Methane Exchange Flux Monitoring between Potential Source Sewage Inspection Wells and the Atmosphere Based on Laser Spectroscopy Method

Abstract

Greenhouse gas emissions are changing the climate and affecting human activities. In cities, the anaerobic environment in sewage inspection wells produces CH4, which is exchanged with the atmosphere and causes pollution. Moreover, although the number of inspection wells has increased with the development of cities, people have not paid enough attention to this potential source of CH4 and ignored it in the statistics of greenhouse gas inventories. Conventional gas monitoring methods like gas chromatography are complex and expensive. Based on the portable TDLAS CH4 sensor developed by our team, combined with a gas velocity analyser, we realised in situ continuous flux monitoring. We corrected the effect of temperature on the results according to the theory of infrared thermometry. We showed that the measurement results of the sensor fluctuate within the range of ±0.1 ppm of the measured standard gas concentration. We also verified its repeatability and ensured its reliability in field applications by comparing its results with the results of gas chromatography analysis. In addition to flux monitoring, based on the monitoring data of 56 inspection wells in the study area, the average concentration was estimated using the Bootstrap method, and combined with the average value of gas velocity, the average flux was estimated to be 2.19 × 10−6 mol/s, and the daily exchange mass was 3.03 g CH4 d−1. Combined with information such as the length of sewage pipes, we estimate that the annual CH4 exchange mass in the city is about 5.49 × 105 kg CH4 yr−1. This monitoring method will help us to understand climate change and improve greenhouse gas inventories.