Ethane is the second-most abundant atmospheric hydrocarbon. Ethane reacts with sunlight and other molecules in the atmosphere to form ozone, which at the surface can cause respiratory problems, eye irritation and other ailments and damage crops.

Surface-level ozone is one of the main pollutants that the national Air Quality Index measures in its effort to let the public know when breathing outside for long periods of time could be harmful. Low-altitude ozone also plays a role in climate change, as it is a greenhouse gas and the third-largest contributor to human-caused global warming after carbon dioxide and methane.

Two percent might not sound like a lot, but the emissions we observed in this single region are 10 to 100 times larger than reported in inventories. They directly impact air quality across North America. And they’re sufficient to explain much of the global shift in ethane concentrations.

—Eric Kort, first author

The Bakken is part of a 200,000-square-mile basin that underlies parts of Saskatchewan and Manitoba in addition to the two US states. It saw a steep increase in oil and gas activity over the past decade, powered by advances in hydraulic fracturing, or fracking, and horizontal drilling.

Between 2005 and 2014, the Bakken’s oil production jumped by a factor of 3,500, and its gas production by 180. In the past two years, however, production has plateaued.

A snapshot from a simulation of how Bakken oil field hydrocarbon emissions including ethane affect North American ground-level ozone concentrations. Hydrocarbons react with NOx and sunlight to produce ozone. Ground-level ozone leads to poor air quality. This snapshot represents one hour during the summer of 2014 in an air quality model. The reddish hues directly over and downwind of the Bakken show that emissions there accounted for increases of up to 4 ozone molecules per billion air molecules, about 6% of the present EPA standard. The colors don’t indicate that any particular location necessarily experienced an unhealthy air day, but they do show where Bakken emissions had the greatest impacts. Credit: Lee Murray, NASA GISS/Columbia University. Click to enlarge.

In 2010, a mountaintop sensor in Europe registered an ethane uptick. Researchers hypothesized that the boom in US oil and gas brought about by hydraulic fracturing could be the cause—even a continent away. Ethane concentrations have been increasing ever since.

To gather their data, the researchers flew over the Bakken Formation in a NOAA Twin Otter aircraft, sampling air for 12 days in May 2014. Their airborne measurements from directly over and downwind of oil production areas show that the field’s ethane emissions of 0.23 teragrams per year, or roughly 250,000 US tons, effectively cancel out half of the global decline rate.

These findings not only solve an atmospheric mystery—where that extra ethane was coming from—they also help us understand how regional activities sometimes have global impacts. We did not expect a single oil field to affect global levels of this gas.

—co-author Colm Sweeney

Ethane emissions from other US fields, especially the Eagle Ford in Texas, likely contributed as well, the research team says. The findings illustrate the key role of shale oil and gas production in rising ethane levels.

Also contributing were researchers from NOAA, NASA Goddard Institute for Space Studies, Columbia University, Stanford University and Harvard University. The research was funded primarily by NOAA and NASA.

Resources

• Kort, E. A., M. L. Smith, L. T. Murray, A. Gvakharia, A. R. Brandt, J. Peischl, T. B. Ryerson, C. Sweeney, and K. Travis (2016), “Fugitive emissions from the Bakken shale illustrate role of shale production in global ethane shift,” Geophys. Res. Lett., 43, doi: 10.1002/2016GL068703