Long-Term Open-Path Dual-Comb Spectroscopy for Urban CO₂ Monitoring

Abstract. Accurate quantification of urban greenhouse gas (GHG) emissions can benefit from path-averaged, high-precision, high-temporal-resolution measurements that complement point sensors and passive remote sensing. Among open-path techniques, dual-comb spectroscopy (DCS) stands out as a particularly capable candidate, offering simultaneous broadband coverage, an absolute SI-traceable frequency axis, and sufficient spectral radiance for multi-kilometer paths. Here we present an open-path dual-comb spectrometer using two commercial, self-referenced, turn-key frequency combs operated continuously in Heidelberg, Germany, over an urban landscape. The instrument allows to infer column-averaged dry-air mole fractions of CO₂ along a 3.1 km absorption path. During routine observations within the evaluation period from September 2025 to February 2026 the system achieved a data coverage of 85 %, with losses primarily attributable to visibility-limiting weather conditions such as fog and heavy rain. The instrument precision, characterized by the overlapping Allan deviation under stable atmospheric conditions, reaches 4.79 ppm √s for CO₂, equivalent to 0.28 ppm at five minutes averaging time. These values are on par with or better than previous open-path DCS experiments and represent roughly one order of magnitude improvement over a co-deployed open-path Fourier transform spectrometer operating on the same path. The two instruments differ by a small bias of 0.16 ppm for CO₂. The results demonstrate that high-quality, long-term open-path DCS operation is achievable with readily available hardware, making the technique accessible to the broader atmospheric science community for applications ranging from urban-flux monitoring and network-scale observations to the validation of spectroscopic databases.