The Impact of Nonequilibrium and Equilibrium Fractionation on Two Different Deuterium Excess Definitions
The Impact of Nonequilibrium and Equilibrium Fractionation on Two Different Deuterium Excess Definitions
复制标题
非平衡和平衡分馏对两种不同的氘过量定义的影响
DOI：
10.1002/2017jd027085
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
H. Sodemann
中科院分区：
文献类型：
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作者：
M. Dütsch;S. Pfahl;H. Sodemann
氘过量 (d) 是同位素分馏的非平衡效应的有用量度，因此可以提供有关蒸发区域或冰云形成期间的气象条件的信息。除了非平衡分馏之外，在相变期间，其他两种效应也会发生变化。第一个是平衡分馏因子对温度的依赖性，第二个是定义 d 的 δ 尺度的非线性。通过使用基于对数标度的替代定义可以避免第二种效应。然而，在这种情况下，当空气团混合时，d 不守恒，这可能导致没有相变的变化。在这里，我们通过在模拟湿绝热上升过程中空气团的同位素组成的简单瑞利模型中分别量化非平衡效应、温度效应、δ尺度效应和混合效应的影响，对两种氘过量定义的优点和局限性进行了系统分析。 δ 尺度效应在贫化空气中很重要，它可以将剩余蒸气中传统氘过量的符号从负变为正。替代定义主要反映非平衡和温度效应，而混合效应小约2个数量级。因此，在水分耗尽且 δ 尺度效应较大的情况下（例如在高纬度或高海拔地区），替代的氘过量定义似乎是对非平衡效应的更准确测量。
The deuterium excess (d) is a useful measure for nonequilibrium effects of isotopic fractionation and can therefore provide information about the meteorological conditions in evaporation regions or during ice cloud formation. In addition to nonequilibrium fractionation, two other effects can change d during phase transitions. The first is the dependence of the equilibrium fractionation factors on temperature, and the second is the nonlinearity of the δ scale on which d is defined. The second effect can be avoided by using an alternative definition that is based on the logarithmic scale. However, in this case d is not conserved when air parcels mix, which can lead to changes without phase transitions. Here we provide a systematic analysis of the benefits and limitations of both deuterium excess definitions by separately quantifying the impact of the nonequilibrium effect, the temperature effect, the δ‐scale effect, and the mixing effect in a simple Rayleigh model simulating the isotopic composition of air parcels during moist adiabatic ascent. The δ‐scale effect is important in depleted air parcels, for which it can change the sign of the traditional deuterium excess in the remaining vapor from negative to positive. The alternative definition mainly reflects the nonequilibrium and temperature effect, while the mixing effect is about 2 orders of magnitude smaller. Thus, the alternative deuterium excess definition appears to be a more accurate measure for nonequilibrium effects in situations where moisture is depleted and the δ‐scale effect is large, for instance, at high latitudes or altitudes.