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Discussion papers
https://doi.org/10.5194/acp-2019-48
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-48
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Mar 2019

Research article | 13 Mar 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Background Heterogeneity and Other Uncertainties in Estimating Urban Methane Flux: Results from the Indianapolis Flux (INFLUX) Experiment

Nikolay V. Balashov1, Kenneth J. Davis1, Natasha L. Miles1, Thomas Lauvaux1,2, Scott J. Richardson1, Zachary R. Barkley1, and Timothy A. Bonin3,4 Nikolay V. Balashov et al.
  • 1The Pennsylvania State University, University Park, Pennsylvania, USA
  • 2Laboratory of Climate Sciences and Environment, Gif-sur-Yvette, France
  • 3Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, USA
  • 4Chemical Sciences Division, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA

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 tower-based cavity ring-down spectrometers measuring atmospheric CH4 mole fractions at 39 to 136 m above ground as 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) the presence of unknown CH4 sources. 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 % and thereby lessens the discrepancy by bringing total city flux within an error range of one of the inventories. Nevertheless, the inverse modeling estimate still remains about 40 % higher than the inventory value. Hourly urban background CH4 mole fractions are shown to be heterogeneous and temporally variable. Statistically significant, long-term biases in background mole fractions of 2–5 ppb are found from single point observations from most wind directions. Random errors in single point background mole fractions observed for a few hours are 20–30 ppb, but decrease substantially when data are averaged over multiple days. 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 (12–16 LST) approximately twice as large as nighttime emissions (20–5 LST). The strongest CH4 source in Indianapolis is the South Side Landfill. Other point sources, perhaps leaks from the natural gas distribution system, are localized and transient, and do not appear to be a consistently large source of CH4 emissions in Indianapolis. 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 and areal representation of emission estimates is recommended for precise and accurate quantification of urban CH4 emissions.

Nikolay V. Balashov et al.
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Nikolay V. Balashov et al.
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In-situ tower atmospheric measurements of carbon dioxide, methane and carbon monoxide mole fraction for the Indianapolis Flux (INFLUX) project, Indianapolis, IN, USA N. L. Miles, S. J. Richardson, K. J. Davis, and B. J. Haupt https://doi.org/10.18113/D37G6P

Nikolay V. Balashov et al.
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Short summary
Accurate independent verification methodology to estimate methane (powerful greenhouse gas) emissions is essential for effective implementation of policies that are aimed at reducing the impacts of climate change. In this paper, four uncertainties that complicate independent estimation of urban methane emissions are identified. They are definition of urban domain, heterogeneity in background, temporal variability, and prior source knowledge. Ways to improve the emission estimates are suggested.
Accurate independent verification methodology to estimate methane (powerful greenhouse gas)...
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