The ozone layer is still sending out warning signals

Experts predict that, with the ban on the production and release of halogenated gases, the ozone hole problem will gradually wane over the coming decades. But lingering concerns remain

by HENRY HENGVELD

In early October, atmospheric scientists watching the annual development of the “ozone hole” over Antarctica reported that the size of this year’s hole set a new record -- about 28.4 million square kilometres. That’s almost three times the size of Canada!

The new record was a stark reminder that, despite the Montreal Protocol and its success in reducing the emission of ozone-destroying chemicals into the atmosphere, the effects of these gases already in the atmosphere from past emissions are slow to disappear and experts are concerned.

Ozone is an oxygen molecule that has three oxygen atoms within it, rather than the normal two. This molecular structure is very unstable and, in the lower atmosphere, ozone quickly interacts with other gases to break down again, eventually returning to normal oxygen molecules. Good thing too, since ozone is a key component of smog and hence harmful to our lungs and to plants. Therefore, near the surface, ozone is considered a “bad” gas that we want to keep very low in concentration.

However, in the upper atmosphere (primarily in the stratosphere, about 15-50 kilometres above us), ozone is a “good” gas that has a very important life-support function. At this level, it absorbs the most harmful parts of the sun’s ultraviolet radiation (UV-C and UV-B) entering the atmosphere, thus dramatically reducing the amount that reaches the Earth’s surface. That’s important, because excessive ultraviolet radiation can give us sun burns and skin cancer, harm plants and animal life, and damage plastics and paints.

Fortunately, the sun’s energy works overtime to produce lots of ozone in the stratosphere, so that -- despite its rapid decay -- concentrations at those levels remain elevated. As a result, around the world, the total average ozone present throughout that atmosphere (mostly in the stratosphere) is on the order of 300 Dobson Units (DU).
That’s not a lot, since 300 DU, if compressed at surface pressures, would be equivalent to a layer of ozone only three millimetres thick. However, that’s enough to fulfill the atmosphere’s ultraviolet filter function very well.

Scientists have known for several decades that some of the complex halogenated chemicals released through industrial and other processes at the Earth’s surface contain chorine atoms that remain bound within the molecules for decades, until the molecules finally reach the stratosphere and, exposed to intense sunlight, break up. Once released, free chorine atoms rapidly begin to gobble up ozone molecules, too fast for the ozone production system to keep up.

The result is a thinner ozone layer, and thus increased ultraviolet penetration to the Earth’s surface. This decrease has been relatively modest at mid-latitudes. Since the 1970s, for example, the average annual ozone concentration over Ontario has fallen by some three to five per cent, increasing to the order of 10 per cent in the spring season.
However, something more worrisome is happening over Antarctica. During the Southern Hemisphere winter season, temperatures in the stratosphere get so cold that any water in these layers is converted to ice crystals, forming what is generally referred to as polar stratospheric clouds.

In addition, during this cold season, the air circulates around the sunless South Pole in a strong vortex, or eddy, with very little air of the air within this vortex mixing with the atmosphere over the rest of the planet. This allows the chlorine that is gradually escaping from the halogenated gases to build up, awaiting the arrival of spring and the energizing sunlight that comes within. Once the sun begins to shine again in the region, the combination of chorine, ice crystals and solar energy provides an explosive combination that causes a rapid collapse of ozone concentrations within the vortex.

This year, by Sept. 24, the ozone concentration at the centre of the Antarctic vortex dropped to about 102 DU. That’s a loss of about two-thirds of the ozone in the region, and almost matches the previous low concentration record of 98 DU set in 1998. However, the total extent of the “hole” (which isn’t really a hole but is defined as the region where the amount of ozone throughout the depth of the atmosphere has thinned to below 220 DU) slightly exceeded the previous record set in 2003.

The good news is that most of the only land area below the ozone hole -- Antarctica -- is unoccupied by plants, animals or humans. A similar ozone hole over the Northern Hemisphere would be far more dangerous, since ecosystems and humans exist far closer to the pole.

So why don’t we hear more about Arctic ozone holes? The answer lies primarily in the vastly different geography and climate of the Arctic. The large ocean -- rather than a continent -- that is centred around the North Pole, although covered with ice, releases significant amounts of heat into the atmosphere above. Hence the winter air above the Pole remains much warmer that that over the South Pole.

As a result, the large atmosphere vortex that moves air around the pole is weaker and unstable. It often breaks down, releasing outbreaks of cold arctic air that moves southward and bringing warmer air from the south to replace the cold air. We regularly hear about these outbreaks of cold Arctic air in our weather forecasts, since they can sweep across Canada into Ontario and beyond. But these periodic breakdowns of the Arctic vortex also prevent the seasonal build-up of chlorine. Furthermore, a warmer stratosphere also reduces the formation of polar stratospheric clouds in the north.

Periodically, conditions can be just right to form a weak ozone hole of the Arctic as well. However, these never have the intensity and extent of those in Antarctica, for which we can be most grateful!

Experts predict that, with the ban on the production and release of halogenated gases, the ozone hole problem will gradually wane over the coming decades. There is, however, a lingering concern.

As increasing concentrations of greenhouse gases insulate the Earth’s surface against heat loss to space, they cause the Earth’s surface and lower atmosphere to warm. However, the reduced flow of heat upward causes the stratosphere to become colder. This increases the risk of polar stratospheric clouds forming over the Arctic as well.

Some experts argue that this could enhance the frequency and intensity of Arctic ozone holes during the next 50 years or so -- until the past build-up of halogenated gases in the atmosphere begins to significantly decline. That means we shouldn’t throw out that sun screen lotion just yet! BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada