The recent development and improvement of commercial laser-based spectrometers have expanded in situ continuous observations of water vapour (H₂O) stable isotope compositions (e.g. δ¹⁸O and δ²H) 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, which includes the correction of H₂O concentration dependence ([H₂O] dependence) of isotopic measurements. Past studies have assessed the [H₂O] dependence for air with H₂O concentrations of 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 concentration 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 conducting a two-point calibration with four different H₂O concentration levels, carried out at the beginning and end of the calibration interval. 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 the best results. The uncertainties in [H₂O] dependence calibration did not show any significant difference using calibration intervals from 28 up to 196 h; this suggested that one [H₂O] dependence calibration per week for the L2130i and L1102i analysers is sufficient. This study shows that the cavity ring-down spectroscopy (CRDS) analysers, appropriately calibrated for [H₂O] dependence, allow the detection of natural signals of stable water vapour isotopes at very high humidity levels, which has promising implications for water cycle studies in areas like the central Amazon rainforest and other tropical regions.

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

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）的稳定同位素组合物（例如δ ¹⁸ O和δ ²全世界2H）中的各种位点。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都会严重影响大气和水文循环。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，其中包括校正同位素测量的H ₂ O浓度依赖性（[H ₂ O]依赖性）。过去的研究评估了[H ₂O]对H ₂ O浓度高达35000 ppm的空气的依赖性，在热带雨林环境（如我们计划进行连续观测的亚马逊中部地区）中，该值经常会超过O] 。在这里，我们研究了两种商业分析仪（L1102i和L2130i模型，Picarro，Inc.，美国），用于测量的性能δ ¹⁸ O和δ ²的H大气湿气在四个不同的H _2个从21 500 O级别至41 1,000ppm的。这些H ₂ O含量是由专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与较旧的L1102i相比，H ₂ O浓度对H ₂ O浓度对测量精度的影响较小。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。最佳策略需要在校准间隔的开始和结束时使用四种不同的H ₂ O浓度水平进行两点校准。在校准[H最小的不确定性₂两个分析器的同位素精度O]依赖分别使用线性曲面拟合方法和28h的校准间隔来实现，除了δ ¹⁸L1102i分析仪的O精度，三次拟合法给出了最佳结果。[H ₂ O]依赖性校准的不确定性在使用28至196 h的校准间隔时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。这项研究表明，对[H ₂ O]依赖性进行了适当校准的腔衰荡光谱分析仪（CRDS）可以检测出在非常高的湿度下稳定的水蒸气同位素的自然信号，这对水循环研究具有潜在的意义在亚马逊中部雨林和其他热带地区。