Cities may be leaking more heat-trapping methane than previously thought

A new approach to measuring methane leaks revealed that the Boston area alone lost enough natural gas during one year to fuel 200,000 homes.

Some of the nation's metropolitan areas may be losing tens of millions of dollars a year in natural gas leaks, which add a potent greenhouse gas to the atmosphere. That's one implication from a study offering a new approach to monitoring natural-gas emissions from urban areas.

The approach, which blends measurements from judiciously placed air-sampling equipment with weather-forecast models, has shown that the rate at which natural gas leaks into the atmosphere, specifically in the Boston metropolitan area, may be nearly three times higher than previously estimated. 

The researchers devising the approach note that the gas lost during their yearlong study was worth about $90 million. By some estimates, the wasted gas could have supplied 200,000 homes for a year.

The losses are of more than economic interest.

Natural gas is seen in some circles as a "bridge fuel" in efforts to limit human-triggered global warming by shifting from fossil fuels to renewable energy sources. When burned, natural gas releases less carbon dioxide per unit of energy than does coal, which accounts for about 40 percent of the fuels used to generate electricity globally as well as in the United States.

But methane, the main component in natural gas, also is a building block for smog. It also is a more powerful greenhouse gas, molecule for molecule, than carbon dioxide. Although methane's residence time in the atmosphere is far shorter than residence time for the excess CO2 building in the atmosphere, clamping down on methane emissions also will provide a climate benefit, researchers say.

The federal government and some states are moving in that direction. Earlier this month, President Obama unveiled a plan to reduce methane emissions from the oil and gas sector 45 percent below 2012 levels by 2025.

But "people haven't paid as much attention to what happens after natural gas is produced and it is distributed and used," says A.R. Ravishankara, an atmospheric chemist at Colorado State University in Fort Collins and former head of the chemical sciences division at the National Oceanic and Atmospheric Administration's Earth Systems Research Laboratory in Boulder, Colo.

The new study, appearing in the online edition of the Proceedings of the National Academy of Sciences, addresses the need to more accurately estimate the rate at which gas is escaping at this business end of pipelines and distribution networks, says Dr. Ravishankara, who was not a member of the team conducting the study.

Interest has been growing for several years in the potential for natural gas leaks in the Boston area, given the region's aging infrastructure and an increasing focus on gas as a bridge fuel. But numbers for evaluating the problem were hard to come by.

"There really weren't any numbers," says Kathryn McKain, a graduate student in the School of Engineering and Applied Sciences at Harvard University and the lead author of the PNAS paper describing the results.

The team used detection sensors at four locations. Two of the four sensors were located beyond the confines of Boston to allow the researchers to correct their estimates for natural gas that wafted in from outside the metro area.

Ethane, a byproduct of natural-gas production, was of particular interest. The ratio of ethane to methane in the team's air samples allowed the team to distinguish the "fingerprint" of methane in natural gas from that of methane from natural sources.

The team, which included researchers from Harvard, Duke, Hofstra, and Stanford Universities as well as two companies with expertise in instrumentation and weather forecasting, found that the leak rate attributable to Boston was toward the low end of the rates for a series of major cities around the world that have tried to make such estimates over the past 25 years.

Still, emissions in the study area during the 2012-2013 study period stood at 8 percent of national emissions tied to distribution, transport, and storage, and 23 percent of the country's total emissions tied to distribution alone. The region consumes about 3 percent of the gas the US use for residential or commercial purposes.

Given the higher loss rate than previously estimated, the study may signal that "the sources may be more complex than we had realized and we don't totally understand what the largest sources are in the region," Ms. McKain says. The port of Boston hosts liquid-natural-gas and petroleum terminals, and a LNG storage facility sits along the shore of Boston's Dorchester neighborhood.

Moreover, current greenhouse-gas inventories don't include sources on a user's end of the gas meter.

"If there are emissions coming from homes, businesses, or industrial facilities, those are not captured at all" with existing approaches to estimating losses, she says, noting that with further improvements, the approach the team used -- which does capture any losses from these sources -- could be adopted by other cities for long-term emissions monitoring.

The proof-of-concept study for estimating urban natural-gas emissions appears to fill an important gap in measurements, Colorado State's Ravishankara says. Measurements have been made covering broad regions, such as the geological formations where drilling and extraction occur. Traveling city streets with sensor-laden cars can sniff out local sources in great detail. The new study fills the space in between, allowing for more-accurate estimates from an urban area than either of the other two may provide.

Last year, Google and the Environmental Defense Fund unveiled a pilot project involving street by street mapping of urban methane sources. But these generally were restricted to tracking leaks in gas-utility mains, says Mark Brownstein, associate vice-president for the EDF's US climate and energy program.

The new study "may be giving us insight into other sources of emissions that aren't solely attributable to leaks in utility infrastructure," he says.

The combination of both approaches provides a powerful tool for gauging the size of the gas-leak problem as well as identifying the most important targets for repairs or upgrades, he suggests.