The observable differences in FFPs of total emissions in U.S. cities are likely driven by the presence and distribution of different sources in the areas studied. Other factors such as climate and weather (i.e., increased natural gas use at different times of the year), fuel mix for electricity and heating, and the age and material composition of distribution pipelines and other natural gas infrastructure may also contribute to the observed differences . Moreover, FFPs may be somewhat modulated by the spatial domains sampled, and if landfills potentially located in urban fringe areas are included in estimates. In general, it is challenging to draw further conclusions on the potential reasons contributing to higher FFPs in some U.S. cities relative to others given the heterogeneity of sources within the spatial domains measured and without a structured causal inference method.