Assessing the Gap Between Top-down and Bottom-up Measured Methane Emissions in Indianapolis, IN. 

Friday, 19 December 2014
Brian K Lamb1, Kuldeep Prasad2, Maria Obiminda L Cambaliza3, Paul B Shepson4, Brian H Stirm3, Olivia Elizabeth Salmon3, Tegan Noel Lavoie3, Thomas Lauvaux5, Thomas Ferrara6, Touche Howard6, Steven L Edburg1 and James R Whetstone2, (1)Washington State University, Pullman, WA, United States, (2)National Institute of Standards and Technology Gaithersburg, Gaithersburg, MD, United States, (3)Purdue University, West Lafayette, IN, United States, (4)Purdue Univ, West Lafayette, IN, United States, (5)Pennsylvania State University Main Campus, University Park, PA, United States, (6)Conestoga-Rovers & Associates, Waterloo, ON, Canada
Releases of methane (CH4) from the natural gas supply chain in the United States account for approximately 30% of the total US CH4 emissions. However, there continues to be large questions regarding the accuracy of current emission inventories for methane emissions from natural gas usage. In this paper, we describe results from top-down and bottom-up measurements of methane emissions from the large isolated city of Indianapolis. The top-down results are based on aircraft mass balance and tower based inverse modeling methods, while the bottom-up results are based on direct component sampling at metering and regulating stations, surface enclosure measurements of surveyed pipeline leaks, and tracer/modeling methods for other urban sources. Mobile mapping of methane urban concentrations was also used to identify significant sources and to show an urban-wide low level enhancement of methane levels.  The residual difference between top-down and bottom-up measured emissions is large and cannot be fully explained in terms of the uncertainties in top-down and bottom-up emission measurements and estimates. Thus, the residual appears to be, at least partly, attributed to a significant wide-spread diffusive source. Analyses are included to estimate the size and nature of this diffusive source.