A Determination of Krypton and Xenon Solubilities in Seawater for Oceanic Air-Sea Exchange and Ventilation Studies

最近的研究表明，我们对一些惰性气体溶解度的认识存在重大错误，不同测定的偏移量接近1？2%。Ne和Ar最近被重新确定，并出现？一切顺利吗？然而，关于Kr和Xe的溶解度存在疑问，这是有问题的，因为这些气体在稀有气体族中包含一个重要的端元。事实上，唯一记录在案的海水氙溶解度测定声称不确定度为2%左右，根据最近的一项研究，似乎有系统地偏离。这种误差很大，与观测到的混合层异常相当，因此极大地限制了稀有气体测量对诊断海洋过程的价值。在这项研究中，伍兹霍尔海洋研究所的研究人员将使用专门设计的平衡装置和双同位素稀释质谱结合高质量的低温分离和净化技术，以0.1%或更高的精度确定Kr和Xe的溶解度。他们将确定这些气体在淡水中的溶解度，以及在接近冰点到30摄氏度的温度范围内的海水盐度。总共有40个点的温度和盐度空间将在重复样本中测量。他们计划同时分析溶解气体和顶空气体，以便在本生溶解度的计算中抵消一些系统的不确定性。在同一个实验中，他们还将通过单一同位素稀释来测量Ne和Ar的溶解度，以与之前的测量结果进行比较。预计这项研究将产生许多重要的更广泛的影响。海洋通风和海气交换过程是许多重要生物地球化学循环的关键组成部分，在全球变化中起着重要作用。例如，人为产生的化石燃料二氧化碳库存的很大一部分已经通过这些过程转移到海洋中。另一个需要了解海气交换率的关键应用是使用季节性发生的浅层氧气最大值来估计生物产量，这依赖于通过海气界面估算光合作用氧气的损失，并考虑气泡捕获对氧气过饱和的贡献。因此，识别、描述和量化与海气交换有关的物理过程是建立可靠的气候和碳循环模型的关键步骤，特别是对于诊断生物地球化学过程以及评估和预测人为影响而言。尽管这些过程很重要，但由于科学和社会/政策的需要，这些过程没有得到充分的描述和量化。惰性气体是诊断海气交换和水团形成过程的潜在有力工具，因为它们的分布受纯物理机制控制。此外，作为一个群体，它们跨越了广泛的分子扩散率和大约一个数量级的溶解度。分子扩散率在扩散海气交换和气泡注入过程中都起着重要的作用。溶解度，特别是其对温度的依赖，是发生显著热传递时气体交换的重要驱动因素，无论是在夏季的辐射增温过程中，还是在冬季的水团形成过程中。稀有气体分布最近被用于量化海气交换速率，限制生物（净群落）生产，诊断温跃层中的双周期混合，以及表征与水团形成相关的海气不平衡。这些都是重要的生物地球化学和物理过程。

Recent work has revealed that there are significant errors in our knowledge of the solubilities of a number of the noble gases, with offsets in different determinations approaching 1? 2%. Ne and Ar have recently been re-determined, and appear ?well in hand?. However, there are questions about the solubilities of Kr and Xe, which is problematic as these gases comprise an important end-member in the noble gas group. In fact, the only documented determinations of seawater xenon solubility claim an uncertainty of order 2%, and appear to be systematically off according to a recent study. Such errors are large and comparable in magnitude to observed mixed layer anomalies, and hence significantly limit the value of noble gas measurements for diagnosing oceanic processes. In this study, researchers at the Woods Hole Oceanographic Institution will determine the solubility of Kr and Xe to an accuracy of 0.1% or better, using a purpose-designed equilibration apparatus and dual isotope dilution mass spectrometry combined with high-quality cryogenic separation and purification technology. They will determine the solubilities of these gases in fresh water as well as seawater salinities over a range of temperatures spanning from near-freezing to 30ºC. In all, 40 points in temperature and salinity space will be measured in replicate samples. They plan to analyze both the dissolved gases and the head-space gases so that some systematic uncertainties will cancel out in the calculation of Bunsen solubilities. In the same experiment, they will also measure the solubilities of Ne and Ar by single isotope dilution as a comparison of previous measurements. This research is expected to have a number of important broader impacts. Ocean ventilation and air-sea gas exchange processes are key components in a number of major biogeochemical cycles and play an important role in global change. For example, a significant portion of the anthropogenic fossil-fuel CO2 inventory has been transferred to the ocean by these processes. Another critical application requiring knowledge of air-sea gas exchange rates is the use of seasonally occurring shallow oxygen maxima to estimate biological production, which relies on estimating the loss of photosynthetic oxygen through the air-sea interface and accounting for the contribution of bubble trapping to oxygen supersaturation. Thus Identifying, characterizing, and quantifying the physical processes associated with air-sea exchange of gases is a vital step in building credible models of climate and carbon cycles, particularly for diagnosing biogeochemical processes and for assessing and predicting anthropogenic impacts. Despite their importance, these processes are not adequately characterized and quantified for scientific and societal/policy needs. Noble gases are potentially powerful tools for diagnosing air-sea exchange and water-mass formation processes, since their distributions are controlled by purely physical mechanisms. Moreover, as a group they span a wide range of molecular diffusivities and an approximately order of magnitude in solubilities. Molecular diffusivity plays an important role both in diffusive air-sea gas exchange, and in bubble injection processes. Solubility, and in particular its dependence on temperature, is an important driver for gas exchange when significant heat transfer occurs, either during radiative warming in the summer months, or in water mass formation processes during the winter. Noble gas distributions have recently been used to quantify air-sea gas exchange rates, constrain biological (net community) production, diagnose diapycnal mixing in the thermocline, and characterize air-sea disequilibrium associated with water mass formation. All of these are important biogeochemical and physical processes.