Open-Path Mid-Infrared Remote Sensing of Atmospheric Gases Using a Broadband Optical Parametric Oscillator
Fugitive hydrocarbon emissions cost the energy sector $5B per year, account for 12% of greenhouse gas emissions and jeopardize safety and public health. Current gas monitoring methods lack the ability to take measurements in moderate winds, at varying heights and to provide simultaneous measurements of a diverse range of hydrocarbon species. The most advanced technique for remote emission measurements is differential absorption lidar (DIAL), in which intense IR pulses are fired into the atmosphere and returned to a ground-based detector by weak scattering from airborne particles. DIAL has many limitations due to the use of narrow-line (1 cm-1) dye-laser technology which restricts it to measuring only one chemical at a time, and scanning for multiple compounds that absorb at different wavelengths is time consuming. Furthermore, DIAL is a complex system and hard to maintain because it uses dye laser technology; it is also large and inefficient. By contrast, Fourier transform infrared (FTIR) spectroscopy, already a gold standard for laboratory chemical identification, is naturally broadband, offering far wider species coverage to DIAL, but the beam quality of the thermal sources in lab instruments is inadequate for free-space remote spectroscopy. Laser-based active FTIR spectroscopy therefore presents immediate opportunities for simultaneous and quantitative hydrocarbon emissions monitoring in and around petrochemical sites, at landfill sites and in agricultural contexts. Critically, the ability to measure methane and ethane simultaneously makes it possible to separate methane contributions from oil and gas sources-which are accompanied by a weaker correlated ethane emission-from biogenic sources (cattle, landfill, compost) which produce only methane.
Here we present an eye-safe active FTIR spectroscopy system based on a broadband ultrafast optical parametric oscillator (OPO) operating in the 3.1–3.5-µm wavelength range (where common hydrocarbons display absorption peaks) and capable of acquiring gas absorption spectra from a simple target at ranges exceeding 30 metres. We demonstrate that simultaneous quantitative measurement of atmospheric background levels of water and methane is readily achieved by this system, even in the presence of strong absorption from a control cell of 1.5% ethane gas.
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Derryck T. Reid (Heriot-Watt University)
Prof. Derryck T. Reid leads Heriot-Watt's Ultrafast Optics Group, with interests in mid-infrared lasers and their applications in gas sensing and imaging. In 2012 he co-founded the UK laser company Chromacity Ltd., which manufactures broadband tunable visible and infrared lasers with applications in imaging and gas sensing.
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