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

International audienceAs natural gas extraction and use continues to increase , the need to quantify emissions of methane (CH 4), a powerful greenhouse gas, has grown. Large discrepancies in Indianapolis CH 4 emissions have been observed when comparing inventory, aircraft mass balance, and tower inverse modeling estimates. Four years of continuous CH 4 mole fraction observations from a network of nine towers as a part of the Indianapolis Flux Experiment (INFLUX) are utilized to investigate four possible reasons for the abovementioned inconsistencies: (1) differences in definition of the city domain, (2) a highly temporally variable and spatially non-uniform CH 4 background, (3) temporal variability in CH 4 emissions, and (4) CH 4 sources that are not accounted for in the inventory. Reducing the Indianapolis urban domain size to be consistent with the inventory domain size decreases the CH 4 emission estimation of the inverse modeling methodology by about 35 %, thereby lessening the discrepancy and bringing total 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 CH 4 mole fractions are shown to be spatially heterogeneous and temporally variable. Variability in background mole fractions observed at any given moment and a single location could be up to about 50 ppb depending on a wind direction but decreases substantially when averaged over multiple days. Statistically significant, long-term biases in background mole fractions of 2-5 ppb are found from single-point observations for most wind directions. Boundary layer budget estimates suggest that Indi-anapolis CH 4 emissions did not change significantly when comparing 2014 to 2016. However, it appears that CH 4 emissions may follow a diurnal cycle, with daytime emissions (12:00-16:00 LST) approximately twice as large as night-time emissions (20:00-05:00 LST). We found no evidence for large CH 4 point sources that are otherwise missing from the inventories. The data from the towers confirm that the strongest CH 4 source in Indianapolis is South Side landfill. Leaks from the natural gas distribution system that were detected with the tower network appeared localized and non-permanent. Our simple atmospheric budget analyses estimate the magnitude of the diffuse natural gas source to be 70 % higher than inventory estimates, but more comprehensive analyses are needed. Long-term averaging, spatially extensive upwind mole fraction observations, mesoscale atmo-Published by Copernicus Publications on behalf of the European Geosciences Union. 4546 N. V. Balashov et al.: Background heterogeneity and other uncertainties spheric modeling of the regional emissions environment, and careful treatment of the times of day are recommended for precise and accurate quantification of urban CH 4 emissions