30 Aug 2023
 | 30 Aug 2023
Status: this preprint is currently under review for the journal AMT.

An open-path observatory for greenhouse gases based on near-infrared Fourier transform spectroscopy

Tobias D. Schmitt, Jonas Kuhn, Ralph Kleinschek, Benedikt A. Löw, Stefan Schmitt, William Cranton, Martina Schmidt, Sanam N. Vardag, Frank Hase, David W. T. Griffith, and André Butz

Abstract. Monitoring the atmospheric concentrations of the greenhouse gases (GHG) carbon dioxide CO2 and methane CH4 is a key ingredient for fostering our understanding of the mechanisms behind the sources and sinks of these gases and for verifying and quantitatively attributing their anthropogenic emissions. Here, we present the instrumental setup and performance evaluation of an open-path GHG observatory in the city of Heidelberg, Germany. The observatory measures path-averaged concentrations of CO2 and CH4 along a 1.55 km path in the urban boundary layer above the city. We are combining these open-path data with local in-situ measurements to evaluate the representativeness of these observation types on the kilometer-scale. This representativeness is necessary to accurately quantify emissions, since atmospheric models tasked with this job typically operate on kilometer-scale horizontal grids.

For the operational period between Feb. 8 and Jul. 11, 2023, we find a precision of 2.7 ppm (0.58 %) and 18 ppb (0.89 %) for the dry air mole fractions of CO2 (xCO2) and CH4 (xCH4) in 5-minute measurements, respectively. After calibration, the open-path measurements show excellent agreement with the local in-situ data under atmospheric background conditions. Both datasets show clear signals of traffic CO2 emissions on the diurnal CO2 cycle. However, there are particular situations, such as under south-easterly wind conditions, where the in-situ and open-path data reveal distinct differences up to 20 ppm in xCO2 most likely related to their different sensitivity to local emission and transport patterns.

Our setup is based on a Bruker IFS 125 HR Fourier transform spectrometer, which offers a spacious and modular design providing ample opportunities for future refinements of the technique with respect to finer spectral resolution and wider spectral coverage to inform on gases such as carbon monoxide and nitrogen dioxide.

Tobias D. Schmitt et al.

Status: open (until 11 Oct 2023)

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  • RC1: 'Comment on amt-2023-185', Anonymous Referee #1, 11 Sep 2023 reply

Tobias D. Schmitt et al.

Tobias D. Schmitt et al.


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Short summary
Our new observatory measures greenhouse gases concentrations of carbon dioxide CO2 and methane CH4 along a 1.55 km long light path over the city of Heidelberg, Germany. We compared our measurements with measurements that were taken at a single point at one end of our path. Both agreed mostly, but show a significant difference for CO2 with certain wind directions. This is important when using greenhouse gas concentration measurements to observe greenhouse gas emissions of cities.