Multiseason Thermogenic Methane Emission Fraction Determination from a Survey of Seven U.S. Cities

City and local policy makers have recently shown a willingness to regulate methane (CH₄) emissions among their constituents. Previous studies have shown that urban CH₄ emissions can be dominated by either biogenic (emitted as a metabolic byproduct) or thermogenic (emitted as a result of natural gas use) sources and that there can be dramatic differences in the source distribution from city to city and season to season. As more cities explore implementing regulations, it is imperative that the scientific community provide comprehensive information about city level CH₄ emissions. We present high resolution measurements of CH₄ and ethane (C₂H₆) (a component of natural gas) from a multi-season (winter and summer) flight campaign that focused on measuring emissions from 7 urban areas, Indianapolis, IN, Chicago, IL, Washington, DC & Baltimore, MD, Philadelphia, PA, New York, NY, Boston, MA, and Richmond, VA. For each of the flights presented in this study, a Lagrangian Particle Dispersion Model (LPDM) was used to quantify and subtract the effect of emissions from upwind of the study domain. We also used the LPDM to unambiguously define the sensitivity of the relative fluxes measured by the aircraft to emissions within the sample domain, a technique which can be applied broadly to conventional regional scale aircraft measurements. Our study shows that fractional thermogenic:biogenic CH₄ emissions from urban areas can vary dramatically from 20% contribution of natural gas to nearly 100% depending on the sample city and the season it was measured. We compare our results to current CH₄ inventories which, in almost all cases, predict a CH₄ emission profile that dramatically overestimates the fractional emissions of biological sources.