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

F Hase,R Sussmann,T Blumenstock,F Desmet,D G Feist,D Wunch,V Sherlock,B J Drouin,Y Té,J Robinson,M Schneider,M De Mazière,G C Toon,C M Roehl,P Heikkinen,C Weinzierl,P O Wennberg,M Rettinger,T Warneke

doi:10.5194/amt-6-3527-2013

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 C2H2. 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.

Highlights

The Total Carbon Column Observing Network (TCCON) is an international network of ground-based Fourier transform spectrometers that record direct solar spectra in the near-infrared (Wunch et al, 2011)
Cell #17 is a unique specimen, as the cell body has been provided by Caltech, but filling and sealing has been performed by a German company (Neoplas, Greifswald)
We have presented a novel, accurate method for calibrating the sealed HCl cells used by TCCON for instrumental line shape (ILS) monitoring

Summary

Introduction

The Total Carbon Column Observing Network (TCCON) is an international network of ground-based Fourier transform spectrometers that record direct solar spectra in the near-infrared (Wunch et al, 2011). The column-averaged abundances of the radiatively important greenhouse gases CO2 and CH4, as well as of a suite of other atmospherically significant trace gases (CO, N2O, H2O, HDO, and HF), are derived from these spectra. Investigating the global distribution of greenhouse gases such as CO2 and CH4 has, until recently, primarily relied on in situ measurements from surface station networks. Remote sensing of greenhouse gas columns (or vertically integrated mixing ratios) is thought to improve flux estimates since variations in the gas columns are more directly relatable to mass fluxes than surface concentration measurements (Keppel-Aleks et al, 2011). The gradients in column CO2, for example, are small, requiring a precision and accuracy of < 1 ppm (< 0.25 %, Olsen and Randerson, 2004; Miller et al 2007).

Methods

Results

Conclusion