A model is presented that gives a quantitative description of the dynamic behavior of a low-humidity water vapor generator in terms of water vapor concentration (humidity) and isotope ratios. The generator is based on the evaporation of a nanoliter-sized droplet produced at the end of a syringe needle by balancing the inlet water flow and the evaporation of water from the droplet surface into a dry-air stream. The humidity level is adjusted by changing the speed of the high-precision syringe pump and, if needed, the dry-air flow. The generator was developed specifically for use with laser-based water isotope analyzers in Antarctica, and it was recently described in Leroy-Dos Santos et al. (2021). Apart from operating parameters such as temperature, pressure, and water and dry-air flows, the model has as “free” input parameters: water isotope fractionation factors and the evaporation rate. We show that the experimental data constrain these parameters to physically realistic values that are in reasonable to good agreement with available literature values. With the advent of new ultraprecise isotope ratio spectrometers, the approach used here may permit the measurement of not only the evaporation rate but also the effective fractionation factors and isotopologue-dependent diffusivity ratios, in the evaporation of small droplets.

中文翻译：

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模拟用于输送同位素校准的低湿度水蒸气的液滴蒸发装置的动态行为

提出了一个模型，该模型根据水蒸气浓度（湿度）和同位素比对低湿度水蒸气发生器的动态行为进行了定量描述。该发生器基于在注射器针头末端产生的纳升大小的液滴的蒸发，通过平衡入口水流和水从液滴表面蒸发到干燥空气流中。通过改变高精度注射泵的速度和干燥空气流量（如果需要）来调节湿度水平。该发生器专为南极洲基于激光的水同位素分析仪而开发，最近在Leroy-Dos Santos 等人中有所描述。(2021). 除了温度、压力、水和干空气流量等操作参数外，该模型还有“自由”输入参数：水同位素分馏因子和蒸发率。我们表明，实验数据将这些参数限制为与现有文献值合理到良好一致的物理现实值。随着新的超精密同位素比光谱仪的出现，这里使用的方法不仅可以测量蒸发速率，还可以测量小液滴蒸发中的有效分馏因子和与同位素体相关的扩散率。