Thursday, July 14, 2016

The Ozone Hole is Shrinking

In a recent paper “Emergence of healing in the Antarctic ozone layer,” published in Science by Susan Solomon (MIT), Diane J. Ivy (MIT), Doug Kinnison (National Center for Atmospheric Research, Boulder, CO), Michael J. Mills (National Center for Atmospheric Research, Boulder, CO), Ryan R. Neely III (University of Leeds) and Anja Schmidt (University of Leeds). Using a 3D model of the atmosphere the team of scientists found that the September ozone hole has shrunk by more than 4 million square kilometers since 2000, when ozone depletion was at its peak.
the ozone hole in 2016 July from NASA

In 2015, the ozone hole reached a record size, despite the fact that atmospheric chlorine continued to drop. In response, scientists had questioned whether any healing could be determined. Using their model and reviewing the data, Dr. Solomon and the other scientists determined that the 2015 spike in ozone depletion could be attributed to the eruption of the Chilean volcano. While volcanoes don’t release significant amounts of chlorine into the stratosphere, they do increase small particles, which, in turn, increase the amount of polar stratospheric clouds with which chlorine reacts.

Solomon and the other scientists believed they would get a clearer picture of chlorine’s effects by looking at ozone levels in September, when cold winter temperatures still prevail at Antartica and the ozone hole is just opening up. The team showed that as the chlorine has decreased, the rate at which the hole opens has slowed down. The researchers tracked the yearly opening of the Antarctic ozone hole in the month of September, from 2000 to 2015. They analyzed ozone measurements taken from weather balloons and satellites, as well as satellite measurements of sulfur dioxide emitted by volcanoes, which can also enhance ozone depletion. And, they tracked meteorological changes, such as temperature and wind, which can impact the ozone hole.

Dr. Solomon felt this study was proof that the U.N. Montreal Protocol, an international agreement to protect the ozone layer by regulating chloroflurocarbons had been effective in impacting the planet. Earlier work, by Dr. Solomon and colleagues when she was at the NOAA Earth Systems Research Laboratory postulated the mechanism that created the Antarctic ozone hole. According to their work it was created by a reaction of ozone and chlorofluorocarbons free radicals on the surface of ice particles in the high altitude clouds that form over Antarctica.

In 1986 and 1987 Solomon led the National Ozone Expedition where the team gathered the evidence to confirm the accelerated reactions. The team, Susan Solomon, Rolando R. Garcia, F. Sherwood Rowland & Donald J. Wuebbles published, “On the depletion of Antarctic ozone.” In the 1980’s this was groundbreaking at the time and formed the basis of the Montreal Protocol that ultimately banned chloroflurocarbons.

Ozone is a gas made up of three oxygen atoms (O3). It occurs naturally in small (trace) amounts in the upper atmosphere (the stratosphere). Ozone protects life on Earth from the Sun’s ultraviolet (UV) radiation. In the lower atmosphere (the troposphere) near the Earth’s surface, ozone is created by chemical reactions between air pollutants from vehicle exhaust, gasoline vapors, and other emissions. At ground level, high concentrations of ozone are toxic to people and plants. However, 90% of the ozone in the atmosphere sits in the stratosphere, the layer of atmosphere between about 10 and 50 kilometers above ground. The natural level of ozone in the stratosphere is a result of a balance between sunlight that creates ozone and chemical reactions that destroy it.

Still, Dr. Solomon’s findings are puzzling to other scientists. The 3D model they developed founding that only half of the 4 million square kilometer shrinkage in the ozone hole was due to the reduction in chlorine and bromine in the atmosphere. The other half appeared to be due to weather. Weather effects should not create a trend, they should be random over time and cancel each other out. This could be an indication of weather shift due to climate change, but climate change should not be visible on so small a time scale.

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