The duration of the measurement campaigns reported in these studies spanned May to November 2013 for , May to June 2020 for , and August 2019 to March 2021 for . It is unclear from if any measurements were performed at distribution facilities in winter because the duration of the field campaign was reported coarsely and spanned 3 years; actual measurement dates were not reported. Larger natural gas distribution facilities like city-gate stations and M&R facilities are known to have line heaters that vent burner emissions, which may include slip and other CH4 emissions. These are presumably more active during colder periods. This may affect the emissions profile of these facilities, where emissions may be higher in winter. Emissions factors (EFs) derived from the measurements in these studies are used in national BU estimates (e.g., . EFs may underestimate total emissions if seasonality is not captured. Winter sampling at distribution facilities could improve knowledge of emissions sources and rates at different periods of the year and enable more accurate assessment of their seasonal contribution to total city-level CH4 emissions. 3.2.2.3. Natural gas combustion Despite our estimates based on the data in several studies suggesting that natural gas combustion slip is a potentially large source of CH4 emissions at the city-level ( §3.1.2., , we found that specific sources of slip emissions are not yet clear based on source-level studies in the review. Slip emissions may occur during the pressure regulation and distribution of natural gas or downstream of customer gas meters and may vary between cities according to differences in the age of infrastructure and end uses.