Open‐Path Dual‐Comb Spectroscopy for Multispecies Trace Gas Detection in the 4.5–5 µm Spectral Region

Abstract Open‐path dual‐comb spectroscopy provides multispecies atmospheric gas concentration measurements with high precision. Here, open‐path dual‐comb spectroscopy is extended to the mid‐infrared 5 µm atmospheric window, enabling atmospheric concentration retrievals of the primary greenhouse gases, N 2 O, CO 2 , and H 2 O, as well as the criteria air pollutants O 3 and CO across 600 m and 2 km round‐trip paths. Measurements are demonstrated over a five‐day period at 2 min temporal resolution with 80% uptime. The achieved precision is sufficient to resolve the atmospheric concentration variations of the multiple gas species; retrieved dry mixing ratios of CO and N 2 O are in good agreement with a colocated point sensor. In addition, the retrieved ratio of excess CO versus CO 2 agrees with similar urban studies but disagrees with the US National Emission Inventory by a factor of 3. The retrieved ratio of excess N 2 O versus CO 2 exhibits a plume‐dependent value, indicating the variability of sources of the greenhouse gas N 2 O.