New methods for oil and gas development such as fracking may be responsible for unusually high levels of ozone in some regions. This appears to be the conclusion reached by a U.S. study that was conducted under the supervision of the National Oceanic and Atmospheric Administration.

Ozone, which is a molecule consisting of three oxygen atoms, is normally present in the upper atmosphere and shields the earth and its inhabitants from UV radiation in the form of the well-known ozone layer. Close to the ground, ozone in high concentrations is an air pollutant and the main ingredient of smog. Ground-level ozone is formed when volatile organic compounds (VOCs) and nitrogen oxides, mainly from vehicle exhaust gases, react with each other in the air. However, smog usually occurs during the summer, which is due to high levels of solar radiation, temperature and humidity that promote these reactions.

For this reason, it is very unusual that high ozone levels have been measured near the ground during the winter in the western United States. This occurred in oil and gas fields. An international research team decided to investigate the incidents in the Uintah Basin in Utah.

Martin Graus, who now works for the Institute of Meteorology and Geophysics at the University of Innsbruck, was involved in the study as a former employee of the CIRES Institute at the University of Colorado Boulder. He explains how researchers approached the matter: «The task was to identify the main ozone precursors and chemical intermediates in this remote area during the winter months between 2012 and 2014 using a variety of complex scientific instruments, and to create a quantitative time series.»

This revealed that the emission of VOCs in the oil and gas fields is very high due to the nature of the processes they are used in as well as leaks. Together with the nitrogen oxides from trucks, compressors, pumps and other equipment with internal combustion engines, two of the conditions for ozone formation are already fulfilled. The precursors of ozone formation also do not dissipate so readily because the ground is continuously covered with snow in this area. This means that there is hardly any air interchange, so ground-level air is saturated with VOCs and nitrogen oxides. These ‘sit and wait’ for a convenient time to react with each other. As soon as the sun shines strongly enough on the blanket of snow and is reflected back, conditions are right to produce ozone near the ground, even during the winter.

Based on the data that was collected, researchers working together with Peter Edwards at the University of York in England developed a model that they used to find an explanation for the ozone formation. The formation of ozone during the winter is different from the formation of smog during the summer because VOCs are more concentrated and need less solar radiation and humidity to react. This new model for ozone formation will now help to make decisions on measures to be taken by the government and industry to reduce ground-level ozone during the winter. However, in and of itself, this seems to be a further argument against fracking and similar methods of fossil fuel extraction.

The study was published as a Nature Advance Online Publication. (DOI: 10.1038/nature13767)

Tanja Peschel