Calibration of isotopologue-specific optical trace gas analysers: a practical guide
<p>The isotopic composition of atmospheric trace gases such as CO<sub>2</sub> and CH<sub>4</sub> 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...
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2018-11-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/11/6189/2018/amt-11-6189-2018.pdf |
| Summary: | <p>The isotopic composition of atmospheric trace gases such as CO<sub>2</sub> and
CH<sub>4</sub> 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.</p><p>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.</p> |
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| ISSN: | 1867-1381 1867-8548 |