May 24, 2013
Mystery solved: Previously unexplained higher levels of greenhouse gas in L.A. from fossil-fuel sources
The missing link—exactly where the extra methane, a powerful greenhouse gas, is coming from in Los Angeles—has finally been identified, according to a study led by a scientist at NOAA’s Cooperative Institute for Research in Environmental Sciences (CIRES). The research explains why the estimates of methane given off by various sources are 35 percent lower than the levels that have actually been measured in the atmosphere by scientists.
“We identified methane sources based on their unique chemical signatures in the atmosphere much like you’d identify a person from their fingerprints,” said lead author Jeff Peischl, a CIRES scientist who works at NOAA’s Earth System Research Laboratory in Boulder, Colo.
Using an innovative experimental technique, the scientists were able to find out that methane quantities coming from activities related to fossil fuels contributed to the discrepancy. Leaks from natural gas (methane) delivery systems in the urban area, geologic seeps such as from the La Brea tar pits, and leaks from local oil and gas exploration activities account for the “missing methane sources” in Los Angeles.
“Our findings can help both industry and the State of California more accurately assess methane emissions,” Peischl said, which would inform the state’s efforts related to climate protection.
While methane is still only the second-most prevalent greenhouse gas emitted in the United States from human activities, it is more efficient at trapping heat in the atmosphere than carbon dioxide. Pound for pound, the comparative impact of methane on climate change is more than 20 times greater than carbon dioxide over a 100-year period, according to the Environmental Protection Agency (EPA).
Methane is given off into the atmosphere from natural sources, such as geologic sources and wetlands, and from human-related sources, such as landfills, leakage from natural gas systems, and the raising of livestock. In California, the California Air Resources Board (CARB) estimates emissions of methane, along with the other major greenhouse gases, and collates the estimates in the Greenhouse Gas (GHG) Inventory. However, several research studies have discovered that the actual methane released into the atmosphere exceeds those estimates. When Peischl and his team flew over Los Angeles in the NOAA P-3 research aircraft in the summer of 2010, they, too, discovered that the amount of methane emitted in the L.A. area exceeded state inventories—in this case, by 35 percent.
To determine the key contributors to the discrepancy, the scientists then used a novel technique based on the fact that each source of methane typically also gives off other gases in distinctive proportions. Landfills emit methane but not much else, Peischl said, but natural gas supplied to people’s homes and the La Brea tar pits emits mostly methane and then a little bit of ethane and even less propane. “So depending on how much ethane you see relative to methane, you figure out whether it came from a landfill or from natural gas,” he said.
The researchers, therefore, measured multiple chemicals, such as the hydrocarbons propane, ethane, and butane, in the air, and from analyzing the types and ratios of gases present, they were able to use a mathematical technique to determine what sources had given rise to the gases and the accompanying methane.
“We use that data to apportion the sources of methane in the L.A. area—as far as I know, that is the first time that has been done,” Peischl said.
The scientists found that more than 85 percent of the L.A. basin’s methane emissions comes from a combination of sources—leaks from pipeline-quality natural gas, geologic seeps (such as the La Brea tar pits), dairies, and landfills. In addition, they were able to identify that 8 percent of the methane emissions in the L.A. basin is due to leaks from the local oil and gas industry, which corresponds to a 17 percent leak rate for the Los Angeles–area oil and gas operations. This leak rate for the operations was similar to the findings of an independent study carried out by CARB.
Most importantly, however, the scientists were able to pin down what sources contributed to the discrepancy between the state’s estimates of methane emissions and the values measured in the atmosphere. “Methane leaks from pipeline-quality natural gas from urban-distribution systems and from geologic seeps, as well as emissions from the local oil and gas industry, account for inventory shortfalls in the Los Angeles area,” Peischl said. The study was published May 14 in the Journal of Geophysical Research.
Would the scientists find these same sources of methane and discrepancies in other parts of the United States?
“We won’t know until we go there,” Peischl said. “But our technique now gives us a way to determine the sources of the methane in the atmosphere, whether from ground-based measurements or from aircraft.”
CIRES coauthors on the study are J. Peischl, J. Brioude, K. C. Aikin, J. A. de Gouw, G. J. Frost, J. B. Gilman, J. S. Holloway, J. Kofler, and W. C. Kuster. Scientists from NOAA’s Earth System Research Laboratory, the University of Miami, the University of California, and Harvard University are also coauthors on the study.
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