Toward a Comprehensive Model of Snow Crystal Growth: 5. Measurements of Changes in Attachment Kinetics from Background Gas Interactions at -5 C

We present measurements of the diffusion-limited growth of ice crystals from water vapor at a temperature of -5 C, in air at a pressure of $p_{air}=1$ bar. Starting with thin, c-axis ice needle crystals, the subsequent growth morphologies ranged from solid prismatic columns to hollow columns to complex "fishbone" dendritic structures as the supersaturation was increased. We modeled the simpler morphologies using analytical techniques together with a cellular-automata method that yields faceted crystalline structures in diffusion-limited growth. We found that the molecular attachment coefficient $\alpha_{prism}$ on faceted prism surfaces in air at -5 C is substantially lower than that measured at low background air pressure. Our data show that increasing $p_{air}$ from 0.01 to 1 bar reduces $\alpha_{prism}$ by nearly two orders of magnitude at this temperature. In contrast, we find that $\alpha_{basil}$ is essentially unaffected by air pressure over this range. These and other measurements indicate that ice surfaces near the melting point undergo a series of complex structural and dynamical changes with temperature that remain largely unexplained at even a qualitative level.