MEASUREMENT OF VOCs USING OPEN-PATH MID-INFRARED DUAL-COMB SPECTROSCOPY

Dual frequency comb spectroscopy (DCS) is a rapidly evolving technique that provides a high-resolution, broadband spectrometer with no instrument lineshape and near perfect frequency calibration\footnote{Coddington I, Newbury N, Swann W. Dual-comb spectroscopy. Optica. 2016 Apr 20;3(4):414. }. These features make DCS well suited for accurate measurements of multiple species simultaneously. Because the frequency comb lasers can be well collimated, such a system can be used for long open-path measurements with path lengths ranging from hundreds of meters to several kilometers. Open-path measurements of atmospheric gas species over these path lengths are well suited to quantify emissions from sources like oil and gas, forest fires, and industry. Previous demonstrations of open-path DCS have primarily been in the 1-2 $\mu$m spectral region; however, in order to reach the sensitivity necessary to detect many atmospheric trace constituents, including volatile organic compounds, operation in the mid-infrared is required. Here, we show a mid-infrared open-path dual comb spectrometer operating in the 3-5 $\mu$m spectral region. We have used this spectrometer to measure methane, ethane, propane, and butane isomers (arising primarily from oil and gas activity) across a 1-km-long path in Boulder, CO for 1 week with an ethane sensitivity of $\sim$0.1 ppb for a 2-minute time resolution. In addition, we show quantitative measurements of intentionally released acetone and isopropanol with a 1-$\sigma$ sensitivity of 5.7 ppm · m and 2.4 ppm · m, respectively