Abstract
Abstract. As natural gas extraction and use continues to increase, the need to quantify emissions of methane (CH4), a powerful greenhouse gas, has grown. Large discrepancies in Indianapolis CH4 emissions have been observed when comparing inventory, aircraft mass balance, and tower inverse modeling estimates. Four years of continuous CH4 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 CH4 background, (3) temporal variability in CH4 emissions, and (4) CH4 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 CH4 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 CH4 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 CH4 emissions did not change 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). We found no evidence for large CH4 point sources that are otherwise missing from the inventories. The data from the towers confirm that the strongest CH4 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 atmospheric modeling of the regional emissions environment, and careful treatment of the times of day are recommended for precise and accurate quantification of urban CH4 emissions.
Highlights
From the beginning of the Industrial Revolution to 2011, atmospheric methane (CH4) mole fractions increased by a factor of 2.5 due to anthropogenic processes such as fossil fuel production, waste management, and agricultural activities (Ciais et al, 2013; Hmiel et al, 2020)
We examine four different potential explanations for the CH4 flux discrepancies reported in Lamb et al (2016): (1) inconsistent geographic boundaries between top-down and bottom-up studies, (2) heterogeneity in the urban-scale CH4 background and (3) temporal variability in urban emissions, which is not captured by the existing top-down studies, and (4) CH4 sources that are not accounted for in the inventories
When reduced to domain 2, the inverse modeling emission estimate decreases to 107 mol s−1, which falls within an error bar of the Lamb et al (2016) inventory estimate
Summary
From the beginning of the Industrial Revolution to 2011, atmospheric methane (CH4) mole fractions increased by a factor of 2.5 due to anthropogenic processes such as fossil fuel production, waste management, and agricultural activities (Ciais et al, 2013; Hmiel et al, 2020). The magnitudes of component CH4 sources responsible for the recent increase in the global CH4 budget are not well understood, with theories attributing these changes to biogenic, thermogenic, and pyrogenic emissions or a decline in the atmospheric CH4 sink (Nisbet et al, 2016, 2019; Saunois et al, 2016; Hmiel et al, 2020). A recent synthesis of emissions from the US NG supply chain demonstrated similar success and concluded that current inventory estimates of emissions from US NG production are too low and that emission from NG distribution is one of the greatest remaining sources of uncertainty in the NG supply chain (Alvarez et al, 2018)
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