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

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 ± 58 (1σ) moles s^-1 (7800 ± 3300 kg hr^-1). The effective per capita CH₄ emission rate for Indianapolis is 77 kg CH₄ person-1 yr^-1, a figure that is less than the national anthropogenic CH₄ emission (∼91 kg CH₄ person-1 yr^-1) but considerably larger than the global figure (∼48 kg CH₄ person-1 yr^-1). 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. Street level mobile measurements downwind of the landfill and the TRS supported the results of aircraft-based data, and were used to quantify the relative contributions from the two sources. We find that the CH₄ emission from the TRS was negligible relative to the landfill, which was responsible for 33 ± 10% of the citywide emission flux. A regression of propane versus methane from aircraft flask samples suggests that the remaining citywide CH₄ emissions (∼67%) 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.