基于稳定氢氧同位素的盐水与纯水蒸发差异分析

Hydrogen and Oxygen Stable Isotope Study on the Difference of Evaporation between Salt and Pure Water

水面蒸发是水循环的重要部分,目前大量的研究集中在淡水或低盐度咸水体蒸发,仅以淡水的蒸发特性或计算方法应用于全部水体并不准确。研究盐水蒸发与淡水蒸发的差异对蒸发过程机理研究、推动蒸发模型的创新改进至关重要。研究中使用蒸发皿对比盐水与纯水的蒸发过程,利用稳定氢氧同位素比较盐水与纯水蒸发的动力学分馏过程,结果表明:盐水蒸发量较纯水少,但仍存在蒸发日内变化规律,蒸发速率与气温变化规律同步,并在正午前后达日最大值;随着蒸发的进行,重同位素在液相富集,盐度对于H/D分馏有更显著的抑制作用;盐水与纯水的蒸发线拟合均有良好的线性关系,盐分使水体蒸发受到更强的非平衡分馏影响;热红外拍摄液面观测到蒸发过程中盐水液面温度始终高于纯水0.1～2.2℃,平均温差达1℃,这是因为盐水蒸发量较纯水小,更少的热量通过潜热释放;Craig&Gordon模型计算蒸发水汽同位素特征值,表明随着蒸发的进行,蒸发水汽组分重同位素也在不断富集,但程度不如剩余水体,检验盐水与纯水蒸发水汽氢氧同位素拟合方程却无明显差异,与前述结论相悖,说明该方程在小尺度上的应用还有待研究。

Water evaporation is an essential part in the water cycle. However, a large amount of research focuses on freshwater or low salinity-saltwater bodies. It is inaccurate to apply the evaporation characteristics or estimation methods of freshwater to all the water bodies. Studying the difference of evaporation between salt water and pure water is crucial to the study of the mechanism of evaporation process and the promotion of innovation in evaporation models. In this study, evaporation pan was used to observe the process of salt and pure water evaporation, and steady-state hydrogen isotopes and oxygen isotopes were used to compare the kinetic fractionation in salt and pure water evaporation. The results show： evaporation of salt water reduced compared with pure water, but the daily variation pattern was consistent with the temperature daily change and reached the maximum evaporate rate around noon. Heavy isotopes were enriched in the liquid phase as water evaporated and salinity significantly suppressed H/D fractionation. The evaporation line of pure water and salt water both had a good linear relationship. The salt caused stronger non-equilibrium fractionation. The thermal infrared imaging observed that the temperature of the brine surface was always higher than that of pure water by 0.1 ~ 2.2°C with an average temperature difference of 1°C, this was because evaporation of salt water was smaller than that of pure water and less heat was released by latent heat. The isotopes of evaporative water vapor calculated through Craig ＆ Gordon model indicated that the heavy isotopes of evaporation water vapor were continuously enriched, but not that strong compared to the remaining water. There is no significant difference between the equations for the hydrogen and oxygen isotopes of water vapor in pure and salt water, that is contrary to the above conclusions, indicating that the application of this equation at small scale deserves further study.