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 %, 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 <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 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 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 LST) approximately twice as large as nighttime emissions (20:00-05:00 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 % 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 language | English (US) |
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Article number | 2020 |
Pages (from-to) | 4545-4559 |
Number of pages | 15 |
Journal | Atmospheric Chemistry and Physics |
Volume | 20 |
Issue number | 7 |
DOIs | |
State | Published - Apr 17 2020 |
All Science Journal Classification (ASJC) codes
- Atmospheric Science