Calculating methane emissions using low-cost fenceline sensors

Loss of methane (CH4) during extraction and transmission processes is harmful to both the environment and profit margins of the producer. Operators aim to keep CH4 loss during operation to a minimum, and continuous monitoring using low-cost fenceline sensors are now being commercialized to observe CH4 enhancements downwind of operations. However, it is not clear how useful these systems are and whether they can be used to quantify emissions or simply identify the presence of a leak. To investigate this, we deployed four calibrated low-cost sensors 30 m from emissions of known rates, 19, 75 and 132 g CH4 h-1 for 48 hours. The aims were to 1) determine if CH4 enhancements measured downwind of a leak, can be used to detect single, point source emissions normally seen from fugitive sources at oil and gas wells; 2) how meteorology and micrometeorology affects downwind enhancements; 3) and if fenceline measurement data can be used to generate accurate mass emission estimates. Our results show that during each 48-hour measurement period the sensors do not detect any mixing ratios that exceeded an 8 ppm threshold for the 19 g CH4 h-1 release, but detected 237 events (8% of the time) during the 75 g CH4 h-1 release and 641 events (23% of the time) during the 132 g CH4 h-1 release. Using our data in a simple Gaussian plume model we show that emissions can be overestimated by as much as 4 x 10102 times if data are not filtered properly, with the lateral distance from the center line of the plume to the sensor being the most important variable. Despite this, highly selective filtering can yield emission estimates within 4% of the known emission rate of the source. However, the filtering omitted over 99% of our data. This study provides evidence to support the use of low-cost sensors being used in fenceline monitoring systems to quantify emission. If the caveats of calibration are addressed, modelling is improved and deployment location is optimized for each site, there is reason to believe that fenceline systems could be used routinely to quantify emissions from oil and gas infrastructure.