the surface and bulk states of a molecule condensing onto the disordered interface. In light of the findings from the molecular dynamics, we discuss and re-analyze a recent experimental data set on αice obtained with an environmental molecular beam (EMB) setup [KongX.; J. Phys. Chem. A2014, 118 ( (22), ), 3973−397924814567] using kinetic molecular flux modeling, aiming at a more comprehensive picture of the accommodation process from a molecular perspective. These results indicate that the experimental observations indeed cannot be explained by evaporation alone. At the same time, our results raise the issue of rapidly growing relaxation times upon decreasing temperature, challenging future experimental efforts to cover relevant time scales. Finally, we discuss the relevance of the water accommodation coefficient on ice in the context of atmospheric cloud particle growth processes.
Highlights
The key parameter governing the thermodynamic equilibria between water vapor and its various condensed phases is the saturation vapor pressure above the liquid or solid phase, which is directly linked to the evaporation rate of water molecules from the condensed phase.[1−3] The difference between the ambient water vapor concentration and the equilibrium vapor pressure determines to a large degree the net growth or evaporation of water hydrometeors.[1,2]
The mass accommodation coefficient is alternatively related to a reorientation of the condensing molecule.[8]
We focus on the accommodation process of single molecules condensing onto ice surfaces at different temperatures
Summary
Condensation and deposition of water vapor onto liquid water and ice surfaces is a key process in the Earth’s atmosphere, driving, e.g., the growth of cloud droplets and ice crystals and influencing the evolution and properties of clouds.[1,2] The key parameter governing the thermodynamic equilibria between water vapor and its various condensed phases is the saturation vapor pressure above the liquid or solid phase, which is directly linked to the evaporation rate of water molecules from the condensed phase.[1−3] The difference between the ambient water vapor concentration and the equilibrium vapor pressure determines to a large degree the net growth or evaporation of water hydrometeors.[1,2] An additional coefficient known as the mass accommodation coefficient α ( sometimes called the condensation or evaporation coefficient) has been proposed to modulate the condensation and evaporation fluxes at the free molecular regime.[3,4] The most common formulation defines α as the fraction of incoming molecules, as determined by kinetic gas theory, that stick and accommodate into the bulk condensed phase in the absence of evaporation, or in reverse, the ratio between the evaporation rate and the maximum kinetic evaporation into vacuum (i.e., in the absence of condensation or deposition).[3,5] Being a primarily kinetic parameter, the effect of α disappears at the limit of the continuum regime where the condensation or deposition becomes diffusion-limited and can be described by the macroscopic transport equations (see, e.g., ref 6 and references therein). Most studies relate α to an additional energetic barrier related to the restructuring of the surface as it takes up a molecule.[6,7] For ice surfaces, the mass accommodation coefficient is alternatively related to a reorientation of the condensing molecule.[8] Some studies describe particle-phase transport through an effective mass accommodation coefficient (e.g., refs 9, 10). The ambiguity in the definition of α makes it difficult to compare and interpret experimental data, Received: October 15, 2020 Revised: December 5, 2020 Published: December 15, 2020
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