| Summary: | We present field deployment results of a portable optical absorption spectrometer for localization and quantification of fugitive methane (CH<sub>4</sub>) emissions. Our near-infrared sensor targets the 2ν<sub>3</sub> R(4) CH<sub>4</sub> transition at 6057.1 cm<sup>−1</sup> (1651 nm) via line-scanned tunable diode-laser absorption spectroscopy (TDLAS), with Allan deviation analysis yielding a normalized 2.0 ppmv∙Hz<sup>−1/2</sup> sensitivity (4.5 × 10<sup>−6</sup> Hz<sup>−1/2</sup> noise-equivalent absorption) over 5 cm open-path length. Controlled CH<sub>4</sub> leak experiments are performed at the METEC CSU engineering facility, where concurrent deployment of our TDLAS and a customized volatile organic compound (VOC) sensor demonstrates good linear correlation (R<sup>2</sup> = 0.74) over high-flow (>60 SCFH) CH<sub>4</sub> releases spanning 4.4 h. In conjunction with simultaneous wind velocity measurements, the leak angle-of-arrival (AOA) is ascertained via correlation of CH<sub>4</sub> concentration and wind angle, demonstrating the efficacy of single-sensor line-of-sight (LOS) determination of leak sources. Source magnitude estimation based on a Gaussian plume model is demonstrated, with good correspondence (R<sup>2</sup> = 0.74) between calculated and measured release rates.
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