Improving consistency in methane emission quantification from the natural gas distribution systems across measurement devices

Abstract. Mobile real-time measurements of ambient methane provide a fast and effective method to identify and quantify methane leaks from local gas distribution systems in urban areas. The objectives of these methodologies are to (i) identify leak locations for repair and (ii) construct measurement-based emission rate estimates, which can improve emissions reporting and contribute to monitoring emission changes over time. Currently, the most common method for emission quantification uses the maximum methane enhancement detected while crossing a methane plume. However, the recorded maximum depends on instrument characteristics, such as measurement cell size, pump speed, and measurement frequency. Consequently, the current approach can only be used by instruments with similar characteristics. We suggest that the integrated spatial peak area is a more suitable quantity that can eliminate the bias between different instruments. Based on controlled release experiments conducted with various devices in four cities (London, Toronto, Rotterdam, and Utrecht), emission estimation methodologies were evaluated. Indeed, when different analysers were measuring in the same vehicle and from the same air inlet, the integrated spatial peak area was found to be a more robust metric across different methane gas analyser devices than the maximum methane enhancement. A statistical function based on integrated spatial peak area is proposed for more consistent emission estimations when using different instruments. On top of this systematic relation between actual emission rate and recorded spatial peak area, large variations in methane spatial peak area were observed for the multiple transects across the same release point, in line with previous experiments. This variability is the main contributor to uncertainty in efforts to use mobile measurements to prioritize leak repair. We show that repeated transects can reduce this uncertainty and improve the categorization into different leak categories. We recommend a minimum of three and an optimal range of five–seven plume transects for effective emission quantification to prioritize repair actions.