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Quantification and source apportionment of the methane emission flux from the city of Indianapolis

M. O. L. Cambaliza, P. B. Shepson, J. Bogner, D. R. Caulton, B. Stirm, C. Sweeney, S. A. Montzka, K. R. Gurney, K. Spokas, O. E. Salmon
Elementa: science of the Anthropocene 2015 v.3 no.000037 pp. 1-18
aircraft, anthropogenic activities, cities, elevated atmospheric gases, greenhouse gas emissions, inventories, landfills, methane, methane production, models, natural gas, propane, quantitative analysis, urban areas, wind, California, Indiana
We report the CH4 emission flux from the city of Indianapolis, IN, the site of the Indianapolis Flux Experiment (INFLUX) project for developing, assessing, and improving top-down and bottom-up approaches for quantifying urban greenhouse gas emissions. Using an aircraft-based mass balance approach, we find that the average CH4 emission rate from five flight experiments in 2011 is 135 moles s-1, equivalent to a per capita emission of 5800 moles CH4 person-1 year-1. On a per capita basis, the Indianapolis CH4 emission is comparable to the national anthropogenic CH4 emission but a factor of ~2 larger than the global figure. We consistently observed elevated CH4 concentrations at specific coordinates along our flight transects downwind of the city. Inflight investigations as well as back trajectories using measured wind directions showed that the elevated concentrations originated from the southwest side of the city where a landfill and a natural gas transmission regulating station (TRS) are located. Surface mobile measurements supported the results of aircraft-based data, and were used to quantify the relative contributions from the two sources. We find that the emission from the TRS was negligible relative to the landfill, which was responsible for 37% of the citywide emission flux. To evaluate our measurements of the landfill CH4 fraction, we measured the emission rate of four other landfills, and compared the results to the California Landfill Methane Inventory Model. A regression of propane versus methane from aircraft flask samples suggests that much of the remaining citywide emissions derive from the natural gas distribution system. We discuss the combination of surface mobile observations and aircraft city-wide flux measurements to determine the total flux and apportionment to important sources. Finally, we suggest methods for the identification and quantification of the emission strengths of the sources contributing to the remaining ~63% of the citywide emissions.