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

Nikolay V. Balashov, Kenneth J. Davis, Natasha L. Miles, Thomas Lauvaux, Scott J. Richardson, Zachary R. Barkley, Timothy A. Bonin

Research output: Contribution to journalArticlepeer-review

5 Scopus citations

Abstract

As natural gas extraction and use continues to increase, the need to quantify emissions of methane (<span classCombining double low line"inline-formula">CH4</span>), a powerful greenhouse gas, has grown. Large discrepancies in Indianapolis <span classCombining double low line"inline-formula">CH4</span> emissions have been observed when comparing inventory, aircraft mass balance, and tower inverse modeling estimates. Four years of continuous <span classCombining double low line"inline-formula">CH4</span> 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 <span classCombining double low line"inline-formula">CH4</span> background, (3) temporal variability in <span classCombining double low line"inline-formula">CH4</span> emissions, and (4) <span classCombining double low line"inline-formula">CH4</span> 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 <span classCombining double low line"inline-formula">CH4</span> emission estimation of the inverse modeling methodology by about 35&thinsp;%, 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&thinsp;% higher than inventory estimates. Hourly urban background <span classCombining double low line"inline-formula">CH4</span> 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&thinsp;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&thinsp;ppb are found from single-point observations for most wind directions. Boundary layer budget estimates suggest that Indianapolis <span classCombining double low line"inline-formula">CH4</span> emissions did not change significantly when comparing 2014 to 2016. However, it appears that <span classCombining double low line"inline-formula">CH4</span> emissions may follow a diurnal cycle, with daytime emissions (12:00-16:00&thinsp;LST) approximately twice as large as nighttime emissions (20:00-05:00&thinsp;LST). We found no evidence for large <span classCombining double low line"inline-formula">CH4</span> point sources that are otherwise missing from the inventories. The data from the towers confirm that the strongest <span classCombining double low line"inline-formula">CH4</span> 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&thinsp;% higher than inventory estimates, but more comprehensive analyses are needed. Long-term averaging, spatially extensive upwind mole fraction observations, mesoscale<span idCombining double low line"page4546"/> atmospheric modeling of the regional emissions environment, and careful treatment of the times of day are recommended for precise and accurate quantification of urban <span classCombining double low line"inline-formula">CH4</span> emissions.

Original languageEnglish (US)
Article number2020
Pages (from-to)4545-4559
Number of pages15
JournalAtmospheric Chemistry and Physics
Volume20
Issue number7
DOIs
StatePublished - Apr 17 2020

All Science Journal Classification (ASJC) codes

  • Atmospheric Science

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