Toward a robust quantification of methane emissions in a medium-sized city: initial results from mobile and stationary measurements

Quantifying methane (CH₄) emissions at the scale of medium-sized European cities remains a significant challenge due to the relatively low annual levels, the spatial and temporal heterogeneity of these emissions. We aim to develop a framework for quantifying urban CH₄ emission including nature gas and waste emissions, following a protocol compatible with the reporting framework of the Oil & Gas Methane Partnership (OGMP) 2.0 for natural gas distribution operators.

Le Mans, a medium-sized city in the Pays de la Loire region of France with a population of approximately 150,000 is selected as a pilot case. The approach combines mobile measurements and fixed monitoring stations to quantify CH₄ emissions and identify natural gas leaks and other potential emission hotspots in the city.

Here, we report on the tests of five Aeris mid infra-red analysers MIRA LDS intended for fixed deployment in the city to monitor CH₄ concentration variations due to the urban emissions. The resulting system demonstrated sufficient precision, with methane and ethane measurement accuracies better than 5 ppb and a few ppt, respectively. We modified the analysers and implemented a two-level calibration strategy with daily injections to achieve sensor stability.

Additionally, mobile surveys were conducted, covering approximately 36% of the streets of the city Le Mans and parts of the roads in the suburbs. These campaigns aimed to: (1) detect and quantify fugitive point source emissions, (2) characterize large-scale CH₄ variations, (3) evaluate the effect of air inlet location, and (4) quantify emissions from major emitting sites. We identified CH₄ point sources linked to natural gas infrastructure and non-natural gas sources. We distinguish between large plumes presumably from large emitters and fugitive spikes indicative of possible local gas leaks. Over 15 transects have been led across the plume from the wastewater treatment plant (WWTP) in Le Mans and allow to estimate the site’s contribution to the city’s emission. Additionally, the analysis of air inlet positions revealed that higher air inlet locations are more suitable for analyzing the plume transects, whereas lower air inlet positions are better suited for detecting peaks associated to near ground or subsurface fugitive emissions.

Finally, exploiting these results, we discuss the capability of a network of fixed stations to measure small variations of CH4 (of the order of tens of ppb) across the city. These results will lead to the implementation of the fixed network over the next 2 years in the city (at the end of 2026), complemented by continued mobile campaigns to analyze emission trends, variability, and sector-specific contributions.