Preprints
https://doi.org/10.5194/amt-2021-91
https://doi.org/10.5194/amt-2021-91

  13 Apr 2021

13 Apr 2021

Review status: a revised version of this preprint was accepted for the journal AMT and is expected to appear here in due course.

Photochemical method for removing methane interference for improved gas analysis

Merve Polat1, Jesper Baldtzer Liisberg2, Morten Krogsbøll1, Thomas Blunier2, and Matthew S. Johnson1 Merve Polat et al.
  • 1Copenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark
  • 2Physics of Ice Climate and Earth, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen Ø, Denmark

Abstract. The development of laser spectroscopy has made it possible to measure minute changes in the concentrations of trace gases and their isotopic analogs. These single or even multiply substituted species occur at ratios from per cent to sub-ppm and contain important information concerning trace gas sources and transformations. Due to their low abundance minimizing spectral interference from other gases in a mixture is essential. Options including traps and membranes are available to remove many specific impurities. Methods for removing CH4 , however, are extremely limited as methane has low reactivity and adsorbs poorly to most materials. Here we demonstrate a novel method for CH4 removal via chlorine-initiated oxidation. Our motivation in developing the technique was to overcome methane interference in measurements of N2O isotopic analogs when using a cavity ring-down spectrometer. We describe the design and validation of a proof-of-concept device and a kinetic model to predict the dependence of the methane removal efficiency on methane concentration [CH4], chlorine photolysis rate JCl2, chlorine concentration [Cl2], and residence time tR. The model was validated by comparison to experimental data and then used to predict the possible formation of troublesome side- and by-products including CCl4 and HCl. The removal of methane could be maintained with a peak removal efficiency > 98 % for ambient levels of methane at a flow rate of 7.5 ml min−1 with [Cl2] at 50 ppm. These tests show that our method is a viable option for continuous methane scrubbing. Additional measures may be needed to avoid complications due to the introduction of Cl2 and formation of HCl. Note that the method will also oxidize most other common volatile organic compounds. The system was tested in combination with a cavity ring-down methane spectrometer, and the developed method was shown to be successful at removing methane interference.

Merve Polat et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Merve Polat et al.

Merve Polat et al.

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Short summary
We have designed a process for removing methane from a gas stream so that nitrous oxide can be measured without interference. These are both key long lived greenhouse gases, and frequently studied in relation to ice cores, plants, water treatment and so on. However, many researchers are not aware of the problem of methane interference and in addition there have not been good methods available for solving the problem. Here we present and evaluate such a method.