Methane Sources in Cities Re-Interpretated Using New Laboratory Results

Many city governments have committed to reducing their emissions of GHGs based on inventories calculated for the city. However, few cities measure their carbon emissions to know if the enacted policies are having the desired effect. The New York Metro Area (pop. 20M) is the most populous urban area in the United States of America (USA) and the largest urban source of CO₂ and methane (CH₄) in the US. Our urban core observatory in Manhattan and background tower sites in rural areas around the city allow us to study the source characteristics of various greenhouse gas and related air pollutant fluxes in the city. Using our observations, an atmospheric transport model (HRRR-STILT), and various prior emissions estimates, we have quantified the city-scale emission rate of methane and carbon monoxide (CO) and during multiple winter-to-spring transitions (Jan-May 2019-2024) and year-round since May 2023. We find that the observed city-scale methane fluxes from NYC are 2-3 times higher than expected from both established and more recent inventories. The fraction of natural gas in mixed methane source areas is often quantified by comparing the observed atmospheric ethane (C₂H₆):CH₄ ratio to that reported in the pipeline. However, this approach assumes no change in the C₂H₆:CH₄ ratio during the natural gas combustion process. We also observed depleted ethane (lower C₂H₆:CH₄ ratios) in combustion plumes with high CO. In order to determine the source of this incomplete combustion, we sampled the stack exhaust from a natural gas boiler and found badly operated boilers can release CH₄, C₂H₆ and CO during the incomplete combustion of natural gas, but with C₂H₆ depleted relative to the C₂H₆:CH₄ratio of the incoming pipeline. Previous studies using ethane:methane would have underestimated the natural gas contribution of methane emission if they assumed an unchanged C₂H₆:CH₄ ratio. We continue to characterize street, rooftop and point source methane emissions within and around the city using this new understanding of C₂H₆:CH₄:CO ratios to understand the source characteristics of pre and post meter natural gas, waste water treatment plants and landfills.