Calibration of sealed HCl cells used for TCCON instrumental line shape monitoring

Abstract

Abstract. The TCCON (Total Carbon Column Observing Network) FTIR (Fourier transform infrared) network provides highly accurate observations of greenhouse gas column-averaged dry-air mole fractions. As an important component of TCCON quality assurance, sealed cells filled with approximately 5 mbar of HCl are used for instrumental line shape (ILS) monitoring at all TCCON sites. Here, we introduce a calibration procedure for the HCl cells which employs a refillable, pressure-monitored reference cell filled with C 2 H 2 . Using this method, we identify variations of HCl purity between the TCCON cells as a non-negligible disturbance. The new calibration procedure introduced here assigns effective pressure values to each individual cell to account for additional broadening of the HCl lines. This approach will improve the consistency of the network by significantly reducing possible station-to-station biases due to inconsistent ILS results from different HCl cells. We demonstrate that the proposed method is accurate enough to turn the ILS uncertainty into an error source of secondary importance from the viewpoint of network consistency.

Intercomparability of X<sub>CO<sub>2</sub></sub> and X<sub>CH<sub>4</sub></sub> from the United States TCCON sites

“…At the sites described in this study, weekly internal lamp measurements of the internal, calibrated HCl cells (Hase et al, 2013) are collected from the 125HR instruments. We use LINEFIT 14.5 (Hase et al, 1999) software on HCl lines from monthly-averaged spectra to characterize the ILS.…”

The mechanical and thermal setup of the GLORIA spectrometer

“…For the guidance of the slide, a preloaded dovetail slide was chosen. Due to its large contact area, it is very stable and largely immune to external loads and vibrations when compared to ball-guided slides (Endemann, 1999) or guide slide bearings with small contact areas as used in high resolution laboratory spectrometers (Hase et al, 2013). These advantages are gained at the cost of higher friction; therefore, the material combination and lubrication have to be chosen carefully.…”

First continuous measurements of δ<sup>18</sup>O-CO<sub>2</sub> in air with a Fourier transform infrared spectrometer

“…To investigate the ILS contribution further, we have recorded spectra of air under the same conditions (temperature, pressure, resolution, cell path length) in a Bruker IFS 125/HR spectrometer at the University of Wollongong. This high resolution spectrometer is maintained in a wellaligned condition as part of the Total Carbon Column Observing Network (TCCON, Wunch et al, 2011) and its ILS is well characterised through high resolution test cell measurements (Hase et al, 2013) to be very close to the theoretically ideal shape calculated in the MALT model (modulation efficiency > 0.99, phase error < 0.5 • ). Fitting these IFS 125/HR spectra resulted in residuals very similar in shape and magnitude to that in Fig.…”