Volcanic gases could deplete ozone layer

Bromine and chlorine are gases that "love to react -- especially with ozone," said Kirstin Krüger, a meteorologist with GEOMAR in Kiel, Germany. "If they reach the upper levels of the atmosphere, they have a high potential of depleting the ozone layer."

New research by Krüger and her colleagues, which she presented June 12 at a scientific conference in Selfoss, Iceland, combined a mixture of field work, geochemistry and existing atmospheric models to look at the previous Nicaraguan eruptions. And the scientists found that the eruptions were explosive enough to reach the stratosphere, and spewed out enough bromine and chlorine in those eruptions, to have an effect on the protective ozone layer. Krüger's talk was at the American Geophysical Union's Chapman Conference on Volcanism and the Atmosphere.

Steffen Kutterolf, a chemical volcanologist with GEOMAR and one of Krüger's colleagues, tackled the question of how much gas was released during the eruptions. He analyzed gases that were trapped by minerals crystallizing in the magma chambers, and applied a novel method that involves using the high-energy radiation from the German Electron Synchrotron in Hamburg to detect trace elements, including bromine. From that, Kutterolf estimated the amount of gas within magma before the eruptions, as well as the gas content in the lava rocks post-eruption. The difference, combined with existing field data about the size of the eruption, allowed the scientists to calculate how much bromine and chlorine are released.

Previous studies have estimated that in large, explosive eruptions -- the type that sends mushroom clouds of ash kilometers high -- up to 25 percent of the halogens ejected can make it to the stratosphere. For this study, the research team used a more conservative estimate of 10 percent reaching the stratosphere, to calculate the potential ozone layer depletion.

Taking an average from 14 Nicaraguan eruptions, the scientists found bromine and chlorine concentrations in the stratosphere jumped to levels that are equivalent to 200 percent to 300 percent of the 2011 concentrations of those gases. The Upper Apoyo eruption 24,500 years ago, for example, released 120 megatons of chlorine and 600 kilotons of bromine into the stratosphere.

Volcanic sulfate aerosols alone can lead to an ozone increase -- if chlorine levels are at low, pre-industrial levels, Krüger said. But bromine and chlorine are halogens, gases whose atoms have seven electrons in the outer ring. To reach a stable, eight-electron configuration, these atoms will rip electrons off of passing molecules, like ozone. So when an eruption also pumps bromine and chlorine levels into the stratosphere, the ozone-depleting properties of the gases together with aerosols is expected to thin the protective layer.

"As we have bromine and chlorine together, we believe that this can lead to substantial depletion," she said. "And this is from one single eruption."

Because the effects are in the stratosphere, where the volcanic gases can be carried across the globe, eruptions of tropical volcanoes could lead to ozone depletion over a large area, Krüger said, potentially even impacting the ozone over polar regions. However, that's a question for future research to address. Some volcanic gases can last in the stratosphere up to six years, she added, although the most significant impacts from eruptions like Mount Pinatubo were within the first two years.

The next step in the research, Krüger said, is to investigate how much damage to the ozone layer the volcanic gases caused in the past -- and what the damage could be from future volcanic eruptions in the active Central American region.

The title of the study is "The Combined Bromine and Chlorine Release From Large Explosive Volcanic Eruptions: a Threat to Stratospheric Ozone."