A dual-comb spectrometer for open-path measurements of greenhouse gases in comparison with Fourier transform spectroscopy.

Path averaged measurements of greenhouse gases (GHG) on the kilometer scale can potentially improve measurement-based estimations of anthropogenic emissions. Path averages are less sensitive to local emission patterns than point-like in-situ measurements. As a result, they could provide more robust information in the face of uncertain prior emission fields, especially if these are highly structured. A typical case are urban areas, which are a major and growing contributor to anthropogenic GHG emissions, but their contribution is also subject to significant uncertainty [1].

Many different techniques are in theory available to perform these path averaged measurements. Between all of them, dual-comb spectroscopy (DCS) comes with a distinct set of features, which make it an ideal tool for the task at hand: high spectral radiance, resulting in high precision, broadband spectral coverage, allowing access to multiple species and a robust spectroscopic evaluation and an extremly high resolution, rendering the spectra basically free of any instrument line function, to name just a few. Additionally, DCS was already demonstrated to be field-deployable [2]. Finally, rapid developments on the commercial availability of DCS systems and their building blocks result in an increased accessibility to this technique, including users without a strong background in Laser Physics and metrology.

Our near-infrared dual comb spectrometer for open-path measurements of greenhouse gases over the city of Heidelberg is centered around two fully stabilized commercially available turn-key frequency combs. We present the results of the first nine months near continuous operation along a 1.55 km long path, including side-by-side measurements with an open-path Fourier transform spectroscopy (FTS) system [3]. With a xCO₂ precision of 1 ppm on a one-minute timescale the DCS system proves five times more precise than the FTS, with a clear path to improvement by at least another factor of two. This puts our system at par with previous, fully home build systems of metrology expert groups [4], all achieved in less than a year after the arrival of the lasers, demonstrating the technological maturity.

References:
[1] Federal Environment Agency, "National Inventory Report for the German Greenhouse Gas Inventory 1990 – 2019" UNFCCC Submission (2021)
[2] Nathan Malarich, et al. "Evaluating CO2 and CH4 absorption models with open-path dual-comb spectroscopy at the Mauna Loa Observatory." Journal of Quantitative Spectroscopy and Radiative Transfer (2025): 109567. https://doi.org/10.1016/j.jqsrt.2025.109567
[3] Tobias D. Schmitt, et al. "An open-path observatory for greenhouse gases based on near-infrared Fourier transform spectroscopy" Atmos. Meas. Tech., 16, 6097-6110 (2023) https://doi.org/10.5194/amt-16-6097-2023
[4] Eleanor M. Waxman, et al. " Estimating vehicle carbon dioxide emissions from Boulder, Colorado" Atmos. Chem. Phys., 19, 4177–4192, (2019) https://doi.org/10.5194/acp-19-4177-2019