Mystery of urban methane emissions
Far more of the potent greenhouse gas methane is being produced by the Los Angeles metropolitan area than was previously realised, according to a new study due to be published in Geophysical Research Letters.
After subtracting all known sources of methane in the Los Angeles region – such as livestock, landfills and sewage – from estimates of total methane production, a research team led by Paul Wennberg, an atmospheric chemist at Caltech in Pasadena, found that about 0.14 to 0.34 megatonnes, or up to half of the total annual emissions, could not be accounted for.
The researchers speculate that the extra methane may come from higher than expected leakage from landfills, sewage treatment plants or natural gas pipelines, or it could represent natural seepage from oil wells or other geological sources. Urban sources would account for seven to 15 per cent of global methane emissions from human activity if other urban areas have a similar share of uncatalogued emissions.
Amy Townsend-Small, a biogeochemist at the University of California at Irvine, believes that isotope analysis could be used to distinguish between the different urban sources; natural gas and other geologically old methane, for example, should be devoid of carbon-14, a relatively short-lived radioactive isotope.
Townsend-Small plans to measure the carbon-isotope composition of methane released from a variety of different sources and use these measurements to calculate the proportion of the emissions that come
from each source.
September 2009
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