On  February 11, 2016, workers in California ended the largest reported natural gas leak in U.S. history. The Aliso Canyon leak released methane and other gases into the atmosphere from an underground-storage facility for over three months, causing the evacuation of more than 5,000 households. Researchers from NOAA, NASA, Scientific Aviation, the University of California, the National Institute of Standards and Technology (NIST), the California Air Resources Board, and South Coast Air Quality Management District mobilized rapidly to assess the environmental impacts of the leak, deploying existing measurement capabilities to quantify how much methane, a potent greenhouse gas, was escaping. The interagency response to this incident demonstrated the application of multiple, independent methane-measurement methods to address challenges ranging from rapid response to unplanned events, to ongoing emissions monitoring and characterization of emissions sources at fine scales.  

Thirteen research flights provided an unprecedented opportunity to document the total amount of methane released over the 112-day leak. A study co-led by NOAA, published in Science just two weeks after the leak was stopped, estimated that about 97,000 tons of methane were released—one quarter of the methane that is typically emitted by the entire Los Angeles Basin over the course of a year—making the leak the largest reported accidental release of methane in U.S. history. In addition, the Hyperion instrument on NASA’s EO-1 satellite and NASA research aircraft successfully detected and quantified the leak plumes at high spatial resolution.

Instrumentation deployed through the Megacities Carbon Project (Highlight 5) provided data on background methane emissions in the area and documented abnormally large methane plumes crossing the Los Angeles basin during the Aliso Canyon incident. Analysis using data from the Megacities tower network and NASA’s California Laboratory for Atmospheric Remote Sensing is underway to develop a record of methane emissions sufficient to attribute fluxes to the vicinity of the Aliso Canyon facility. These analyses will evaluate the potential for smaller emissions in the weeks preceding the leak onset, the potential for highly variable fluxes associated with early “top-kill” attempts to stop the leak, and subsequent evolution of the leak flux before and after the successful “bottom-kill” closure. Data from remote-sensing instrumentation mounted on aircraft is also being combined with NIST plume modeling to estimate emissions fluxes. Future analyses and synthesis of these data sets will further explore the physical mechanisms controlling methane leak rates and their potential broader applicability to other underground gas-storage facilities.