Natural Gas Leaks in Boston

“Atmospheric methane (CH4) is the second-most important greenhouse gas after carbon dioxide (CO2); the Intergovernmental Panel on Climate Change (IPCC) estimates that it was responsible for ∼about 20% of global anthropogenic direct radiative forcing from 2000 to 2010” begins a paper by Maryann R. Sargent et al-Majority of US urban natural gas emissions unaccounted for in inventories. The authors continue “Oil and natural gas systems are estimated to account for 31% of anthropogenic methane emissions in the United States .” 

Globally wetlands contributed 30% of global methane emissions, with oil, gas, and coal activities accounting for 20%. Agriculture, including enteric fermentation (cow belching), manure management, and rice cultivation, made up 24% of emissions, and landfill gas contributed 11%. Sixty-four percent of methane emissions come from the tropical regions of South America, Asia, and Africa, with temperate regions accounting for 32% and the Arctic contributing 4%. According to previous studies the amount of methane in Earth’s atmosphere continues to rise. Concentrations of methane now exceed 1875 parts per billion, about 2.5 times as much as was in the atmosphere in the 1850s.

In the past most efforts to reduce methane emissions in the United States has targeted oil- and natural gas-production, because the U.S. Environmental Protection Agency (EPA) had estimated that to be the source of most of the leakage from natural gas. However, a 2019 NOAA and University of Michigan study using an instrumented airplane found unexpectedly large emissions over five major cities along the East Coast. These cities have natural gas distribution systems and deliver natural gas to households.

This confirmed the earlier work of two scientists, Robert B. Jackson, who was then Professor of Global Environmental Change at Duke University and Nathan Phillips, associate professor at Boston University Department of Earth and Environment collaborated with Robert Ackley of Gas Safety Inc., and Eric Crosson of Picarro Inc., who studied gas leaks in Boston. They mapped the gas leaks under the city using a the Picarro high-precision methane analyzer installed in a GPS-equipped car. Driving all 785 road miles within city limits, the researchers discovered 3,356 leaks. The leaks were found to be associated with old cast-iron underground pipes, rather than neighborhood socioeconomic indicators. Levels of methane in the surface air on Boston’s streets exceeded 15 times the normal atmospheric background value.

"Bottom-up" estimates measure emissions from a sample of devices and multiplies the result by the total number of devices. In contrast, "top-down" measurements can be performed at a regional scale, such as flying an aircraft upwind and downwind of a study area. In most studies, the bottom up method is underestimating the emissions.  The newly published study mentioned above from scientists at Harvard, Boston University and NOAA used 8 years of actual measured methane levels at two locations in the heart of Boston (Boston University and Copley Square) and 3 sites far outside the city to study methane emissions in Boston has made some interesting suggestions of the potential missing sources of methane emissions.

The city of Boston has set a goal of becoming carbon neutral by 2050 (34), and Massachusetts implemented new laws and regulations between 2014 and 2019 requiring utilities to report and repair large leaks based on their size (35) (36). A 2014 law required repair of leaks classified as grade 1 and grade 2, as well as making leak data public. A 2018 law required repair of grade 3 leaks classified as “significant environmental impact” as well. In the eight year period of this study there has been no significant change in the number of grade 1 or 2 leaks on the pipeline system and only a slight reduction in the number of grade 3 leaks. Thus the authors conclude that new leaks are appearing in the aging Boston pipeline system as fast as old ones are being fixed. The methane distribution infrastructure is crumbling.

According to the authors of a 2019 study by the Home Energy Efficiency Team (HEET) predicted that the 2018 Massachusetts law requiring the repair of leaks deemed “significant environmental impact” could reduce pipeline emissions by half, based on a finding that 7% of leaks emit half of all gas by volume. The current study analysis finds that these efforts have not yet resulted in a measurable change in methane emissions which can be attributed to the constant number of leaks reported despite ongoing repairs.

Using their measurements and model the scientists found an average methane loss rate of 2.5% ± 0.5% from 2012 to 2020. They found no statistically significant trend in loss rate over the 8 year period of the study, despite the new leak repair regulations going into effect. In addition, the methane loss rate calculated by the scientists in this study is three times higher than the 0.8% loss rate indicated by their prior bottom up inventory for Boston and six times higher than the Massachusetts Department of Environmental Protection (MassDEP) estimate. 

The scientists found a strong, correlation between methane emissions and consumption in Boston that the scientists said was unexpected. During the winter when natural gas is in higher use, the methane emission are higher. This relationship has also been found in Washington, DC and Los Angeles. This lead the scientists to speculate that either the releases are coming from end use equipment (heaters, stoves, etc.) in homes or there is a fluctuation in the pipeline pressure based on demand that causes and increase in leakage from the distribution system. Only further study of distribution pipeline pressure can answer this.

However, the authors interpreted their data to imply that sources other than pipelines, such as transmission and appliances, are important sources of methane emissions and may require future policy action. They estimated total supply chain losses of 3.3% to 4.7% for methane consumed in urban areas, which significantly increases the climate impacts of natural gas compared to Environmental Protection Agency estimates.

The scientists point out that densely populated urban areas could potentially make big changes in their methane emissions, because they have concentrated populations, infrastructure, and emissions and in in many cases Democratic/ progressive control and the will to implement emission reductions policies. Pipelines, transmission infrastructure, household and commercial appliances, meters, stationary combustion, and service leaks are believed to be the most significant contributors for urban methane emissions. Bottom-up inventories estimate that distribution and end use are responsible for 6% of US emissions from methane this work suggests a higher contribution.

Full Citation:

Majority of US urban natural gas emissions unaccounted for in inventories
Maryann R. Sargent, Cody Floerchinger, Kathryn McKain, John Budney, Elaine W. Gottlieb, Lucy R. Hutyra, Joseph Rudek, Steven C. Wofsy
Proceedings of the National Academy of Sciences Nov 2021, 118 (44) e2105804118; DOI: 10.1073/pnas.2105804118 

Majority of US urban natural gas emissions unaccounted for in inventories | PNAS

Other reading:

R B Jackson, M Saunois, P Bousquet , J G Canadell, B Poulter, A R Stavert , P Bergamaschi, Y Niwa , A Segers and A Tsuruta: Increasing anthropogenic methane emissions arise equally from agricultural and fossil fuel sources, Environ. Res. Lett. 15 (2020) 071002 https://iopscience.iop.org/article/10.1088/1748-9326/ab9ed2/pdf