Open-path multi-species remote sensing with a broadband optical parametric oscillator

Open-path remote sensing is critical for monitoring fugitive emissions from industrial sites, where a variety of volatile organic compounds may be released. At ranges of only a few tens of metres, spatially coherent broadband mid-infrared sources can access sufficiently large absorption cross-sections to quantify hydrocarbon gas fluctuations above ambient background levels at high signal:noise ratios. Here we report path-integrated simultaneous concentration measurements of water, methane and ethane implemented in the 3.1-3.5-µm range using 0.05-cm^-1-resolution Fourier-transform spectroscopy with an ultrafast optical parametric oscillator and a simple, non-compliant target. Real-time concentration changes were observed at a range of 70 m by simulating a fugitive emission with a weak localized release of 2% methane in air. Spectral averaging yielded a methane detection sensitivity of 595 ppb·m, implying a system capability to resolve few-ppb concentrations of many volatile organic compounds at observation ranges of 50-100 m.