The Aliso Canyon natural gas well blowout, first reported on Oct. 23, 2015, released over 100,000 tons of methane—the largest such leak in U.S. history—before the well was sealed on February 11, according to a study published in the journal Science.

The research effort, by teams from the University of California (UC), Davis, the National Oceanic and Atmospheric Administration (NOAA) and partners, showed that during the peak of the Aliso Canyon event, enough methane poured into the air every day to fill a balloon the size of the Rose Bowl. Total emissions during the 112-day event were equal to one-quarter of the annual methane pollution from all other sources in the Los Angeles basin combined—the climate impact equivalent of the annual greenhouse gas emissions from over half a million passenger cars.

“Our results show how failures of natural gas infrastructure can significantly impact greenhouse gas control efforts,” says NOAA’s Tom Ryerson, co-lead scientist on the study.

During the more than three-month event, co-lead scientist Stephen Conley, of Scientific Aviation and UC Davis, made 13 research flights carrying instruments that provided real-time measurements of methane and ethane, two components of natural gas, and captured air samples for more comprehensive analysis later in the laboratory.

UC Davis scientist Stephen Conley flies over Aliso Canyon measuring methane emissions. Image credit: Joe Proudman/UC Davis. In addition to providing real-time air quality data above and around the leak site to state regulators, the measurements will allow researchers the opportunity to check the accuracy of greenhouse gas measurements made using remote sensing systems such as satellites.

Near the well site, study co-author Donald Blake, of the University of California, Irvine, gathered samples of the leaking natural gas to determine its composition and compare to Conley’s airborne measurements. The analysis found a slew of other compounds present in natural gas, including benzene, butanes and pentanes.

The team’s measurements confirmed that high concentrations of methane and ethane were surging from the breached well into the densely populated San Fernando Valley. The analysis found that at its peak, the blowout doubled the rate of methane emissions from the entire Los Angeles basin and temporarily created the largest known human-caused point source of methane in the U.S., twice as large as the next-largest source, an Alabama coal mine.

The study highlights the value of rapid-response airborne sampling for independent, time-critical, accurate and detailed information about major chemical releases, the authors say. Measuring leak rates, plume locations and plume composition can also help officials assess public health risks, the effectiveness of leak control and climate and air quality impacts, according to the researchers.

“If we don’t measure these things quickly, we won’t have any idea what kind of response might be called for,” Conley says.