Theoretical and experimental evaluation of the isotope effect of NDIR analyzer on atmospheric CO₂ measurement

Abstract

[1] Nondispersive infrared (NDIR) CO 2 analyzer could produce erroneous CO 2 mole fraction measurements for an air sample when CO 2 -in-air mixtures having different isotopic compositions than atmospheric CO 2 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 12 C 16 O 2 and (2) absorption bands for the other CO 2 -related isotopologues, for example, 13 C 16 O 2 , 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 13 CO 2 -in-air mixture with CO 2 mole fraction of 380 ppm to experimentally determine the optical filter property. The apparent mole fractions of the 13 CO 2 -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

Evaluation of a new JMA aircraft flask sampling system and laboratory trace gas analysis system

“…On the other hand, the δ 13 C and δ 18 O values of ambient CO 2 were assigned −8 ‰ ( al., 1993) and +42 ‰ (Allison and Francey, 2007), respectively. Using these values, the isotope effect was estimated to be about 0.14-0.16 ppm (CO 2 of 360-420 ppm), which is consistent with a previous estimate of 0.14-0.16 ppm Tohjima et al, 2009).…”

“…Indeed, to experimentally evaluate the NDIR isotope effect, we measured the optical property of the filter by using a gravimetric 13 CO 2 -in-air mixture with CO 2 mole fraction of 380 ppm, following the method reported by Tohjima et al (2009). The apparent mole fraction of the 13 CO 2 -in-air mixture determined by the NDIR analyzer used in this study was determined to be 16 ppm, indicating that its optical filter substantially reduced the response to 13 C 16 O 2 .…”

Atmospheric CO<sub>2</sub> monitoring with single-cell NDIR-based analyzers

“…In this case, if the field measurements are made using a sensor that has different 13 CO 2 sensitivity than that used for laboratory transfer calibrations, biased measurements may result Tohjima et al, 2009). LiCor NDIR analyzers are approximately 10 to 30 % as sensitive to 13 CO 2 as they are to 12 CO 2 (McDermitt et al, 1993;Tohjima et al, 2009). For the PSU system, with field standards calibrated and used on two different LiCor sensors, this is likely a smaller factor, but for the AIRCOA system, with field standards calibrated on a Siemens Ultramat 6F that is essentially blind to 13 CO 2 , this effect requires a correction of approximately −0.04 ppm to field measurements, assuming a 30 % molar response ratio for 13 CO 2 on the LI-820.…”

“…In this case, if the field measurements are made using a sensor that has different 13 CO 2 sensitivity than that used for laboratory transfer calibrations, biased measurements may result Tohjima et al, 2009). LiCor NDIR analyzers are approximately 10 to 30 % as sensitive to 13 CO 2 as they are to 12 CO 2 (McDermitt et al, 1993;Tohjima et al, 2009).…”

Development of a cavity-enhanced absorption spectrometer for airborne measurements of CH<sub>4</sub> and CO<sub>2</sub>

“…However, if plumes are measured that are significantly enriched in 13 C such as from biomass burning this will result in an underestimation of the total mole fraction (0.5 ppb at δ 13 C − CH 4 = −27 ‰). Other CO 2 and CH 4 isotopologues are less abundant and are thought to have a smaller effect (Tohjima et al, 2009;Chen et al, 2010).…”