Abstract. The recent development and improvement of commercial laser-based spectrometers have expanded in situ continuous observations of water vapour (H₂O) stable isotope ratios (e.g., δ¹⁸O, δ²H, etc.) in a variety of sites worldwide. However, we still lack continuous observations in the Amazon, a region that significantly influences atmospheric and hydrological cycles on local to global scales. In order to achieve accurate on-site observations, commercial water isotope analysers require regular in situ calibration, including H₂O concentration dependence ([H₂O]-dependence) of isotopic accuracy. Past studies have assessed [H₂O]-dependence for air with H₂O concentrations up to 35,000 ppm, a value that is frequently surpassed in tropical rainforest settings like the central Amazon where we plan continuous observations. Here we investigated the performance of two commercial analysers (L1102i and L2130i models, Picarro, Inc., USA) for measuring δ¹⁸O and δ²H in atmospheric moisture at four different H₂O levels from 21,500 to 41,000 ppm. These H₂O levels were created by a custom-built calibration unit designed for regular in situ calibration. Measurements on the newer analyser model (L2130i) had better precision for δ¹⁸O and δ²H and demonstrated less influence of H₂O concentration on the measurement accuracy at each moisture level compared to the older L1102i. Based on our findings, we identified the most appropriate calibration strategy for [H₂O]-dependence, adapted to our calibration system. The best strategy required using two pairs of a two-point calibration with four different H₂O concentration levels. The smallest uncertainties in calibrating [H₂O]-dependence of isotopic accuracy of the two analysers were achieved using a linear-surface fitting method and a 28 h calibration interval, except for the δ¹⁸O accuracy of the L1102i analyser for which the cubic fitting method gave best results. The uncertainties in [H₂O]-dependence calibration did not show any significant difference using calibration intervals from 28 h up to 196 h; this suggested that one [H₂O]-dependence calibration per week for the L2130i and L1102i analysers is sufficient.

中文翻译：

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在热带地区进行连续现场大气水蒸气同位素测量的商用腔衰荡光谱仪的水蒸气浓度依赖性特征

摘要。最近商业的基于激光的光谱仪的发展和改进中的水蒸汽的原位连续观测扩大（H ₂ O）稳定同位素比率（例如，δ ¹⁸ O，δ ²在全球范围内的各种位点的H，等等）。但是，我们仍然缺乏对亚马逊地区的连续观测，该地区在局部到全球范围内对大气和水文循环产生重大影响。为了实现准确的现场观测，商用水同位素分析仪需要定期进行原位校准，包括同位素精度的H ₂ O浓度依赖性（[H ₂ O]依赖性）。过去的研究评估了空气中H _2的[H ₂ O]依赖性O的浓度高达35,000 ppm，在热带雨林环境中（例如我们计划进行连续观测的亚马逊中部地区）经常会超过该值。在这里，我们调查了两个商业分析仪（L1102i和L2130i模型，Picarro，Inc.，美国），用于测量δ性能¹⁸ O和δ ²的H大气湿度在四个不同的H _2个O级别从21500 41,000 ppm的。这些H ₂ O含量是由专门为常规原位校准设计的定制校准单元创建的。在较新的分析器模型（L2130i）测量具有更好的精确度为δ ¹⁸ O和δ ² H和证实h的影响少₂与较旧的L1102i相比，O浓度在每种湿度水平下的测量精度都高。根据我们的发现，我们确定了最适合[H ₂ O]依赖性的校准策略，并适合我们的校准系统。最佳策略需要使用两对具有四个不同H ₂ O浓度水平的两点校准。在校准[H最小的不确定性₂ O]在两个分析器的同位素精度-dependence使用一个线性表面拟合方法和一个28 H校准间隔来实现，除了δ ¹⁸的L1102i分析器的O-精度的量，立方拟合方法给出了最佳结果。[H ₂O]依赖性校准在28小时至196小时的校准间隔中没有显示任何显着差异。这表明L2130i和L1102i分析仪每周进行一次[H ₂ O]依赖性校准就足够了。