<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/noaa_airborne_research_072419.jpeg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/noaa_airborne_research_072419.jpeg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/noaa_airborne_research_072419.jpeg?v=ap&w=980&h=551 800w" > A NOAA Twin Otter aircraft is photographed flying over the lower Hudson River during an airborne research project in 2018. (Eric Kort, University of Michigan)

At a Glance Five major cities along the U.S. East Coast are emitting twice as much methane as estimated by the EPA.

Methane is the second-most-abundant greenhouse gas in the Earth's atmosphere.

The decay of organic material in landfills and leaks from pipes and appliances are the two main sources of methane emissions from cities.

Five major cities along the Interstate 95 corridor have an alarming trait in common: They are all emitting a surprising amount of methane, a powerful greenhouse gas, according to scientists at NOAA and the University of Michigan.

Methane is 25 times more effective than carbon dioxide at absorbing infrared radiation, trapping it in the atmosphere rather than letting it escape out to space. Atmospheric methane also contributes to ground-level ozone pollution.

The scientists used an instrumented airplane to measure concentrations of methane emissions over Boston, New York, Philadelphia, Baltimore and Washington, D.C., and the results were staggering.

The researchers found unexpectedly large methane emissions over the cities – twice the total amount of methane estimated by the Environmental Protection Agency and almost 10 times the amount estimated from the burning of natural gas, the cleanest-burning fossil fuel. Methane is a primary component of natural gas.

The measurements were taken by NOAA Twin Otter aircraft over the course of 20 research flights between April 8 and May 12, 2018. Instruments aboard the aircraft measured levels of methane, carbon dioxide, carbon monoxide and ethane upwind and downwind of urban areas. The flights occurred at 1,000 to 1,500 feet above ground level.

“The amount of natural gas that flows into these older cities is very large,” Genevieve Plant, lead author of the study from the University of Michigan, said in a press release. “That means leakage from old cast-iron pipes and 'end-use' losses from inefficient appliances could potentially be significant. We found methane emissions in the five largest cities that we sampled are significant – about double what the EPA estimates for the total emissions.”

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Measuring methane emissions is key to evaluating this greenhouse gas' impact on climate and air quality now and in the future. There are two main sources of methane emissions from cities, according to the study: methane produced by the decay of organic material in landfills, and methane released in natural gas from old, leaky pipes or incomplete combustion from older appliances.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/noaa_univ_of_michigan_scientists.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/noaa_univ_of_michigan_scientists.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/noaa_univ_of_michigan_scientists.jpg?v=ap&w=980&h=551 800w" > Genevieve Plant of the University of Michigan and NOAA's Colm Sweeney review measurements of methane and other gases during an airborne research project in 2018. (Eric Kort, University of Michigan)

A separate study by NOAA that estimated emissions from the production field all the way to the edges of cities found methane emissions were 60% higher than estimated by the EPA. However, it did not consider the loss during distribution and end-use in homes and businesses.

The recent joint study by NOAA and the University of Michigan, however, took direct measurements of downwind plumes of emissions coming from the five East Coast cities and also took into account leaks from pipes and appliances. This methodology allowed the researchers to determine the total amount of methane coming from each of the cities that were studied. They also analyzed the ethane-to-methane ratio to calculate how much methane resulted from natural-gas leakage.

“There’s a real lack of data on urban emissions, including end-use losses,” Colm Sweeney, a NOAA scientist involved in the study, said. “In our study, we found leaks and end-use losses in just five cities was larger than what EPA estimates for leaks in the entire country."

Determining where exactly the leakage is coming from is one of the biggest problems, he added. If the source can be confirmed, emissions could be reduced.

Numerous other studies on methane emissions have been conducted at various urban areas around the United States, but the NOAA and University of Michigan study is particularly important because it sampled the most heavily populated urban corridor in the country – home to about 12% of the U.S. population.

It's still unclear how representative this data is compared to other U.S. cities. The researchers say there is much more work to be done there.

The NOAA and University of Michigan study was published in the journal Geophysical Research Letters on July 15 .