ESRL Quarterly Newsletter - Spring 2009

Ozone Smog in Wyoming

ESRL solves mystery of winter highs

Ozone pollution is normally linked to hot summers in big cities, so researchers were puzzled when ozone levels soared dangerously high during the past three winters in a rural Wyoming gas field. At high levels, surface ozone can damage people’s lungs and harm ecosystems.

Now, ESRL scientists have figured out how the pollutant forms in the cold, and their results suggest that wintertime ozone formation could be a more widespread problem than believed.

Jonah Pinedale Anticline gas field in Wyoming.

Jonah Pinedale Anticline gas field in Wyoming.

“Rapid production of wintertime ozone is probably occurring in other regions of the western United States, in Canada, and around the world,” said ESRL’s Russ Schnell, Deputy Director of the Global Monitoring Division. Schnell was lead author of a new paper on the Wyoming ozone problem, published in the February 2 issue of Nature Geosciences.

To form ozone, researchers and air quality regulators have long known that bright sunlight and hot weather can “cook” precursor chemicals—motor vehicle exhaust, industrial gases and other urban and natural emissions—into ozone. For that reason, the chemical is routinely monitored only between April and October in the United States.

In Wyoming’s Jonah and Pinedale Anticline natural gas production site, JPA, ozone precursors were probably emitted in many ways by the 24-hour operation—by engines that powered compressors, by drilling rigs, by vehicles, and possibly through leaks and other processes. JPA is one of the largest and most concentrated natural gas fields in the United States.

Ozone precursors emitted there, however, were thought to rise quickly enough in the atmosphere that they shouldn’t contribute to ground-level ozone. Regulators also believed that in winter, the Sun was too low in the sky to jump start the chemical reactions that lead to ozone. “Some thought the ozone must be coming from the stratosphere, or from Utah or Los Angeles, or maybe the instruments were defective,” said Schnell.

His team focused on several frigid days in February 2008, when instruments near the JPA natural gas fields recorded ozone levels significantly higher than the Environmental Protection Agency standard of 75 parts per billion, ppb, averaged over 8 hours. At times that month, single-hour ozone averages topped 140 ppb, rivaling peak summertime levels of 150 ppb measured in highly polluted US cities.

The researchers discovered that ozone was rapidly produced in the fields when three factors converged: the presence of ozone-forming chemicals from natural gas operations, a strong temperature inversion that trapped the chemicals close to the ground, and extensive snow cover, which provided enough reflected sunlight to ignite the chemical reactions.

Time-series analyses showed a repeated pattern leading to high-ozone events: Temperature inversions occurred, chemical-rich air became trapped near the ground, the sun rose, light scattered off the white snow, ozone levels surged, and precursor chemicals fell, consumed by the chemical reactions creating ozone.

“It’s as if at night, you spill a bunch of gasoline, and then you just wait for the match of the Sun in the morning,” Schnell said.

Schnell’s co-authors were Sam Oltmans, Ryan Neely, and Allen White of ESRL; Maggie Endres of the Wyoming Department of Environmental Quality; and John Molenar of Air Resource Specialists, Inc., in Fort Collins, Colo. Ryan Neely is a graduate student at the University of Colorado at Boulder, and is the first recipient of the ESRL-CIRES Graduate Research Fellowship.