A109-07
Eight Years of Urban Methane Emissions From a Top-down Framework in Boston, MA

Friday, 11 December 2020: 04:43
Virtual
Maryann R Sargent, Harvard University, Cambridge, MA, United States, Cody R Floerchinger, Aerodyne Research Inc., Billerica, MA, United States, Steve M Raciti, Boston University, Arlington, MA, United States, Lucy Hutyra, Boston University, Earth & Environment, Boston, MA, United States, Kathryn McKain, NOAA ESRL Global Monitoring Division, Boulder, CO, United States and Steven C Wofsy, Harvard University, John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA, United States
Recorded Presentation
Abstract:
Across many cities, top-down estimates of methane emissions from the natural gas distribution system are typically an order of magnitude or more larger than bottom-up estimates. We present a new eight-year top-down study of natural gas emissions from the Boston urban region to assess natural gas emissions, their seasonality, and trends. We used observations from 5 sites in and around Boston, combined with the HYSPLIT model and a high-resolution bottom-up inventory to calculate top-down methane emissions. By combining those results with co-located ethane measurements, we were also able to calculate emissions from natural gas infrastructure and the leak rate from the distribution system. We found a strong correlation between methane emissions and natural gas consumption in Massachusetts. We also found a ~30% decrease in natural gas emissions in the area of influence of the 29 m Boston University site during the Covid-19 shutdown in April-May 2020, but no appreciable change in emissions for the 215 m Copley Square site compared to previous years. We found no significant trend in natural gas loss rate over eight years, despite efforts from the state to increase the rate of repairing natural gas pipeline leaks. For the 8-year period we found an average loss rate of 2.3±0.4%, which is about 6 times higher than the Massachusetts Department of Environmental Protection’s bottom-up estimated loss rate of 0.4%.

We also compared top-down natural gas emission estimates from Boston, Indianapolis, Los Angeles, New York, and Washington D.C. with new studies estimating bottom-up emissions from pipeline leaks and in-house losses to quantitatively assess how well we can balance the urban methane budget. We find that while the new studies increase estimated bottom-up emissions and reduce the gap between top-down and bottom-up studies, significant gaps remain between the two methods, pointing to significant missing sources in bottom-up inventories.

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