Direct measurements show decreasing methane emissions from natural gas local distribution systems in the United States

Fugitive methane emissions from natural gas distribution systems are a significant source of anthropogenic greenhouse gases. Here we present data from a national sampling program to measure methane emissions from 13 local natural gas distribution systems in cities across the United States. Emission factors were derived from direct source measurements at 230 underground pipeline leaks and 229 metering and regulating facilities targeted using a stratified random selection approach. When these new emission factors for pipeline leaks and M&R facilities are combined at the national level with emissions from customer meters, maintenance blow-downs, and dig-ins, the total estimate for these source categories is 393 Gg/yr with an upper 95% confidence limit of 854 Gg/yr, or 0.10% to 0.22% of the methane delivered to natural gas customers nationwide. The large upper confidence limit accounts for the characteristic skewed distribution of measured emission rates within each source category where a few large emitters accounted for a large percentage of the emissions. The upper range of this emission estimate is approximately 30% less than current EPA estimates for these categories, and clearly reflects significant equipment upgrades at metering and regulating stations, improvements in leak detection and maintenance activities, and replacement of older pipeline materials in the last 20 years. The data reported here address emissions only from pipeline leaks and metering and regulating stations, and thus, can be used to provide a lower bound on emissions in urban areas to help guide future efforts to assess other sources which contribute to the residual between these direct source measurements and top-down emission estimates.