# In-Situ Testing of Methane Emissions from Landfills Using Laser Absorption Spectroscopy
## 1 Introduction  
The world generates 2.01 billion tons of municipal solid waste (MSW) annually with a per capita rate of 0.74 kg/day [1]. By 2030 and 2050, the world is expected to generate 2.59 billion and 3.40 tons of MSW annually, respectively. Around the world, almost 40 percent of MSW is disposed of in landfills. Landfilled MSW contains large amounts of organic matter, which will biodegrade in anaerobic and aerobic conditions to produce large amounts of landfill gas (LKG) [2; 3; 4; 5]. The main components of LKG are methane (volume ratio of 55-60%) and carbon dioxide (volume ratio of 40-45%) [6; 7; 8; 9]. The annual production of LKG in China is about 13.2 billion m\({}^{3}\)[10]. Landfill is ranked as the third-largest resource of anthropogenic methane emissions. It has been reported that the annual methane emission from landfills accounted for 22% of the total anthropogenic methane emissions during 2000-2009 [11]. Landfill methane is not only a greenhouse gas, but it also could be reused as a clean fuel for power generation [12; 13]. To reduce methane emissions, the surface of landfills is generally covered with a final cover system (i.e., soil layers) or a temporary cover system (i.e., geomembrane). When the landfill is covered well, the gas collection efficiency, which is defined as the ratio of the collected amount of gas to the amount generated, can reach 90% [14]. However, methane emissions from many landfills remain high due to a number of factors, including improper cover structure, holes in geomembrane, uncovered working area, etc. [15; 16]. Field monitoring of methane emissions from landfills is of great importance for both environmental concern and economic benefit. The static chamber method (SCM) is most widely used to monitor methane emission flux in landfills due to its simple approach and low-cost [17; 18; 19]. The principle of SCM is to seal a known volume chamber above a gas-emitting surface such that the emitted gas cannot escape and its accumulated concentration in the chamber can be tested. [PERSON] et al. [20] measured the methane and carbon dioxide surface emissions from a landfill in Burkina Faso using the static chamber method. The flux rate of methane was in a range of 657-1210 mg/m\({}^{2}\)/h at the fully covered area, which was higher than the tolerable value. [PERSON] et al. [21] investigated the methane emission fluxes from two landfills with different surface cover conditions in China by SCM. The influence of vegetation and climatic conditions on methane emission was discussed. [PERSON] et al. [22] developed a static chamber method to measure the emissions of CH\({}_{4}\) and CO\({}_{2}\) from landfill covers constructed with loess and gravel. The in situ testing results were consistent with those of the laboratory calibration. Although the static chamber method is proved to be feasible in methane emission monitoring, there are still some disadvantages. For the whole landfill site monitoring, a large number of points are needed to quantify the representative flux of the whole site [23]. In this case, the chamber method is time and labor-intensive [24]. Meanwhile, the sampling chamber closure may create some overpressures, which can lead to overestimation of methane concentration. To overcome the disadvantages of SCM, optical sensing techniques based on infrared absorption spectroscopy are recently developed [25]. [PERSON] et al. [26] adopted a portable laser measurement detector (LMD) to monitor the methane concentration at a large-scale landfill in China. The average methane concentration above the temporary cover (geomembrane) was higher than that observed on the final cover (loess layer). LMD measured the methane concentration of landfill gas in a range of 1 m.  
In this study, a device for measuring methane concentration is designed based on tunable diode laser absorption spectroscopy (TDLAS). Compared with SCM and LMD, the TDLAS method has higher test efficiency.