McNeill V Faye, Geiger Franz M, Loerting Thomas, Trout Bernhardt L, Molina Luisa T, Molina Mario J
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Phys Chem A. 2007 Jul 19;111(28):6274-84. doi: 10.1021/jp068914g. Epub 2007 Jun 22.
Characterization of the interaction of hydrogen chloride (HCl) with polar stratospheric cloud (PSC) ice particles is essential to understanding the processes responsible for ozone depletion. The interaction of HCl with ice was studied using a coated-wall flow tube with chemical ionization mass spectrometry (CIMS) between 5x10(-8) and 10(-4) Torr HCl and between 186 and 223 K, including conditions recently shown to induce quasi-liquid layer (QLL) formation on single crystalline ice samples. Measurements were performed on smooth and rough (vapor-deposited) polycrystalline ice films. A numerical model of the coated-wall flow reactor was used to interpret these results and results of studies on zone-refined ice cylinders with grain sizes on the order of several millimeters (reported elsewhere). We found that HCl adsorption on polycrystalline ice films typically used in laboratory studies under conditions not known to induce surface disordering consists of two modes: one relatively strong mode leading to irreversible adsorption, and one relatively weak binding mode leading to reversible adsorption. We have indirect experimental evidence that these two modes of adsorption correspond to adsorption to sites at crystal faces and those at grain boundaries, but there is not enough information to enable us to conclusively assign each adsorption mode to a type of site. Unlike what was observed in the zone-refined ice study, there was no strong qualitative contrast found between the HCl uptake curves under QLL versus non-QLL conditions for adsorption on smooth and vapor-deposited ices. We also found indirect evidence that HCl hexahydrate formation on ice between 3x10(-7) and 2x10(-6) Torr HCl and between 186 and 190 K is a process involving hydrate nucleation and propagation on the crystal surface, rather than one originating in grain boundaries, as has been suggested for ice formed at lower temperatures. These results underscore the dependence of the HCl-ice interaction on the characteristics of the ice substrate.
表征氯化氢（HCl）与极地平流层云（PSC）冰粒之间的相互作用对于理解导致臭氧消耗的过程至关重要。使用带有化学电离质谱（CIMS）的涂壁流动管，在5×10⁻⁸至10⁻⁴托的HCl以及186至223 K的温度范围内研究了HCl与冰的相互作用，包括最近显示能在单晶冰样品上诱导准液层（QLL）形成的条件。对光滑和粗糙（气相沉积）的多晶冰膜进行了测量。使用涂壁流动反应器的数值模型来解释这些结果以及对晶粒尺寸在几毫米量级的区域精炼冰柱的研究结果（其他地方有报道）。我们发现，在实验室研究中通常使用的多晶冰膜上，在未知会诱导表面无序的条件下，HCl的吸附由两种模式组成：一种相对较强的模式导致不可逆吸附，另一种相对较弱的结合模式导致可逆吸附。我们有间接实验证据表明这两种吸附模式分别对应于在晶面和晶界处的吸附，但没有足够信息使我们能够确凿地将每种吸附模式归属于一种类型的位点。与在区域精炼冰的研究中所观察到的不同，在QLL与非QLL条件下，对于光滑和气相沉积冰上的吸附，HCl吸收曲线之间没有发现强烈的定性差异。我们还发现间接证据表明，在3×10⁻⁷至2×10⁻⁶托的HCl以及186至190 K的温度范围内，冰上HCl六水合物的形成是一个涉及水合物在晶体表面成核和生长的过程，而不是像在较低温度下形成的冰那样起源于晶界。这些结果强调了HCl - 冰相互作用对冰基质特性的依赖性。
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