In Southern California, scientists knew the missing methane had to be coming from somewhere. Was it dairies? Landfills? Natural seeps? Oil and gas operations?
Emissions of methane from the Los Angeles basin had been estimated in the mid-2000s as part of the state's landmark cap-and-trade bill, known as A.B. 32, which regulates emissions of the powerful greenhouse gas. But later measurements of the air in the region showed there was a lot more methane being emitted than was accounted for, more than a third as much.
Where was this "missing" methane coming from? Methane has 20 times the global warming potency of CO2. If regulators could identify the source, they could also get those methane emitters to reduce the amount of the gas they release.
Now, Jeff Peischl, an associate scientist at the National Oceanic and Atmospheric Administration's Cooperative Institute for Research in Environmental Sciences, has solved this puzzle, outlining the sources of the missing methane in a paper published yesterday in the Journal of Geophysical Research.
While methane in the Los Angeles basin comes from many sources, there are three main sectors responsible for the difference in what the state of California had estimated and what is actually being emitted.
These are leaks from pipeline distribution systems, natural seeps from areas such as the La Brea Tar Pits and emissions from oil and gas drilling operations in the area.
Peischl was able to determine where the methane came from because different sources of methane contain different ratios of compounds called alkanes, like propane and butane.
By examining the ratios of alkanes in samples collected from 16 overflights of the Los Angeles basin in 2010, using NOAA's P-3 aircraft, a flying laboratory with sensitive air-sampling equipment, Peischl learned whether methane came from a landfill, dairy or fossil fuel source.
In his analysis, Peischl found that the methane leak rate from Los Angeles-area oil and gas operations was 17 percent. This is high; leakage rates of "fugitive emissions" from oil and gas drilling operations are currently estimated at 4 percent by U.S. EPA (ClimateWire, April 4).
The California Air Resources Board has been working toward pinning down this leakage percentage in its own way, too. Peischl's paper cites a survey it did of oil and gas producers in which it queried them about the equipment they use. Based on those results, CARB's revised estimate for fugitive emissions from the Los Angeles basin is 12 percent.
While this new figure is not yet incorporated into the state's greenhouse gas inventory, Peischl was encouraged that his calculated emissions and the board's were in the same ballpark, even though they used an entirely different methodology.
Paul Wennberg, a scientist at the California Institute of Technology who did two of the previous studies in the region that showed methane emissions were much higher than the earlier inventory, was encouraged by Peischl's results. He pointed out that if the missing methane was from fossil fuel extraction, this could be a "win-win" situation. Methane, or natural gas, is valuable. Oil and gas producers would certainly benefit from reducing their leakage.
"This is an important finding that we need to follow further," Wennberg wrote in an email.
CARB is doing just that. According to Bart Croes, chief of the agency's research division, Peischl had presented his work to the agency at an earlier date, and it is continuing research in this vein.
Peischl's results were surprising, Croes said, because many people thought dairies and landfills were likely sources for the mystery methane. "There was this suspicion that they could be a major contributor, but they don't seem to be," he said.