Methane (CH₄) is the primary component of natural gas, accounting for more than 90% of it. CH₄is a colorless, odorless, flammable, and explosive gas. It is a safety hazard during the production and transportation of natural gas, coal, and oil. CH₄ leakage is difficult to detect and can result in suffocation or even death. When the CH₄ concentration(volume fraction) in the air reaches a certain limit (5%-16%), it can cause combustion and explosion in the presence of a heat source or open fire. According to the data from the U.S. Environmental Protection Agency in 2020, CH₄ is a strong, short-term greenhouse gas with a rapid warming effect, accounting for 20% of global emissions, ranking second only to carbon dioxide (CO₂). Although CH₄ exists in the atmosphere for only approximately 12 years, its heat storage capacity in the atmosphere is 28-34 times that of CO₂. Therefore, the development of a CH₄ sensor system for real-time monitoring of CH₄ concentration is essential for mitigating climate change, finding the sources of natural gas leakage quickly, and minimizing the loss of life and property.

The selection of gas detection technology is essential for detecting trace concentrations of CH₄ in the atmosphere. In terms of time resolution, size, and cost, optical methods based on mid-infrared laser spectroscopy are advantageous for CH₄ sensing. Tunable diode laser absorption spectroscopy (TDLAS) is demonstrated to be an excellent tool for trace gas detection because of its high sensitivity, selectivity, and real-time performance. In this paper, a mid-infrared CH₄ sensor system is developed based on TDLAS. The light source is an interband cascade laser that is used to target the absorption line of CH₄ at 3038.5 cm^-1. For gas absorption enhancement, a multi-pass gas cell with an effective optical path of 26.4 m is used. To increase detection range and sensitivity, the wavelength modulation spectroscopy (WMS) scheme is used. The optical structure is simple, stable, and compact, making it ideal for vehicle-borne movement measurement. A mobile natural gas leakage source location method based on the LabVIEW data processing platform along with the gas turbulence model and particle swarm optimization algorithm is developed. Vehicle-borne movement measurement and leakage source location experiments are performed on Jilin University campus using the CH₄ sensor system to validate its practical applicability.

The modulation depth optimization experiment is performed to optimize the sensor's performance, and the optimal modulation depth is 0.235 cm^-1 (Fig.5). To evaluate the stability of the CH₄ sensor, pure N₂ is measured for approximately 16 min using the WMS scheme. The Allan variance is used to calculate the detection limit of the CH₄ sensor based on the measured data. The detection limit is 4.1×10^-9 with the average time of 0.5 s, and the optimal detection limit is 6.2×10^-10 with the average time of 54 s (Fig.7). The CH₄ sensor has the response time of 1.92 s (Fig.8). The results of the vehicle-borne movement measurement experiment show that the atmospheric CH₄ concentration range is 2.04×10^-6-2.16×10^-6 (Fig.9). The field testing is conducted to validate the effectiveness of the mobile natural gas leakage source location method. The localization error is 3.43 m.

This study demonstrates a promising method for detecting trace CH₄ concentrations in the atmosphere and identifying the source of natural gas leakage for field applications. The experimental results demonstrate that the proposed method has promising future application in the detection of natural gas leakage.