Abstract: The purpose of this article is to present some facts of interest for the quantification of gas emissions in active landfills obtained from a series of field campaigns in a case study landfill where spatial and temporal patterns of methane emissions were analyzed. Nine campaigns were carried out to measure diffuse surface methane emissions at an European municipal waste landfill in operation using the static flow chamber technique in different seasons over three years. Results obtained show a global annual diffuse flux of 733.26 t CH4/year for the year 2020. Certain points on the surface, where concentrations reached values above 1000 ppm, were observed during the campaigns. These points, called "hotspots", represented only 10% of all the points measured but accounted for 73% of the total diffuse methane emissions (506 t CH4/year). Furthermore, localized emissions, such as those from landfill gas extraction wells, which were not connected to the general extraction network, were also analyzed. These localized emissions represent more than twice the total diffuse emissions measured on the surface (1500 t CH4/year). These results highlight the importance of identifying high emission points to design effective mitigation measures. Moreover, the influence of certain meteorological conditions such as atmospheric pressure, temperature or rainfall was also studied. A new particular effect has been detected regarding precipitation, which favors or hinders methane emissions depending on the volume accumulated during the previous weeks. Pressure was found to be the factor that most affects methane emission variations in the case studied, presenting a clear inverse correlation with the field data that was collected. This suggests the need to consider the meteorological fluctuations over time to calculate the field emission estimates. Correcting the annual estimation in the case studied by considering the atmospheric pressure fluctuations over the year led to a 14% change in the estimate, obtaining a final result of 836.73 t CH4/year for the total diffuse emissions.

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