UNIVERSITY PARK, Pa. — Most natural gas leakage in Indianapolis likely originates from homes and other buildings, not exterior pipelines and large-scale distribution infrastructure that deliver the fossil fuel, according to Penn State researchers who assessed nearly a decade of data.
Their observations could signal similar emissions patterns across U.S. cities and shape mitigation strategies, members of the team said. They evaluated continuous measurements of methane, the main component of natural gas, across the Midwest city to estimate emissions from natural gas distribution.
The analysis relied on a network of monitoring devices affixed to tall towers that informed one of the world’s most detailed records of urban methane, the team wrote recently in the Journal of Geophysical Research: Atmospheres. Its replicable approach offers cities a reliable way to identify and measure emissions of methane — the second-most impactful greenhouse gas generated by humans — and to inform public policy on climate, said the researchers in the College of Earth and Mineral Sciences.
They estimated about 2% of all natural gas piped into Indianapolis is eventually leaked. Over 100 years, methane has 28 times greater impact than carbon dioxide in trapping radiation, according to data from the Intergovernmental Panel on Climate Change and cited by the U.S. Environmental Protection Agency.
“Two percent leakage is massive from a climate standpoint,” said lead author Zachary Barkley, an associate research professor in the Department of Meteorology and Atmospheric Science. He explained that the estimate represents a significant volume of uncounted emissions and outpaces an EPA estimate of such leakage nationally. “Our findings add to indicators that more methane than imagined may be escaping into the atmosphere from U.S. cities, even cities with relatively modern distribution systems like Indianapolis.”
The research paper emerged from a roughly 15-year collaboration among the National Institute of Standards and Technology (NIST), Penn State and other partners including the University of Maryland. Starting in 2011, NIST supported the installation of the extensive monitoring network in Indianapolis to quantify greenhouse gas emissions in an urban environment.
Until its wind-down in 2025, the network of up to 12 tower-based instruments documented trace gases including carbon dioxide, carbon monoxide and methane at heights up to 136 meters, or nearly 450 feet. The Indianapolis Flux Experiment — known as INFLUX — was designed from the start to create and strengthen models for quantifying carbon dioxide and methane released in urban environments. Tower-based measurements represent one approach to monitoring emissions and can be used to test, for example, methods that rely on aircraft, drones or satellite-based indicators. Different methods of measurement have different strengths, and comparisons can establish confidence in the results.
“The idea was to create methods that can measure and monitor urban greenhouse gas emissions,” said Kenneth J. Davis, a professor of atmospheric and climate science, the lead investigator of INFLUX and a co-author on the recent paper.
In part by incorporating aircraft- and ground-based measurements in their analysis, researchers determined the consistency and reliability of tower-based readings over extended periods, the team wrote. The qualities are vital in identifying long-term patterns.
“We found it really doesn’t take much to monitor emissions effectively,” Barkley said. “Sometimes all you need is a few good, solid instruments and measurement systems downwind of a city, and you can really figure out what’s going on.”
NIST also supported urban testbeds, analyzed by other teams, in Los Angeles and in the Washington, D.C., region. For the methane analysis in Indianapolis, researchers focused on measurements taken from 2014 through 2023, when four or more towers collected observations of the gas. The team used statistical modeling to estimate total emissions and their shifts over time.
The researchers determined that the city released an average of 3.4 metric tons of methane per hour, nearly half of that coming from a landfill known as the South Side Landfill — the single largest source of methane emissions in the area. Methane emissions were higher by 13% during the winter, but seasonal variations were less pronounced than those recorded in Los Angeles and the D.C. region in other studies during the past decade, according to the paper.
Results also suggested relative year-to-year consistency in Indianapolis’ overall methane emissions, despite wide variations in calculations that the EPA used to assess the landfill emissions through 2023, Barkley said. EPA figures are available through a data portal tracking greenhouse gas emissions.
While cities in the Northeast tend to have older, more leak-prone exterior gas lines, researchers expected less leakage in Indianapolis because of its more modern, plastic transmission lines. But their discovery of what they called “an ambient plume of methane everywhere in Indianapolis” suggests leaks from inside buildings, in sources such as gas-fired furnaces and interior gas pipes, Barkley said.
“We know where the emissions are. Public policy can reduce them,” he said, pointing to incentives for technologies such as heat pumps.
At Penn State, contributors to the paper include Natasha M. Miles and Scott J. Richardson, research professors in the Department of Meteorology and Atmospheric Science, and Kevin Fletcher and James Marlow, doctoral candidates in the department. Other collaborators are Nikolay Balashov, assistant research scientist in the Earth System Science Interdisciplinary Center at the University of Maryland, and Aijun Deng, senior principal meteorologist at Vestas Wind Systems.
This work was supported by NIST under grant numbers 70NANB10H245, 70NANB19H128 and 70NANB23H188. Data collection from the tower network in Indianapolis concluded with the end of federal funding. The Penn State Institute for Energy and the Environment also supported the work. This content is the sole responsibility of the authors and does not necessarily reflect the views of the funders.
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