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. As natural gas extraction and use continues to increase, the need toquantify emissions of methane ( CH4 ), a powerful greenhouse gas, hasgrown. Large discrepancies in Indianapolis  CH4  emissions have beenobserved when comparing inventory, aircraft mass balance, and tower inversemodeling estimates. Four years of continuous  CH4  mole fractionobservations from a network of nine towers as a part of the IndianapolisFlux Experiment (INFLUX) are utilized to investigate four possible reasonsfor the abovementioned inconsistencies: (1) differences in definition of thecity domain, (2) a highly temporally variable and spatially non-uniform CH4  background, (3) temporal variability in  CH4  emissions, and (4)  CH4  sources that are not accounted for in the inventory. Reducing theIndianapolis urban domain size to be consistent with the inventory domainsize decreases the  CH4  emission estimation of the inverse modelingmethodology by about 35 %, thereby lessening the discrepancy and bringingtotal city flux within the error range of one of the two inventories.Nevertheless, the inverse modeling estimate still remains about 91 %higher than inventory estimates. Hourly urban background  CH4  molefractions are shown to be spatially heterogeneous and temporally variable.Variability in background mole fractions observed at any given moment and asingle location could be up to about 50 ppb depending on a wind directionbut decreases substantially when averaged over multiple days. Statisticallysignificant, long-term biases in background mole fractions of 2–5 ppb arefound from single-point observations for most wind directions. Boundarylayer budget estimates suggest that Indianapolis  CH4  emissions did notchange significantly when comparing 2014 to 2016. However, it appears that CH4  emissions may follow a diurnal cycle, with daytime emissions (12:00–16:00 LST) approximately twice as large as nighttime emissions (20:00–05:00 LST). Wefound no evidence for large  CH4  point sources that are otherwisemissing from the inventories. The data from the towers confirm that thestrongest  CH4  source in Indianapolis is South Side landfill. Leaks fromthe natural gas distribution system that were detected with the towernetwork appeared localized and non-permanent. Our simple atmospheric budgetanalyses estimate the magnitude of the diffuse natural gas source to be 70 % higherthan inventory estimates, but more comprehensive analyses are needed.Long-term averaging, spatially extensive upwind mole fraction observations,mesoscale  atmospheric modeling of the regional emissions environment, andcareful treatment of the times of day are recommended for precise andaccurate quantification of urban  CH4  emissions.

Background heterogeneity and other uncertainties in estimating urban methane flux: results from the Indianapolis Flux Experiment (INFLUX)