Tunable diode laser absorption spectroscopy for stable isotope studies of ecosystem-atmosphere CO2 exchange
Tunable diode laser absorption spectroscopy for stable isotope studies of ecosystem-atmosphere CO2 exchange
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DOI：
10.1016/s0168-1923(03)00074-1
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发表时间：
2003-08-30
影响因子：
6.2
通讯作者：
Ehleringer, JR
中科院分区：
文献类型：
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作者：
Bowling, DR;Sargent, SD;Ehleringer, JR
大气CO2的稳定同位素含量提供了生态系统碳-水关系和生物圈-大气碳交换的信息。这些领域内的几乎每一项同位素研究都需要在偏远地点采集空气样本，然后在实验室进行同位素分析。这一要求严重限制了采样频率和实验持续时间。在本文中，我们评价了可调谐半导体激光吸收光谱仪(TDL)，用于测量大气摩尔分数(350-700摩尔(-1))和同位素丰度(增量(13)C为-6到-16ppm)的二氧化碳的碳同位素含量。利用红外吸收，TDL系统独立地测定(CO2)-C-12和(CO2)-C-13的摩尔分数，而不是像在质谱仪(MS)中那样测定它们的比例。该仪器在野外草原上测试了测量同位素比率(增量(13)C)的能力，并与野外采集的烧瓶样品进行的标准实验室MS测量进行了比较。TDL的进样流速为230mlmin(-1)，进样间隔为2min，进样高度为2。在TDL和MS测量之间，Delta(13)C的一致偏差为1.77ppm，误差的标准偏差(MS-TDL)为0.35ppm(n=82)。剔除两个异常值使标准偏差提高到千分之0.25(n=80)。去除偏移量后，82个样品中有62个样品的绝对差异小于千分之0.3。随后的实验室实验表明，TDL/MS偏差是由压力展宽引起的，未来可以通过用二氧化碳与空气而不是氮气混合来校准TDL来避免。基于这些结果，我们估计在我们的抽样方案中，增量(13)C的精度为千分之0.25。与用校准后的红外气体分析仪测量的CO2摩尔分数((CO2)-C-12+(CO2)-C-13)进行了类似的比较，结果表明，TDL的精度为0.4%(在400摩尔摩尔(-1)时为1.6摩尔摩尔(-1))。空气中二氧化碳的增量(13)C的测量是在四个不同的夜晚进行的，分别位于离地面1厘米和60厘米的高度。313CR不随身高变化，但随夜间不同而不同。对313个CR的每小时测量表明，一夜之间变化高达6.4ppm(-29.1+/-0.4到-22.7+/-0.8ppm)。(13)C(R)在夜间的时间变化在以前的研究中还没有报道。这种观察可以为研究生态系统呼吸所用碳基质的时间动态提供一种新的方法。(C)2003 Elsevier Science B.V.保留所有权利。
The stable isotope content of atmospheric CO2 Provides information about ecosystem carbon-water relations and biosphere-atmosphere carbon exchange. Virtually every isotope study within these fields has required air sample collection at remote locations followed by isotope analysis at a laboratory. This requirement severely limits sampling frequency and experiment duration. In this paper, we evaluate a tunable diode laser absorption spectrometer (TDL) for measuring the carbon isotope content of CO2 at atmospheric mole fractions (350-700 mumol mol(-1)) and isotopic abundance (delta(13)C of -6 to - 16parts per thousand). Using infrared absorption, the TDL system determines the mole fractions of (CO2)-C-12 and (CO2)-C-13 independently, rather than their ratio as in mass spectrometry (MS).The ability of the instrument to measure isotope ratios (delta(13)C) was tested outdoors in a grassland and compared to standard laboratory-based MS measurements made on field-collected flask samples. The TDL was operated at a sampling flow rate of 230 ml min(-1) and a sampling interval of 2 min for two intake heights. There was a consistent offset for delta(13)C of 1.77parts per thousand between the TDL and MS measurements, and the standard deviation of the error (MS - TDL) was 0.35parts per thousand (n = 82). Removal of two outliers improved this standard deviation to 0.25parts per thousand (n = 80). After removing the offset, 62 out of 82 samples had absolute differences less than 0.3parts per thousand. Subsequent laboratory experiments indicated that the TDL/MS offset was caused by pressure broadening, and can be avoided in the future by calibrating the TDL with CO2 mixed with air rather than nitrogen. Based on these results we estimate the precision for delta(13)C to be 0.25parts per thousand for our sampling scheme. A similar comparison with flask-based measurements of CO2 mole fraction ((CO2)-C-12 + (CO2)-C-13) made with a calibrated infrared gas analyzer indicated a TDL precision of 0.4% (1.6 mumol mol(-1) at 400 mumol mol(-1)).The TDL was used to investigate the vertical and temporal variation in the carbon isotope content of respired CO2 (delta(13) C-R) from the grassland. Measurements of delta(13)C of CO2 in air were made during four separate nights at 1 and 60 cm height above ground. 313 CR did not vary with height, but it did vary from one night to the next. Hourly measurements of 313 CR showed it changed as much as 6.4parts per thousand (-29.1 +/- 0.4 to -22.7 +/- 0.8parts per thousand) in a single night. Temporal changes in delta(13)C(R) during the night have not been reported in prior studies. Such observations could provide a new way to investigate temporal dynamics of the carbon substrates utilized for ecosystem respiration. (C) 2003 Elsevier Science B.V. All rights reserved.