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Gaby Petron didn't set out to challenge industry and government assumptions about how much pollution comes from natural gas drilling.

She was just doing what she always does as an air pollution data sleuth for the National Oceanic and Atmospheric Administration.

"I look for a story in the data," says Petron. "You give me a data set, I will study it back and forth and left and right for weeks, and I will find something to tell about it."

Petron saw high levels of methane in readings from a NOAA observation tower north of Denver. And through painstaking, on-the-ground detective work, she tied that pollution to the sprawling oil and gas fields in northeastern Colorado.

The story she stumbled into suggests that government may be far underestimating air pollution from natural gas production. Her measurements, which were published in the Journal of Geophysical Research, suggest that methane, a potent greenhouse gas, is leaking at at least twice the rate reported by the industry.

Coal Vs. Natural Gas: Which Is Cleaner?

Her paper was the latest volley in an intense estimate war under way in the scientific community about whether natural gas really is cleaner than the coal it's already starting to replace on the electric grid.

A lot of research shows power plants pump out fewer greenhouse gases when they run on gas instead of coal. But no one really knows how much natural gas leaks out when companies are drilling for gas and getting it to power plants. Natural gas is primarily methane.

"We need to know a lot about methane itself, which is natural gas, if we're worried about climate change, so that we don't automatically think that gas is so much cleaner than coal," says energy consultant Sue Tierney at Analysis Group.

Methane is very effective at trapping heat in the atmosphere. And already, natural gas production is the biggest manmade U.S. source of methane.

"Fifty years from now, are we really going to be wondering if we really screwed up because we went on this big gas boom? You really wouldn't want to be messing that up," Tierney says.

Tierney says the time to study air pollution from natural gas is now, before the U.S. makes major investments in new power plants and factories that would use natural gas.

The way it is now, the Environmental Protection Agency relies on estimates of methane emissions. They're based on some measurements of emissions from individual pieces of equipment and lots of complicated math.

"What the official estimates are based on generally are not so many measurements, but rather estimates," says Greg Frost, an atmospheric scientist and air pollution expert for NOAA. "They really are based on maybe a measurement here or there, but then they're largely based on extrapolation. So in other words, you make a measurement in one place or for one particular source, and you do a complicated calculation to assess what does that mean across a whole region, across 20,000 wells?"

A Revealing Tower

A tall tower at the foot of the Rocky Mountains tipped off scientists that these estimates are poor substitutes for measuring.

Imagine an open metal structure as tall as the Eiffel Tower and in the shape of a Toblerone chocolate box. A tiny elevator runs up the middle.

For the past few years, that tower has been Gaby Petron's muse, spewing out numbers about air pollution.

Petron was studying those numbers at NOAA's lab in Boulder, Colo., just 15 miles from the tower. What she saw amazed her.

"Oh, my God, we were looking at something really different than anywhere else where we were taking measurements in this country," Petron recalls. "And at first I didn't know what it was."

What she saw was very high levels of methane gas, not all the time, but often. And every time methane was high, she saw a consistent mix of other chemicals with it — a sort of chemical cocktail — that included methane, propane and pentane in specific proportions.

Levels as high as she was seeing suggested a lot more methane than anyone realized was coming from somewhere, but from where?

She talked a colleague into turning his Prius into a mobile lab so she could sniff out the source.

"You want to see the invisible. You want to see what's in the air, and you want to know exactly where the air is coming from," Petron says.

As they drove east, toward the tower, methane levels would increase.

Seeking The Source

They saw lots of potential sources of methane: a landfill, cows and lots of telltale signs of the oil and gas industry, such as storage tanks, drill rigs and lots of bright red Halliburton trucks.

"And then you come here and you see cows," Petron says. "OK, maybe it's the cows — they burp methane."

She collected canisters of air near cows to see if they were her mysterious source. But when the analysis came back from the lab, there was methane, but not the other chemicals in the cocktail from the tower. No match.

The landfill was not a match, either.

When she drove around the oil and gas field northeast of the tower, the methane levels spiked.

"They would increase a lot, and that's why we saw we were looking at something really important and really big," Petron recalls. "We would say, 'Oh, my god, let's stop and take a sample.' "

When her lab analyzed the canisters of air from the gas field, she had a match. The tower is on the southwest corner of the Denver-Julesburg basin, where at the time there were more than 20,000 active natural gas and condensate wells.

She got detailed wind direction data from the tower and confirmed that methane at the tower was highest when the wind was blowing from the direction of the gas fields. She also got a hold of the industry's analysis of the mixture of chemicals that come out of the ground with the natural gas here. It also matched what she found in the tower air.

"So that's when you have your moment. All right, the story is right there. It's really not the landfill. It's really not the cows. It's really all the oil and gas equipment and activities that are going on in the region. And it's not new — it's always been there. We were just not measuring it." Petron said.

Then came the hard part: trying to figure out how much methane was leaking from the gas fields. That took a few years and a lot of input from industry and regulators. The science and calculations were complicated.

"That's why it took so long to write this story," Petron recalls.

Petron says even the lowest range of her estimate was higher than the leak rate industry and regulators were reporting.

"Really, what our story is telling in our paper is the leak rate is twice what the industry thinks it is," she says.

Petron's work also suggests that the industry is underestimating its releases of other chemicals, including benzene, which, if present at high enough levels, can cause cancer. The industry reports negligible benzene emissions. But her calculations show it is likely the region's largest source of benzene.

Measuring A Sprawling Industry

So why don't gas companies measure the pollution they pump into the air?

Companies can have thousands of gas wells, storage tanks and equipment that leak air pollution sprawled over hundreds of square miles.

Allen says it would take too much work for companies to maintain air pollution monitors near each well site.

"Direct emission measurement is extremely expensive. It's not realistic to install such devices on every single emissions source that there is," says Cindy Allen, who heads the environment team for a drilling company called Encana.

The American Petroleum Institute says companies are trying to improve their air pollution estimates. The trade group is working on a new survey of methane emissions from tens of thousands of wells.

The API's Howard Feldman says more measurements like the ones that came from that NOAA tower are needed, too.

"Both are valid, and both add to the information that we have," he says.

Feldman says it's in the industry's interest to find leaks and capture methane, because methane — which is natural gas — is their product, and they don't want to lose it to the atmosphere.

Road Trip

Petron believes scientists need to play a much bigger role in measuring air pollution from natural gas production — at well sites and compressor stations, and over entire gas fields.

"I think the atmosphere, it's not lying," Petron says.

This winter, she tricked out a van with a lot of sensitive instruments and hit the road in Utah and Colorado. She's collaborating with other scientists measuring the air over gas fields from aircraft.

She believes pollution levels could vary a lot from one gas field to another. So Petron wants to take her van to other gas fields in Utah, Texas and Pennsylvania.

"There's a lot of booming oil and gas activities around the country," Petron says. "If I could dream, I would be going to all these places."

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