Uptake of Fe, Na and K atoms on low-temperature ice: implications for metal atom scavenging in the vicinity of polar mesospheric clouds

Abstract

Ice clouds form in the mesosphere between 80 and 90 km, at high latitudes during summer when the temperature falls below 150 K. There is strong evidence that the water–ice particles in these clouds scavenge metal atoms that are produced in the mesosphere by meteoric ablation. In the present study the uptake of Fe, Na and K on an ice film was studied in a fast flow tube over a temperature range of 80–150 K, covering the temperatures over which ice clouds form in the upper mesosphere. The uptake was found to be highly efficient and mostly in the diffusion-limited regime, requiring accurate measurements of the diffusion coefficients of the metal atoms in He: D_FeHe = 366 (±17) (T/296 K)^{1.85 ± 0.07}, D_NaHe 286 (±13) (T/296 K)^{1.68 ± 0.04} and D_KHe = 247 (±15) (T/296 K)^{1.69 ± 0.07} Torr cm² s⁻¹. Measured values of the diffusion coefficients in N₂ are 112 (±4), 125 (±4) and 88 (±4) Torr cm² s⁻¹ at 293 K for Fe, Na and K, respectively. The uptake of Na and K was observed to be extremely efficient from 80–150 K, with lower limits of γ_Na > 0.09 and γ_K > 0.05, although it is likely that γ is much closer to unity. The uptake of Fe on cubic ice is close to unity efficiency above 135 K, but γ_Fe decreases to only 3 × 10⁻³ at 80 K. Uptake of Fe on amorphous ice films is much more efficient than on cubic ice films below 130 K. These results are interpreted using quantum calculations of the metal atoms adsorbed onto a 12-H₂O model ice surface. Finally, it is shown that the uptake of Fe, Na and K on low-temperature ice is sufficiently fast to explain the substantial depletions in the mesospheric metal layers that are observed in the presence of mesospheric ice clouds.