Mobile Vehicle-Mounted Mid-Infrared Dual-Comb Spectrometer for Outdoor Trace Gas Detection

Advances in mid-infrared (MIR) dual-comb spectroscopy (DCS) have greatly boosted molecular detection in recent years. The ability of DCS to precisely identify and quantify outdoor atmospheric trace gases makes it attractive for applications in agriculture, energy, and industrial monitoring. Here, we demonstrate a vehicle-mounted MIR DCS system based on optical-optical modulated frequency combs (OMFCs), which achieve passive mutual coherence between the two combs. The system enables point-by-point and drive-by measurements under various outdoor conditions, achieving minimum detection limits of 45 parts per billion (ppb) for methane (CH$_4$) and 46 parts per million (ppm) for water vapor (H$_2$O) at 100-s averaging time. Finally, we mapped the two-dimensional concentration field and the concentration probability distribution near a CH$_4$ source and compared them with the contemporaneous wind rose. These experiments demonstrate the system's capability for localized gas identification and in situ quantification with high temporal and spatial resolution.