Evaluation of Urban Methane Emissions along the Eastern United States

Methane is a potent greenhouse gas. Due to a large degree of uncertainty in regards to methane emissions, specifically near urban areas, there are discrepancies between greenhouse gas inventories. This study compares airborne observational data to inventory data from two methane inventories to gain a better understanding of urban methane emissions along the eastern coast of the United States. In doing so, this study seeks to evaluate how well urban methane emissions along the East Coast are represented in greenhouse gas inventories.

This study presents airborne methane and carbon dioxide measurements between April and May 2018. This data was collected along the eastern coast of the United States using the National Oceanic and Atmospheric Administration (NOAA) DHC-6 Twin Otter aircraft equipped with a flight-ready Picarro instrument. Using measurements downwind from the city, the observed methane to carbon dioxide ratio was determined for New York City, Philadelphia, and Washington DC. Since errors in the carbon dioxide emissions are significantly smaller than methane, it is possible to use the carbon dioxide emission estimates to assess whether the methane inventories are consistent with observations. In order to compare the observed ratios to inventory ratios, the Emissions Database for Global Atmospheric Research version 4.2 (EDGARv42) methane inventory and the Environmental Protection Agency (EPA) methane inventory where divided by the Open-Data Inventory for Anthropogenic Carbon Dioxide (ODIAC). If sufficient mixing has occurred, the observed ratio will directly correspond to the inventory ratio.

Primarily results suggest that the EPA methane inventory, which currently represents our most comprehensive understanding of greenhouse gas emissions, consistently assigned very little emissions near the three urban areas in this study. Conversely, the EDGARv42 methane inventory which is an older inventory with less accurate proxies for representing methane emissions is closer to the observed ratios. Taken collectively, this highlights that urban methane emissions are higher than the current best inventory-based estimates.