Collaborative Research: Testing Controls on Source, Sink, and Lifetime of Atmospheric Water with Numerical Tags and Stable Isotope Ratios

人们一眼就能认出水的化学式H2O，但这个简单的化学式掩盖了一个事实：并非所有的氢原子都是一样的。地球上的一小部分水可以更准确地称为HDO，其中D代表氘，一种较重的氢同位素，其原子核中包含一个中子和一个质子。同样，一些水分子含有氧的同位素18O，由于增加了两个中子，它比普通氧重。这些较重形式的水与普通的水有一个微妙但重要的区别：它们蒸发得更慢，凝结得更容易，更倾向于固态和液态而不是气相。当水从陆地或海洋表面蒸发时，水蒸气中较重的同位素会稍微减少，而且每次从云中落下雨水时，留在大气中的水蒸气比雨中排出的水稍微轻一些。降水对重同位素的去除表明，水蒸气和降水中重同位素的丰度为理解水文循环及其与环境条件的联系提供了重要线索。这一领域的研究结果是有希望的，但由于影响同位素组成的因素众多，研究结果变得复杂。其中包括降水发生地点的温度，产生降水的云层高度，水蒸气蒸发的海洋上空的相对湿度，以及雨滴在干燥空气中落下时较轻的同位素的蒸发。此外，许多关于水同位素的研究都是基于同位素组成与与物理机制有关的变量（例如，降雨地区的温度）之间的相关性。这些相关性可以提供相当多的信息，但相关性本身不能确定地建立因果关系。该奖项下的工作旨在建立与同位素组成有关的物理机制与水蒸气和沉淀水的实际组成之间的直接因果关系。这项研究是利用群落地球系统模型（CESM）进行的，在这个模型中，机制及其对同位素组成的影响是精确已知的。CESM已经配备了模拟物理过程的设备，以确定重同位素与轻同位素的比例，并且它还跟踪在模拟大气中循环的水蒸气的同位素组成。该项目开发了一种新的“水标签”功能，允许模拟水蒸气记录有关其蒸发条件的信息，例如蒸发的地点和时间，以及它是否经过云层。当水蒸气凝结时，这些信息也被传递到产生的降水中。标签信息可以直接与同位素组成进行比较，从而可以直接评估标签所代表的物理条件和过程的影响。一些模拟使用数据同化系统（称为DART）来确保模拟的气候和环流准确地代表它们的真实对应。额外的工作使用一维能量平衡模型，它提供了一个降低复杂性的设置，以发展新的想法和测试假设。由于广泛使用水同位素作为了解气候和水文循环功能的窗口，这项工作具有更广泛的科学影响。例如，冰川冰芯中的同位素比率已被用于估计冰期和间冰期温暖期之间的温差，从而为了解当前的气候变化提供了有价值的参考点。这里开发的水标记算法将提供给CESM的全球用户社区，为用户提供一个解释同位素数据并将其应用于各种研究问题的强大工具。此外，该奖项支持的研究人员还通过一项以德克尔湖青年中心为重点的项目开展教育推广活动，德克尔湖青年中心是盐湖城山谷的一个安置有犯罪前科的青少年的设施。外联活动涉及臭氧生物指示花园，展示了臭氧污染的危害，这是盐湖城山谷的一个普遍问题。惩教设施中的教育机会已被证明可以减少再犯。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

H2O is instantly recognizable as the chemical formula for water, but this simple formula masks the fact that not all hydrogen atoms are alike. A small fraction of the water on earth could be more accurately called HDO, where D stands for deuterium, the heavier hydrogen isotope in which the nucleus contains a neutron as well as a proton. Likewise, some water molecules contain 18O, an isotope of oxygen which is heavier than ordinary oxygen by the addition of two neutrons. These heavier forms of water share a subtle but important difference from ordinary H2O: they evaporate more sluggishly and condense more readily, preferring the solid and liquid phases to the vapor phase. When water evaporates from the land or ocean surface the vapor is slightly depleted of the heavier isotopes, and each time rain falls from a cloud the water vapor remaining in the atmosphere is slightly lighter than the water that rained out. The removal of heavier isotopes by precipitation suggests that the abundances of heavier isotopes in water vapor and precipitated water contain important clues for understanding the hydrological cycle and its connections to environmental conditions. Results of research in this area are promising but they are complicated by the numerous factors that can influence isotopic composition. Among these are the temperature at the location where the precipitation occurred, the height of the cloud that produced the precipitation, the relative humidity over the ocean where the water vapor evaporated, and the evaporation of lighter isotopes from raindrops as they fall through dry air. Moreover, much of the research on water isotopes is based on correlations between isotopic composition and variables relevant to physical mechanisms (temperature in the region of rainout, for instance). These correlations can be quite informative but correlations by themselves cannot definitively establish causality.Work under this award seeks to establish direct, causal relationships between the physical mechanisms relevant to isotopic composition and the actual composition of water vapor and precipitated water. The research is conducted using a model, the Community Earth System Model (CESM), in which the mechanisms and their influence on isotopic composition are known exactly. CESM is already equipped to simulate the physical processes that determine the ratio of heavier to lighter isotopes, and it also tracks the isotopic composition of water vapor as it circulates in the simulated atmosphere. This project develops a new "water tagging" capability which allows simulated water vapor to record information about conditions under which it evaporated, for instance where and when it evaporated and whether it was processed through clouds. This information is also transferred to the precipitation produced when water vapor condenses. The tag information can be directly compared to isotopic composition, allowing the influence of the physical conditions and processes represented by the tags to be directly assessed. Some simulations use a data assimilation system (known as DART) to ensure that the simulated climate and circulation accurately represent their real-world counterparts. Additional work uses a one-dimensional energy balance model, which provides a reduced-complexity setting to develop new ideas and test hypotheses.The work has scientific broader impacts due to the widespread use of water isotopes as a window on the functioning of climate and the hydrological cycle. For instance isotope ratios in glacial ice cores have been used to estimate temperature differences between ice ages and interglacial warm periods, thus providing a valuable point of reference for understanding current climate change. The water tagging algorithms developed here will be made available to the worldwide user community of CESM, providing users with a powerful tool for the interpretation of isotope data and its application to a variety of research questions.In addition, researchers supported under this award conduct educational outreach through a program focused on the Decker Lake Youth Center, a facility housing post-conviction youth in the Salt Lake Valley. The outreach involves ozone bioindicator gardens which demonstrate the hazards of ozone pollution, a common problem in the Salt Lake Valley. Educational opportunities in correctional facilities have been shown to reduce recidivism.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.