Ozone loss and recovery in the Arctic

Antarctic ozone hole 2012 — The ozone hole is the region over Antarctica with total ozone of 220 Dobson Units or lower. This map shows the Antarctic ozone hole as of September 22, 2012. / Courtesy: NASA Goddard Space Flight Center

The ozone hole is a problem which plagues the skies above Antarctica. Yet in 2011, Arctic skies experienced the most severe ozone depletion ever measured in the north. The reasons why are now explained in a paper published in the Journal of Geophysical Research-Atmospheres headed by lead author Susan E. Strahan, an atmospheric scientists at the NASA Goddard Space Flight Center.

Understanding ozone loss begins with a look at the ozone layer. Located high in the stratosphere (which begins five miles up in the air near the north pole) ozone (O₃) shields us from dangerous solar ultraviolet radiation. Functioning normally, it absorbs the UV rays and in the process splits into O and O₂; later those two forms of oxygen recombine into ozone (O₃).

Aura satellite NASA — Artist’s rendition of NASA’s Aura satellite. The instruments onboard comprise a powerful chemistry laboratory for studying the Earth’s atmosphere. / Courtesy: NASA

The cycle is interrupted, though, when man-made chlorofluorocarbons (CFCs) are present. When temperatures fall below -80°Celsius (-112°Fahrenheit) like they can over the poles, clouds form in the stratosphere, where conditions are right for CFCs to transform into active compounds containing chlorine. Chlorine in the stratosphere bonds with O, leaving O₂ partnerless and unable to return to its state as ozone (O₃). Other agents like bromine and methane can also play a role. The resulting ozone depletion results in ozone depletion – it can even form a hole, like the one which still forms over Antarctica, since CFCs linger in the atmosphere for a decade or more after they are released. The ozone hole has been decreasing, though, because of the 1987 Montreal Protocol in which over 190 nations agreed to reduce their CFC emissions. It’s an ozone success story.

The lingering man-made CFCs at the poles is a key ingredient in ozone loss, but so are frigid temperatures and sunlight, which explains why the Arctic usually experiences less ozone depletion than the Antarctic. First, the north pole tilts away from the sun during winter and sunlight doesn’t return in force until spring when temperatures are warmer. Second, the swirling cold winds of the polar vortex that keeps temperatures low, as well as locking out warmer southerly winds carrying replacement ozone, are usually weakened and pushed south by strong atmospheric currents by the time the sunlight returns to the Arctic.

Conditions were different in 2011, though. Strahan’s study used supercomputer simulations to simulate the conditions, and compared compared their findings to real ozone observations from NASA’s Aura satellite.

Arctic ozone hole 2011 — Maps of ozone concentrations over the Arctic come from the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite. The left image shows March 19, 2010, and the right shows the same date in 2011. March 2010 had relatively high ozone, while March 2011 has low levels. / Courtesy: NASA Goddard Space Flight Center

In late 2011, ozone concentrations in the Arctic atmosphere were about 20 percent lower than average… the opposite direction from the ozone recovery we expect to see. The atmospheric scientists found that chlorine pollution and unusually cold temperatures in the stratosphere were responsible for two-thirds of 2011’s radical Arctic ozone loss. The last third could be blamed on the unusually stagnant atmospheric winds which failed to break through the polar vortex and deliver replacement ozone north from the tropics until April.

Meanwhile, the European Union project RECONCILE completed in February 2013 have their own comprehensive findings, and also a word of caution about climate change. “Even if the ozone layer recovers, climate change could alter the underlying conditions,” stated environmental chemist Dr. Marc von Hobe. The project page explains: “Climate change could alter the temperature, circulation patterns and chemical composition in the stratosphere. This also influences the ozone layer, which in turn has a bearing on temperature.” They note that although CFC compounds may linger in the atmosphere longer than previously thought and climate change may further delay recovery, the ozone layer to can still be expected to recover by the end of the century. Good news for planet Earth.

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References:

‘NASA pinpoints causes of 2011 Arctic ozone hole’ NASA: National Aeronautics and Space Administration
http://climate.nasa.gov/news/885

‘New Findings on Ozone Depletion’ Forschungszentrum Jülich & project RECONCILE
http://www.fz-juelich.de/SharedDocs/Pressemitteilungen/UK/EN/2013/13-03-11reconcile.html