# In-Situ Testing of Methane Emissions from Landfills Using Laser Absorption Spectroscopy
## 5 Conclusions  
This study presents a method to measure methane concentration using tunable diode laser absorption spectroscopy (TDLAS). Beer-Lambert law is adopted to analyze the wavelength-dependent absorption behavior of light through a gas medium. Based on TDLAS, the testing apparatus is designed and assembled for in situ testing.  
The relationship between methane concentration and the optical signal was calibrated in the laboratory. When the laser-scanned to CH\({}_{4}\) absorption peak, part of the laser was absorbed by the CH\({}_{4}\) molecular, causing the detected light intensity to decrease. As the CH\({}_{4}\) concentration increased from 400 ppm to 5000 ppm, the absorption spectrum amplitude increased linearly from 0.0005 to 0.0046. This linear relationship was used to determine the CH\({}_{4}\) mass concentration during the in situ testing.  
The in situ testing of methane emissions at the Xiaping landfill demonstrated the accuracy of the TDLAS method. The methane concentrations in the well-covered areas were generally below 100 ppm. In the working area or the coverage area with holes, the methane concentration was about 700 ppm. The methane concentration was up to 1900 ppm, where the gas collection pipe is disconnected. Owing to the accuracy and simplicity, the TDLAS method is suitable to detect methane emissions on a large-scale from landfills.  
Although the TDLAS method showed good performance in quick testing of methane emission from landfills, this method also has some weaknesses. During the field test, the laser signal is easily disturbed by aerosol scattering, humidity and other environmental  
Figure 12: Methane concentration distribution above the surface of the Xiaping landfill.  
factors, which will affect the testing accuracy. For example, the scattering phenomenon is serious in foggy weather, and the accuracy of the long-distance measurement by TDLAS will be significantly reduced. In bad weather, such as heavy fog, heavy rain and strong wind, the static chamber method is less disturbed and has higher accuracy.  
Conceptualization, H.H. and S.H.; methodology, H.H., S.H. and J.H.; validation, H.H., Y.S. and T.Z.; formal analysis, H.H.; investigation, S.G. and T.W.; resources, H.H., S.H., C.Z. and T.Z.; writing--original draft preparation, H.H.; writing--review and editing, Y.S., S.H. and J.H.; supervision, Z.Q.; project administration, H.H. and Y.S.; funding acquisition, H.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Zhejiang Provincial Key Research and Development Program (2021C03178), Key projects of the Natural Science Foundation of Zhejiang Province (LCZ19E080001), Science and technology project of the Ministry of Housing and Urban-Rural Development (2020-K-026), Exploration project of Natural Science Foundation of Zhejiang Province (LQ21E080003), Taizhou science and technology project (1902 gy18), and Science and technology project of Department of housing and urban-rural development of Zhejiang Province (2020-K-164).  
Not applicable.  
The data that support the findings of this study are available from the corresponding author upon reasonable request.  
The authors acknowledge financial supports from the Zhejiang Provincial Key Research and Development Program (2021C03178), Key projects of the Natural Science Foundation of Zhejiang Province (LCZ19E080001), Science and technology project of the Ministry of Housing and Urban-Rural Development (2020-K-026), Exploration project of Natural Science Foundation of Zhejiang Province (LQ21E080003), Taizhou science and technology project (1902 gy18), and Science and technology project of Department of housing and urban-rural development of Zhejiang Province (2020-K-164). The TDLAS equipment was developed in the laser spectroscopy lab of the Center for Optical and Electromagnetic Research, Zhejiang University.  
The authors declare no conflict of interest.