Laboratory measurements of heterogeneous CO2 ice nucleation on nanoparticles under conditions relevant to the Martian mesosphere
Laboratory measurements of heterogeneous CO2 ice nucleation on nanoparticles under conditions relevant to the Martian mesosphere
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DOI：
10.1002/2015je004978
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发表时间：
2016-05-01
影响因子：
4.8
通讯作者：
Leisner, Thomas
中科院分区：
文献类型：
--
作者：
Nachbar, Mario;Duft, Denis;Leisner, Thomas
在火星中层观察到二氧化碳冰粒云。这些云被认为是通过二氧化碳在纳米级流星烟雾颗粒（MSP）或向上传播的火星尘埃颗粒（MDP）上异质成核形成的。由于关键物理化学参数尚不清楚，因此对这些云的微物理形成过程进行参数化仍然存在很大的不确定性。我们展示了在接近火星中层的条件下，CO2 冰在代表 MSP 的亚 4 nm 半径氧化铁和二氧化硅颗粒上的成核和生长的测量结果。对于这两种颗粒材料，我们确定 CO2 的解吸能为 Delta F-des = (18.5 +/- 0.2) kJ mol(-1)，对应于 Delta F-des = (0.192 +/- 0.002) eV，并通过使用经典异质成核理论分析测量结果，获得控制异质成核的接触参数 m = 0.78 +/- 0.02。我们没有发现在所检查的温度范围（64 K 至 73 K）内接触参数有任何温度依赖性。通过应用火星中间层 MSP 的这些值，我们得出了 CO2 冰成核开始的特征温度，该温度比 CO2 霜点温度低 8-18 K，具体取决于颗粒大小。这与在没有观测到云的情况下延伸至霜点温度以下 20 K 的高度过饱和条件的发生是一致的。此外，CO2 在固体 CO2 上的粘附系数被确定为接近 1。我们进一步认为，相同的参数可以应用于向上传播的 MDP 上的 CO2 成核。
Clouds of CO2 ice particles have been observed in the Martian mesosphere. These clouds are believed to be formed through heterogeneous nucleation of CO2 on nanometer-sized meteoric smoke particles (MSPs) or upward propagated Martian dust particles (MDPs). Large uncertainties still exist in parameterizing the microphysical formation process of these clouds as key physicochemical parameters are not well known. We present measurements on the nucleation and growth of CO2 ice on sub-4nm radius iron oxide and silica particles representing MSPs at conditions close to the mesosphere of Mars. For both particle materials we determine the desorption energy of CO2 to be Delta F-des = (18.5 +/- 0.2) kJ mol(-1) corresponding to Delta F-des = (0.192 +/- 0.002) eV and obtain m = 0.78 +/- 0.02 for the contact parameter that governs heterogeneous nucleation by analyzing the measurements using classical heterogeneous nucleation theory. We did not find any temperature dependence for the contact parameter in the temperature range examined (64 K to 73 K). By applying these values for MSPs in the Martian mesosphere, we derive characteristic temperatures for the onset of CO2 ice nucleation, which are 8-18 K below the CO2 frost point temperature, depending on particle size. This is in line with the occurrence of highly supersaturated conditions extending to 20 K below frost point temperature without the observation of clouds. Moreover, the sticking coefficient of CO2 on solid CO2 was determined to be near unity. We further argue that the same parameters can be applied to CO2 nucleation on upward propagated MDPs.