# In-Situ Testing of Methane Emissions from Landfills Using Laser Absorption Spectroscopy
## 3 Gas Concentration Calibration  
Before the in situ testing, the relationship between methane concentration and the optical signal was calibrated in the laboratory. The testing apparatus emitted a laser to pass through a gas sample with a methane mass concentration of 500 parts per million (ppm). Figure 4 shows the received signal amplitude. There was an obvious dent in data points between 11,000 and 13,000. This was because that when the laser-scanned to CH4 absorption peak, part of the laser was absorbed by the CH4 molecular. It led to the detected intensity being weaker than that without the absorption. Figure 3: Pictures of the testing apparatus: (**a**) schematic view, (**b**) outside view, and (**c**) inside view. After the absorption data of CH\({}_{4}\) gas was measured, the absorption spectrum could be obtained through these data by two methods. One method was to measure a set of background signals (i.e., the signals without CH\({}_{4}\) gas) and then subtracted the measured background signals from the absorption signals so as to obtain the absorption spectrum of CH\({}_{4}\). Another method was to directly use the non-absorbed part of the CH\({}_{4}\) absorption signal to fit the background signal and then subtract the fitted background signal to obtain the absorption spectrum of CH\({}_{4}\). The accuracy of the absorption spectrum obtained by the first method was high, but it needed to collect a background signal for each measurement, which was time-consuming. The second method was to directly use the algorithm to fit the background signal, which was high-efficiency and suitable for practical application. Therefore, the second method was adopted in this study to obtain the absorption spectrum. The background signal was fitted using a linear function, and the results are shown in Figure 4 (with correlation coefficient R2 = 0.98). According to the fitted curve, the accuracy of the method is the CH4 concentration corresponding to the detected signal without CH\({}_{4}\) absorption. The laser wavelength was determined according to the input current. The relationship between the laser wavelength and current was calibrated in the laboratory, and the result is shown in Figure 5. When the current increased from 40 mA to 80 mA under the temperature of 16.2 \({}^{\circ}\)C, the laser wavelength increased from 1653.54 nm to 1654.22 nm. The drift coefficient of wavelength with the current was 0.017 nm/mA. When the laser current was constant at 60 mA, as the temperature increased from 14.8 \({}^{\circ}\)C to 17.7 \({}^{\circ}\)C, the laser wavelength increased from 1653.68 nm to 1654 nm. The drift coefficient of wavelength with temperature was 0.11 nm/\({}^{\circ}\)C. For a given current and temperature, the wavelength was determined based on the calibration relationship. Figure 4: Received signal amplitude after laser passing through the gas sample with methane concentration of 500 ppm. Figure 6 shows the absorption spectrum of CH\({}_{4}\) after subtracting the fitted background signal. Thirteen CH\({}_{4}\) samples were tested with mass concentrations ranged from 400 ppm to 5000 ppm. The absorption spectrum amplitude (at a wavelength of 1653.7 nm) showed an increasing trend with the increasing CH\({}_{4}\) concentration. Under different CH\({}_{4}\) concentrations, the absorption spectrum in the non-absorbed part (i.e., wavelength \(<\) 1653.6 nm and wavelength \(>\)1653.8 nm) was very close. The absorption spectrum amplitudes (which are the peak values of the absorption spectrum in Figure 6) of CH\({}_{4}\) samples at 13 mass concentrations of 5000 ppm, 2590 ppm, 1890 ppm, 1380 ppm, 1040 ppm, 860 ppm, 780 ppm, 650 ppm, 580 ppm, 520 ppm, 470 ppm, 430 ppm and 400 ppm were obtained. Each group of data was collected 5 times, and an average value was calculated. The standard deviation of the five data was less than 5 ppm. Figure 7 shows the variation of absorption spectrum amplitude with CH\({}_{4}\) mass concentration.