Assessment of a multi-species in-situ FTIR for precise atmospheric greenhouse gas observations

Abstract

We thoroughly evaluate the performance of a multi-species, in-situ FTIR analyser with respect to high accuracy needs for greenhouse gas monitoring networks. The in-situ FTIR analyser measures CO<sub>2</sub>, CO, CH<sub>4</sub> and N<sub>2</sub>O mole fractions continuously, all with better reproducibility than requested by the WMO-GAW inter-laboratory compatibility (ILC) goal. Simultaneously determined δ<sup>13</sup>CO<sub>2</sub> reaches reproducibility as good as 0.03‰. This paper focuses on the quantification of residual dependencies between the measured components and the thermodynamic properties of the sample as well as the cross-sensitivities among the sample constituents. The instrument has proven to be linear for all components in the ambient range. The temporal stability of the instrument was investigated by 10 months of continuously collected quality control measures. Based on these measures we conclude that for moderately stable laboratory conditions weekly calibrations of the instrument are sufficient to reach WMO-GAW ILC goals

A Fourier transform infrared trace gas and isotope analyser for atmospheric applications

“…Hammer et al (2012) show similar stability over six days but some species show small significant drifts at the precision limit. Figure 6, also from Hammer et al (2012, Fig.…”

“…Figure 5 shows residuals from linear regressions of FTIRmeasured mole fractions against reference values from a suite of standard tanks maintained at the University of Heidelberg (data from Hammer et al, 2012). Similar measurements over wider mole fraction ranges for a suite of tanks at CSIRO's GASLAB also show no significant deviations from linearity (albeit with lower precision) .…”

“…Cal- ibration of the analyser to an absolute or reference scale is achieved by measurements of two or more tanks of air independently calibrated for each trace gas on the reference scales. Calibration is described in detail in an accompanying paper by Hammer et al (2012); combined uncertainty is shown to exceed GAW compatibility targets (Table 1) for all species except δ 13 C in CO 2 . Griffith et al (2011) and Hammer et al (2012) demonstrate that the raw FTIR scale is linear relative to WMO reference scales over a range of mole fractions typical of ambient air and above.…”

“…Calibration is described in detail in an accompanying paper by Hammer et al (2012); combined uncertainty is shown to exceed GAW compatibility targets (Table 1) for all species except δ 13 C in CO 2 . Griffith et al (2011) and Hammer et al (2012) demonstrate that the raw FTIR scale is linear relative to WMO reference scales over a range of mole fractions typical of ambient air and above. While the calibration regressions are in general linear within the measurement error limits, they have small but significant non-zero y-axis intercepts, so the general calibration equation for each species is expressed as…”

“…They inherently assume that Beer's Law (i.e. that absorbance is proportional to concentrations of absorbers) is obeyed or nearly obeyed, but cannot fit spectral variations due to ILS effects, and are restricted to regions of weak absorption to avoid non-linearities and breakdown of Beer's Law (Anderson and Griffiths, 1975;Haaland, 1987). In non-linear least squares the transmittance spectrum can be fitted in any region, not just one of weak absorption, because there is no assumption of linearity between transmittance and trace gas concentrations.…”