The first thing anyone noticed was the smell—like rotten eggs, or the ocean at low tide. That was before anyone in Porter Ranch, north of Los Angeles, knew their town was near the largest recorded leak of natural gas in California history. But it was enough for Stephen Conley to take to the skies.

Conley is a pilot, president of the aerial survey company Scientific Aviation, and an atmospheric scientist at UC Davis. Over the several years of flying, Conley has carved out a niche hunting down leaks over gas fields and pipelines. But the turbulent flight over Aliso Canyon Natural Gas Storage Site shocked him.

“When I first saw the analyzer,” Conley says, “my first thought was something was wrong.” The methane reading was ten times higher than he had ever seen. But the analyzer was working fine. Conley’s readings, the only source of data on the leak’s size, showed that Aliso Canyon has emitted an estimated 80,000 tons of methane to date.

Two and a half months later, the Aliso Canyon site is still spewing natural gas. Thousands of Porter Ranch families have relocated. Crews are at work on a fix, but the plan is so complex it’ll take until spring. Meanwhile, Conley has flown over Aliso Canyon seven times this week—twice for the California Air Resources Board and the others for the gas company behind the leak. His plane is one of the few on the west coast outfitted for air quality research.

Aerial image of Aliso Canyon Natural Gas Storage Facility shot from Conley's airplane.Aerial image of Aliso Canyon Natural Gas Storage Facility shot from Conley’s airplane. Joe Proudman/UC Davis

Conley got into scientific aviation thanks to a storm. A decade ago, he was running a software company and flying an airplane to get around for meetings. Then he wandered into a thunderstorm. “I was getting thrown in every direction, and it was the only time I’ve flown I didn’t know if I was going to live or die,” he recalls. “I thought I should probably learn more about this atmospheric science.” So he went back to school, to Davis, and got his PhD.

Now, every week when the winds are favorable, Conley flies 400 miles from his base near Sacramento to Aliso Canyon, just north of Los Angeles. That’s a four hour commute back and forth, which means a long playlist. (Lately, it’s been a lot of ABBA.) Conley is the only pilot allowed to fly so low and close to the leak. The Federal Aviation Administration has actually restricted flights directly above Aliso Canyon. Airplane engines are all about combustion, you see, and natural gas—methane—is oh so very combustible.

That’s in part why Conley has to get downwind of the leak on survey flights. He then drops his plane down to just 200 feet, tracing a path on the ground, back and forth, while slowly climbing, until he’s high enough that his instruments no longer detect methane. On some days, that’s as low as 1,000 feet, on others, 4,000 feet. The whole time, his plane, a Mooney TLS, is sucking air through custom inlets into the methane analyzer stowed in the luggage compartment.

Conley looks at methane emission data from the site's leak on Friday, Jan. 8, 2016. Conley, flying in a pollution-detecting airplane, provided the first estimates of methane emissions spewing from the Southern California leak.Conley looks at methane emission data from the site’s leak on Friday, Jan. 8, 2016. Conley, flying in a pollution-detecting airplane, provided the first estimates of methane emissions spewing from the Southern California leak. Joe Proudman/UC Davis

Even without the analyzer, the gas is obvious. “I get a little bit of a headache. It’s nasty,” Conley says. Methane is odorless, but gas companies add sulfurous mercaptans to make the invisible gas easier to detect. And it’s post-mercaptan gas that’s leaking above Aliso Canyon.

The stench isn’t the only unpleasant part of the flight. Aliso Canyon is, true to its name, a canyon, which means hills, which means air bumping into tall things, which means turbulence. “Every single person who has gone on the flight has gotten sick,” he says. But not Conley. “I’m cast iron,” he says.

With his background in aviation and atmospheric science, Conley has had found plenty of work surveying gas pipelines, oil and gas fields, and smog. Usually though, the work comes in the summer months, and the winter months are the down time, when he has time to analyze the data. This year, much to the dismay of Porter Ranch, winter is looking busier and busier.

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The Lone Pilot Flying Over California’s Giant Methane Leak