07 Jun 2023
 | 07 Jun 2023
Status: a revised version of this preprint was accepted for the journal AMT.

A portable reflected-sunlight spectrometer for CO2 and CH4

Benedikt A. Löw, Ralph Kleinschek, Vincent Enders, Stanley P. Sander, Thomas J. Pongetti, Tobias D. Schmitt, Frank Hase, Julian Kostinek, and André Butz

Abstract. Mapping the greenhouse gases carbon dioxide (CO2) and methane (CH4) above source regions such as urban areas can deliver insights into the distribution and dynamics of the local emission patterns. Here, we present the prototype development and an initial performance evaluation of a portable spectrometer that allows for measuring CO2 and CH4 concentrations integrated along a long (>10 km) horizontal path component through the atmospheric boundary layer above a target region. To this end, the spectrometer is positioned at an elevated site from which it points downward at reflection targets in the region collecting the reflected sunlight at shallow viewing angles. The path-integrated CO2 and CH4 concentrations are inferred from the absorption fingerprint in the shortwave-infrared (SWIR) spectral range. While mimicking the concept of the stationary CLARS-FTS (California Laboratory for Atmospheric Remote Sensing - Fourier Transform Spectrometer) at Los Angeles, our portable setup requires minimal infrastructures and is straightforward to duplicate and to operate at various places.

For performance evaluation, we deployed the instrument, termed EM27/SCA, side-by-side with the CLARS-FTS at Mt. Wilson observatory (1670 m a.s.l.) above Los Angeles for a month-long period in Apr./May 2022. We determined the relative precision of the retrieved slant column densities (SCDs) for urban reflection targets to 0.36–0.55 % for O2, CO2 and CH4, where O2 is relevant for lightpath estimation. For the partial vertical columns (VCDs) below instrument level, which is the quantity carrying the emission information, the propagated precision errors amount to 0.75–2 % for the three gases depending on the distance to the reflection target and solar zenith angle. The comparison to simultaneous CLARS-FTS measurements shows good consistency, but the observed diurnal patterns highlight the need for taking into account light scattering to enable detection of emission patterns.

Benedikt A. Löw et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2023-101', Anonymous Referee #1, 04 Jul 2023
  • RC2: 'Comment on amt-2023-101', David Griffith, 17 Jul 2023

Benedikt A. Löw et al.

Benedikt A. Löw et al.


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Short summary
We developed a portable spectrometer (EM27/SCA) that remotely measures greenhouse gases in the lower atmosphere above a target region. The measurements can deliver insights into local emission patterns. To evaluate its performance, we set up the EM27/SCA above the Los Angeles basin side-by-side with a similar non-portable instrument (CLARS-FTS). The precision is promising and the measurements are consistent with CLARS-FTS. In the future, we need to account for light scattering.