Tracking Urban Emissions of Greenhouse Gases during the East Coast Outflow (ECO) Experiment

Urban emissions of greenhouse gases represent an ever increasing fraction of the global emissions and tracking the impact of policies directed at emissions reduction are paramount to emissions mitigation. Here we present and interpret aircraft observations of six cities along the East Coast of the United States. We use direct observations of methane (CH₄), carbon dioxide (CO₂), carbon monoxide (CO), ethane (C₂H₆), and their correlations to quantify CH₄ emissions and attribute to natural gas. We find the five largest cities emit 0.85 (0.63,1.12) Tg CH₄·y^-1, of which 0.75 (0.49,1.10) Tg CH₄·y^-1 is attributed to natural gas. Our estimates, which include all thermogenic methane sources including end-use, are more than twice that reported in the most recent gridded EPA inventory, which does not include end-use emissions. These results highlight that current urban inventory estimates of natural gas emissions are substantially low, either due to under-estimates of leakage, lack of inclusion of end-use emissions, or some combination thereof.

This study demonstrates the power of aircraft observations for determining urban greenhouse gas emissions and the use of observations from commercial aircraft for tracking greenhouse gas emissions in a variety of urban environments.