From an observation station located in Switzerland, researchers from the InfraRed Group of Atmospheric and Solar Physics of the University of Liege have identified a potentially worrying phenomenon that could lead to air quality degradation: since 2009, the level of ethane in the atmosphere has increased by 5% per year while previously, it was decreasing by 1%. The explanation for this increase can be found several thousand kilometers away in the United States where the massive exploitation of shale gas contained underground is certainly not without consequences.

How can drilling in the middle of an American plain have effects that can be observed at an altitude of 3,580 meters in Switzerland? This is quite some butterfly effect which needs to be explained. Since the end of the 2000s, the Americans have been exploiting shale gas. In order to do this it was necessary to drill vertical wells in the hope of accessing the geological layers containing methane. This chemical compound which is highly flammable is not always easy to collect. In order to remove it from usual depths of 1,500 to 3,000 meters where it is well-contained in shale, it is necessary to use the horizontal method of drilling known as fracking or “hydraulic fracturing” which involves the injection of a complex mixture of water, sand, lubricants, biocides and detergents which allow the gas to be collected on the surface.

However, at some point during this process, part of the methane gas (CH₄) escapes. Not only methane but also ethane (C₂H₆), which is closely associated with it, also partly evaporates.

At the summit of Jungfraujoch

Amazingly, this leaking of ethane was not identified in the US but from the snow-capped summits of the Jungfraujoch station, in the middle of the Swiss Alps. The station is an international scientific observatory which is also home to the Laboratory of Atmospheric and Solar Physics of ULg among others. It is here that measurements of infrared radiation were carried out at high altitude in order to avoid interference from water which is abundant in the lower layers of the atmosphere and which could interfere with the results.  

Two young researchers from Liege, Whitney Bader and Bruno Franco carried out an analysis of this solar infrared light. During the course of 2014, the two researchers developed a method for more accurately analyzing information linked to ethane in infrared observations thanks to new spectroscopic parameters. This improved technique made it possible to study certain data sets including those related to the presence of ethane in the atmosphere.  

“While inspecting this series again, we detected a reversal of the trend as it was happening”, explains Emmanuel Mahieu, a researcher at the FNRS and head of GIRPAS (the InfraRed Group of Atmospheric and Solar Physics). “Since the middle of the 1990s, the presence of ethane was dropping each year by around1%. Then, after around 2009, we notice an increase of 5% per year”. In other words, the efforts that had been made to reduce these emissions for more than a decade were rendered useless.  

Already in the 1980s, this atmospheric pollution due to ethane had been noticed, it was the result of uncontrolled “fugitive” emissions linked to the exploration of oilfields. Public authorities reacted by imposing restrictions on oil companies which brought about a progressive improvement of the situation.

Watch out for bad ozone

Ethane should not be taken lightly! While this gas is not polluting in itself, its degradation makes it dangerous. It ends up by forming ozone in the troposphere. This is “bad” ozone which is found nearer to ground level and reaching a height of ten kilometers. It is a major pollutant for human beings and the biosphere which is the exact opposite of “good” ozone present at higher altitudes and protecting us from the sun’s ultraviolet rays. “It is due to this bad ozone that there are sometimes pollution-alert days in summer when people are advised not to practise sport or to go outside if they are suffering from asthma… It is also an oxidant which is dangerous for vegetation and construction materials (roofs, chassis etc.)”, Explains Emmanuel Mahieu. More seriously still, ethane is emitted at the same time as methane, a more efficient greenhouse gas than _CO2.

Why did the level of ethane in the atmosphere rise suddenly from 2010 onwards? At first, the researchers from Liege only had a theory. They noticed that the period coincided with the beginning of large-scale exploitation of shale gas in the US. Because the prevailing winds go from the American continent to Europe and the time it takes to transport this gas is lower than its life-cycle it was plausible that traces of it could be found in the Swiss Alps.