Uptake of Fe, Na and K atoms on low-temperature ice: implications for metal atom scavenging in the vicinity of polar mesospheric clouds.
Uptake of Fe, Na and K atoms on low-temperature ice: implications for metal atom scavenging in the vicinity of polar mesospheric clouds.
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DOI：
10.1039/b508846a
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发表时间：
2005-11
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影响因子：
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通讯作者：
B. Murray;J. Plane
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作者：
B. Murray;J. Plane
夏季，当温度低于 150 K 时，高纬度地区 80 至 90 公里的中间层会形成冰云。有强有力的证据表明，这些云中的水冰颗粒会清除中间层因流星消融而产生的金属原子。在本研究中，在 80-150 K 的温度范围内，在快流管中研究了冰膜上铁、钠和钾的吸收，涵盖了中层上部冰云形成的温度。发现吸收效率很高，并且主要处于扩散限制状态，需要精确测量 He 中金属原子的扩散系数：DFeHe = 366 (+/- 17) (T/296 K)(1.85 +/- 0.07)、DNaHe 286 (+/- 13) (T/296 K)(1.68 +/- 0.04) 和 DKHe = 247 (+/- 0.04) 15) (T/296 K)(1.69 +/- 0.07) 托·厘米2·秒(-1)。在 293 K 下，Fe、Na 和 K 在 N2 中的扩散系数测量值分别为 112 (+/- 4)、125 (+/- 4) 和 88 (+/- 4) Torr cm2 s(-1)。据观察，Na 和 K 的吸收在 80-150 K 范围内非常有效，伽马 Na > 0.09 和伽马 K > 0.05 的下限，尽管伽马可能更接近统一。在 135 K 以上，立方体冰上的 Fe 吸收接近统一效率，但在 80 K 时，γ Fe 降低至仅 3 x 10(-3)。在 130 K 以下，非晶冰膜上 Fe 的吸收比立方体冰膜上的吸收效率高得多。这些结果是使用吸附在 12-H2O 模型冰表面上的金属原子的量子计算来解释的。最后，研究表明，低温冰对铁、钠和钾的吸收速度足够快，足以解释在存在中层冰云的情况下观察到的中层金属层的大量损耗。
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: DFeHe = 366 (+/- 17) (T/296 K)(1.85 +/- 0.07), DNaHe 286 (+/- 13) (T/296 K)(1.68 +/- 0.04) and DKHe = 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 gamma Na > 0.09 and gamma K > 0.05, although it is likely that gamma is much closer to unity. The uptake of Fe on cubic ice is close to unity efficiency above 135 K, but gamma Fe decreases to only 3 x 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.