Theoretical and experimental evaluation of the isotope effect of NDIR analyzer on atmospheric CO2 measurement
Theoretical and experimental evaluation of the isotope effect of NDIR analyzer on atmospheric CO2 measurement
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DOI：
10.1029/2009jd011734
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发表时间：
2009-07
影响因子：
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通讯作者：
Y. Tohjima;K. Katsumata;I. Morino;H. Mukai;T. Machida;Isao Akama;Taketo Amari;U. Tsunogai
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文献类型：
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作者：
Y. Tohjima;K. Katsumata;I. Morino;H. Mukai;T. Machida;Isao Akama;Taketo Amari;U. Tsunogai
[1]当使用同位素组成与大气CO2不同的CO2-空气混合物作为非色散红外（NNDIR）CO2分析仪校准气体时，NDIR CO2分析仪可能对空气样品产生错误的CO2摩尔分数测量。这是因为（1）在典型的N2 O3分析仪中配备的光学带通滤波器基本上被设计成仅透射12 C16 O2的吸收带，以使来自其它红外活性物质的干扰效应最小化，以及（2）其它CO2相关同位素体（例如13 C16 O2）的吸收带根据其同位素效应而被移动到较低波数。为了评估同位素组成对NH3响应的影响，我们基于单个同位素体的红外吸收率计算了仪器对每个同位素体的理论相对摩尔响应。然后，我们制备了重量13 CO2-空气中的混合物与CO2的摩尔分数为380 ppm的实验确定的光学过滤器的属性。在这项研究中使用的三个分析仪测定的13 CO2-空气中的混合物的表观摩尔分数为46，94，和27 ppm，这表明在这些仪器中的光学过滤器大大降低了响应13 C16 O2。基于这些理论和实验分析，我们评估了当使用同位素较轻的空气中CO2混合物（δ 13 C = −32.4‰和δ 18 O = +11.7‰）与大气中CO2（δ 13 C = −8‰和δ 18 O = 1/4 + 40 ‰）相比作为校准气体时，由三台NATRA分析仪测定的CO2摩尔分数与真实值的明显差异。不同的分析仪得出的估计差异不同，对于CO2含量为380 ppm的空气样本，差异范围为−0.04至−0.08 ppm。
[1] Nondispersive infrared (NDIR) CO2 analyzer could produce erroneous CO2 mole fraction measurements for an air sample when CO2-in-air mixtures having different isotopic compositions than atmospheric CO2 are used as the NDIR calibration gases. This is because (1) an optical band-pass filter equipped in a typical NDIR analyzer to minimize the interference effect from the other infrared-active species is basically designed to transmit only the absorption band of 12C16O2 and (2) absorption bands for the other CO2-related isotopologues, for example, 13C16O2, are shifted to lower wave numbers depending on their isotope effects. To evaluate the effect of the isotopic composition on the NDIR response, we computed the theoretical relative molar response of the instrument to each isotopologue based on the infrared absorptance by the individual isotopologues. We then prepared a gravimetric 13CO2-in-air mixture with CO2 mole fraction of 380 ppm to experimentally determine the optical filter property. The apparent mole fractions of the 13CO2-in-air mixture determined by three NDIR analyzers used in this study were 46, 94, and 27 ppm, indicating that the optical filters in these instruments substantially reduced the response to 13C16O2. Based on these theoretical and experimental analyses, we evaluated the apparent difference in the CO2 mole fraction determined by the three NDIR analyzers from the true value when isotopically lighter CO2-in-air mixtures (δ13C = −32.4‰ and δ18O = +11.7‰), as compared to atmospheric CO2 (δ13C = ∼−8‰ and δ18O = ∼+40‰), are used as calibration gases. The estimated difference varied with NDIR analyzers, ranging from −0.04 to −0.08 ppm for air samples with 380 ppm CO2.