Ratios increasingly > 1 suggest several possible effects: (i) CH4 emissions may be missing or underestimated with the BU approaches used, or (ii) TD methods are overestimating emissions. Several examples can be explored. estimate that natural gas-only CH4 emissions in U.S. East Coast cities are ~10 times greater than estimated in inventories, which traditionally have not included CH4 emissions from incomplete combustion at the burner tip, representing a missed emissions source in BU approaches. More recently, found that TD estimates of natural gas emissions in New York were 2.3 times larger than their prior, whereas posterior biogenic emissions were 1.3 times larger. This suggests that natural gas sources -and possibly end-use emissions -are a key contributor to TD > BU in this region. Differences in the age and material type of natural gas distribution infrastructure and substantial residential and commercial natural gas use in U.S. East Coast cities for space and water heating may elevate the median TD > BU ratio for this region. More broadly, CH4 emissions from natural gas may be driving TD > BU across the regions and cities studied. For example, diffuse natural gas emissions in Indianapolis -which we interpret as leaks and end-use emissions distributed widely across the city -are underestimated by a factor of ~1.7 to 6.9 in BU estimates . In contrast to natural gas emissions, BU estimates of landfill emissions in Indianapolis are thought to mostly align with TD estimates . Los Angeles-based studies indicate that emissions matching the chemical signature of dry natural gas in the distribution system and to a lesser extent oil and gas production are mainly responsible for TD > BU . Landfills may have less influence on TD > BU in Los Angeles but are the main driver of TD > BU in the San Francisco Bay Area (SFBA) where landfills dominate total city-level CH4 emissions . Overall, the importance of natural gas sources to total city-level emissions and differences in the age and material type of natural gas distribution infrastructure may be driving the variability in median TD > BU ratios regionally.