Measurement of VOCs using open-path mid-infrared dual comb spectroscopy

Open-path measurements of atmospheric gas species over km-scale path lengths are well suited to quantify emissions from sources like oil and gas, forest fires, and industry. is a relatively new technique that combines high-resolution and broad spectral coverage with no instrument lineshape and near perfect frequency calibration. These features have enabled open-path DCS to provide accurate measurements of multiple trace gas species simultaneously in the near-infrared across path lengths ranging from 100 m to several km. However, in order to reach the sensitivity necessary to detect many atmospheric trace constituents, including volatile organic compounds (VOCs), operation in the mid-infrared (or UV/Vis) is required.

Here, we show a mid-infrared open-path dual comb spectrometer operating in the 3-4 and 4.5-5 μm spectral regions. We have used this spectrometer to measure methane, ethane, and propane (arising primarily from oil and gas activity) across a 1-km-long path in Boulder, CO for 1 week with an ethane sensitivity of ∼0.1 ppb for a 2-minute time resolution. In addition, we show quantitative measurements of intentionally released acetone and isopropanol with a 1-σ sensitivity of 5.7 ppm·m and 2.4 ppm·m, respectively. In the 4.5-5 μm region, we have used this system to detect N₂O, CO, and O₃. Finally, we have developed a second-generation instrument in the 3-4 μm region that is more compact and has improved stability. This system was recently deployed in a van at an active oil and gas drilling operation. We present preliminary measurements of methane, ethane, and higher hydrocarbons from this deployment as well as initial efforts at emissions quantification.