Atmospheric methane is a powerful greenhouse gas that also contributes to ground-level ozone pollution. In the past decade, there has been intense scientific focus on improving the accuracy of methane emissions estimates in the United States, with most of the effort targeting oil- and natural gas-producing basins.
Now, a new NOAA and University of Michigan study using an instrumented airplane has found unexpectedly large emissions over five major cities along the East Coast – twice the total amount of methane and almost 10 times the amount estimated from natural gas. The work was published in the journal Geophysical Research Letters.
The analysis was based on airborne measurements taken from NOAA Twin Otter aircraft along the East Coast of the United States during the course of 20 research flights between April 8, 2018 to May 12, 2018, totaling 120 flight hours. The flights covered Washington, Baltimore, Philadelphia, New York, Providence and Boston, with instruments measuring levels of methane, carbon dioxide, carbon monoxide, and ethane upwind and downwind of urban areas. Methane is a primary component of natural gas.
“The amount of natural gas that flows into these older cities is very large,” said lead author Genevieve Plant, of the University of Michigan. “That means leakage from old cast-iron pipes and ‘end-use’ losses from inefficient appliances could be 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.”
Climate and air-quality impacts
Quantifying methane emissions is important for assessing its current and future impact on climate and air quality. Generally there are two main sources of methane emissions from cities: methane created by the decay of organic material in landfills, and methane in releases of natural gas from old or leaky pipes, incomplete combustion from older appliances, etc.
Ethane is a component of natural gas, but is not generated by landfill, and utilities have precise measurements of how much ethane is in the gas they deliver. So it’s useful as a tracer for separating emissions coming from fossil fuel from those coming from landfills.
One recent synthesis study that estimated emissions from the production field all the way to the edge of cities found methane emissions were 60 percent higher than estimated by the EPA, said NOAA scientist Colm Sweeney. But that study, like most others, was not focused on examining the loss during distribution and end-use in homes and businesses.
The NOAA & UM study, by contrast, directly measured downwind plumes of emissions coming from these five cities, which included leaks from pipes and appliances. The methods used allowed scientists to quantify the total amount of methane coming from the city. Scientists analyzed the ratio of ethane to methane to calculate how much methane was due to natural gas leakage.
A lack of data
“There’s a real lack of data on urban emissions, including end-use losses,” Sweeney 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. That was kind of stunning. The problem is we still do not exactly where it is coming from. Is it the pipes? Is it the appliances? What we do know is that this provides a huge opportunity for emissions reduction if we can determine the source of these leaks”
While there have been a handful of other studies of urban areas scattered around the country, this study sampled one of the largest metropolitan megaplexes, home to about 12 percent of the US population.
“It’s unclear how representative the cities we looked at are compared to other cities in the US,” Plant said. “Clearly, more work needs to be done.”
Plant said the research flights – typically at around 1,000 to 1,500 feet off the ground – made for an unusually scenic research environment. “It was quite the view,” she said. “We flew up the Hudson a few times. Flying off the coast is also quite picturesque, to say the least.”