1 Elementa: Science of the Anthropocene 3: 000037 doi: 10.12952/journal.elementa.000037 elementascience.org Quantification and source apportionment ofthe methane emission lux from the city ofIndianapolis M. O. L. Cambaliza 1 * P. B. Shepson 1,2 J. Bogner 3 D. R. Caulton 1 B. Stirm 4 C. Sweeney 5,6 S. A. Montzka 6 K. R. Gurney 7 K. Spokas 8 O. E. Salmon 1 T. N. Lavoie 1 A. Hendricks 1 K. Mays 1 J. Turnbull 9 B. R. Miller 6 T. Lauvaux 10 K. Davis 10 A. Karion 5,6 B. Moser 1 C. Miller 1 C. Obermeyer 1 J. Whetstone 11 K. Prasad 11 N. Miles 10 S. Richardson 10 1 Department of Chemistry, Purdue University, West Lafayette, Indiana, United States 2 Department of Earth, Atmospheric and Planetary Science & Purdue Climate Change Research Center, Purdue University, West Lafayette, Indiana, United States 3 Department of Earth & Environmental Sciences, University of Illinois at Chicago, Illinois, United States 4 Department of Aviation Technology, Purdue University, West Lafayette, Indiana, United States 5 University of Colorado, Boulder, Colorado, United States 6 NOAA/ESRL, Boulder, Colorado, United States 7 School of Life Sciences, Arizona State University, Tempe, Arizona, United States 8 U.S. Department of Agriculture, St. Paul, Minnesota, United States 9 National Isotope Centre, GNS Science, Lower Hutt, New Zealand 10 Department of Meteorology, e Pennsylvania State University, University Park, Pennsylvania, United States 11 NIST, Gaithersburg, Maryland, United States *mcambali@purdue.edu/mocambaliza@gmail.com Abstract We report the CH 4 emission flux from the city of Indianapolis, IN, the site of the Indianapolis Flux Experi- ment (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 CH 4 emission rate from five flight experiments in 2011 is 135 ± 58 (1σ) moles s –1 (7800 ± 3300 kg hr –1 ). The effective per capita CH 4 emission rate for Indianapolis is 77 kg CH 4 person –1 yr –1 , a figure that is less than the national anthropogenic CH 4 emission (∼91 kg CH 4 person –1 yr –1 ) but considerably larger than the global figure (∼48 kg CH 4 person –1 yr –1 ). We consistently observed elevated CH 4 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 CH 4 emission from the TRS was negligible relative to the landfill, which was responsible for 33 ± 10% ofthe citywide emission flux. A regression of propane versus methane from aircraft flask samples suggests that theremaining citywide CH 4 emissions (∼67%) derive from the natural gas distribution system. We discussthe combination of surface mobile observations and aircraft city-wide flux measurements to determine the total flux and apportionment to important sources. Introduction Next to carbon dioxide, methane (CH 4 ) is the second most important anthropogenic greenhouse gas, contrib- uting ∼0.48 W m –2 (∼17%) to the total direct radiative forcing by long-lived greenhouse gases (Myhre et al., Domain Editor-in-Chief Detlev Helmig, University of Colorado Boulder Associate Editor Isobel Jane Simpson, University of California, Irvine Knowledge Domain Atmospheric Science Article Type Research Article Received: June 20, 2014 Accepted: November 13, 2014 Published: January 7, 2015