Beyond-the-Meter: Unaccounted Sources of Methane Emissions in the Natural Gas Distribution Sector.

The United States Environmental Protection Agency maintains an inventory of greenhouse gas emissions in accordance with the Intergovernmental Panel on Climate Change. Methane (CH), a potent gas with a global warming potential 86-125× that of carbon dioxide (CO) over a twenty-year period, is the main component of natural gas (NG). As NG becomes an increasingly larger percentage of the energy resources used in the United States, it is ever more important to evaluate the CH emissions inventory. However, the inventory also does not account for all possible sources of CH leaks, contributing to uncertainty in the national CH inventory. Discrepancies between top-down and bottom-up inventories of CH emissions imply that there are significant unaccounted-for sources of CH leaks, especially over cities. Diffuse CH plumes above cities that are not attributable to distribution pipelines or other NG infrastructure suggest many small beyond-the-meter leaks together contribute to large emissions. Here, we evaluate the distribution sector of the CH emissions inventory and make suggestions to improve the inventory by analyzing end-user emissions. Preliminary research into beyond-the-meter emissions suggests that while individually small, the appliances and buildings that make up the residential sector could contribute significantly to national scale emissions. Furnaces are the most leak-prone of appliances, contributing to 0.14% of total CH emissions from the NG sector in the United States. Combining measurements from whole house emissions and steady-state operation of appliances, we estimate that residential homes and appliances could release 9.1 Gg CH yearly in the United States, totaling over 2% of the CH released from the NG sector. While factors such as appliance age and usage, climate, and residential setting could influence the emissions profile of individual appliances, these preliminary estimates justify further exploration of beyond-the-meter emissions.