Calibration of isotopologue-specific optical trace gas analysers: a practical guide

Abstract

Abstract. The isotopic composition of atmospheric trace gases such as CO2 and CH4 provides a valuable tracer for the sources and sinks that contribute to atmospheric trace gas budgets. In the past, isotopic composition has typically been measured with high precision and accuracy by isotope ratio mass spectrometry (IRMS) offline and separately from real-time or flask-based measurements of concentrations or mole fractions. In recent years, development of infrared optical spectroscopic techniques based on laser and Fourier-transform infrared spectroscopy (FTIR) has provided high-precision measurements of the concentrations of one or more individual isotopologues of atmospheric trace gas species in continuous field and laboratory measurements, thus providing both concentration and isotopic measurements simultaneously. Several approaches have been taken to the calibration of optical isotopologue-specific analysers to derive both total trace gas amounts and isotopic ratios, converging into two different approaches: calibration via the individual isotopologues as measured by the optical device and calibration via isotope ratios, analogous to IRMS. This paper sets out a practical guide to the calculations required to perform calibrations of isotopologue-specific optical analysers, applicable to both laser and broadband FTIR spectroscopy. Equations to calculate the relevant isotopic and total concentration quantities without approximation are presented, together with worked numerical examples from actual measurements. Potential systematic errors, which may occur when all required isotopic information is not available, or is approximated, are assessed. Fortunately, in most such realistic cases, these systematic errors incurred are acceptably small and within the compatibility limits specified by the World Meteorological Organisation – Global Atmosphere Watch. Isotopologue-based and ratio-based calibration schemes are compared. Calibration based on individual isotopologues is simpler because the analysers fundamentally measure amounts of individual isotopologues, not ratios. Isotopologue calibration does not require a range of isotopic ratios in the reference standards used for the calibration, only a range of concentrations or mole fractions covering the target range. Ratio-based calibration leads to concentration dependence, which must also be characterised.

N<sub>2</sub>O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison

“…There was an offset in measured δ values resulting from the change in [N2O] introduced to the analyzers analyzers tested, which is characteristic of optical analyzers calibrated using a δ calibration scheme (Griffith et al, 2012;Griffith, 2018). However, examination of the residuals from the linear regression revealed varying degrees of residual curvature, highlighting that further non-linear terms would be required to adequately describe, and correct for, this mole fraction dependence (see Griffith et al, 2012).…”

“…Alternative calibration approaches based on isotopocule concentrations or ratio calibration procedures were not included in our study, but have the potential to remove the need for this correction (e.g. Wen et al, 2013;Flores et al, 2017;Griffith, 2018). Isotopocule calibration approaches would 20 require a set of N2O standard gases with high accuracy mole fractions in addition to assigned δ values.…”

“…(3) changes in [N2O] affect N2O isotope results when using the δ-calibration approach (Griffith, 2018); and (4) laser spectroscopic results are affected by mole fraction changes of atmospheric background gases 20 (N2, O2, and Ar), herein called gas matrix effects, due to the difference of pressure-broadening coefficients (Nara et al, 2012), and potentially by spectral interferences from other atmospheric constituents (H2O, CO2, CH4, CO, etc. ), herein called trace gas effects, depending on the selected wavelength region.…”

Combining a Quantum Cascade Laser Spectrometer with an Automated Closed-Chamber System for δ13C Measurements of Forest Soil, Tree Stem and Tree Root CO2 Fluxes

“…Direct comparison of our results with previous studies is not possible because no one else has previously combined an Aerodyne CO 2 isotope QCL system with the LI8100A/LI8150 system for automated closed-chamber measurements. However, the issues of dependence of water vapour and especially CO 2 concentration on measured δ 13 C have been addressed in a few studies using laser spectroscopy methods, including quantum cascade lasers [19,[23][24][25]34]. In a study where δ 13 C was measured with a quantum cascade laser spectrometer, [21] performed 90 closed-chamber measurements and determined the CO 2 concentration dependency but did not report the effect of this calibration on the raw δ 13 C values or the effect on the δ 13 C determined by the Keeling plots.…”

“…Furthermore, the change in CO 2 concentration over time during eddy covariance measurements is relatively small and thus, the concentration dependence may be limited. However, it still needs to be quantified, which has been done by varying the CO 2 concentration in calibration gases while keeping the δ 13 C constant [25,26]. In contrast to eddy covariance, the concentration of both CO 2 and H 2 O, as well as δ 13 C may all change significantly during a closed-chamber measurement.…”