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

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.