Kim Westerskov

The witches’ cauldron

Earth’s protective ozone shield is being destroyed by two processes: the slow eating away of ozone by chlorine around the globe, and dramatic holes that appear over Antarctica every spring.

Written by       Photographed by Kim Westerskov

During the bitterly cold Antarctic winter, when the sun never shines and the tem­perature stays well below zero, strange things happen in the upper atmosphere. An area of cold, still air 10 – 20 kilometres above the frozen continent becomes a witches’ caul­dron of supercooled gases and ice particles; a virulent brew waiting only for the first light of spring to work its dark magic.

As the sun peeps over the horizon in August—its first appearance since April—it triggers a series of photo­chemical reactions which have the net effect of destroying virtually all the ozone in this layer of the strato­sphere. The resulting “hole” has be­come a matter of international con­cern, with far-reaching implications for us all.

It has not been easy to find out what happens to minute traces of gases high above this lonely land during the coldest, darkest time of year, but over the last six years a determined push by scientists from many countries, including New Zea­land, has painted in the broad out­lines of the ozone story.

Ozone is a variant form of oxygen in which each molecule has three atoms of oxygen, as opposed to two in the oxygen we breathe. The total amount of ozone in the atmosphere is very small, but 90 per cent of it is concentrated in a distinct layer in the stratosphere, the portion of the at­mosphere that is between 15 and 50 kilometres above the earth’s surface. There it acts as a shield, protecting the earth from solar ultraviolet radia­tion which is harmful to plant and animal life. Without ozone, life as we know it would not exist.

In the upper stratosphere, ozone is produced from oxygen by the action of high-energy ultraviolet. In the lower stratosphere, it is broken down again—into molecular oxygen plus a free oxygen atom—by photochemi­cal reactions also involving ultravio­let. Both processes happen continu­ously. The important factor as far as life on earth is concerned is that these reactions “mop up” virtually all harmful solar ultraviolet, turning it into heat which never reaches the earth.

Scientists recognise three types of ultraviolet: A, B and C. Low-energy ultraviolet A (wavelength of 320 ­400 nanometres, or millionths of a millimetre) is harmless, and much of it reaches the earth’s surface. High-energy ultraviolet C (100 – 280nm), on the other hand, is deadly to bio­logical material. Fortunately, it is entirely cut out in the upper stratosphere, where it collides with water, oxygen and ozone molecules, either reacting with them chemically or simply being re-radiated as heat. Ul­traviolet B (280 – 320nm) is the ra­diation which is causing concern to­day. Being of lower energy than UVC, it is not cut out by water or oxygen molecules. Only ozone can effec­tively block these wavelengths, which have been shown to be harm­ful to life.

Besides blocking ultraviolet, the ozone layer has an important, but indirect, influence on climate. The heat generated by the ozone/oxygen reactions stays in the stratosphere, making this part of the atmosphere warmer than the layer below, the troposphere (in which the earth’s weather processes operate). The warm upper layer acts as a “lid” on the weather fluctuations happening in the troposphere beneath.

To measure the atmospheric con­centration of trace gases such as ozone, scientists use a spectropho­tometer, a device which records the intensity of various wavelengths of light. The standard machine for measuring ozone is the Dobson spectrophotometer, developed in the 1930s and still in common use. Be­cause ozone is known to absorb ultra­violet light, the Dobson focuses on this part of the spectrum, measuring the reduction in intensity of UV light reaching earth. From these readings the amount of ozone in the atmos­phere can be calculated.

The New Zealand Meteorological Service moved its Dobson spectrophotometer from Invercargill to Scott Base in January 1988, where it now forms part of an international network monitoring ozone depletion over Antarctica. The Dobson, along with two other spectrophotometers, is located at Arrival Heights, four kilometres northwest of the main base. This area has been set aside as a “Site of Special Scientific Interest”, an electrically quiet area ideal for the sensitive work of measuring the parts per million and parts per billion of trace gases.

Many of the important ozone measurements have to be made in winter, when the Antarctic continent seems devoid of life and the scien­tific bases are in “winter-over” mode. Measuring light intensities during the constant darkness of the Antarc­tic winter may seem a contradiction, but the Dobson can make adequate, though less accurate, ozone readings from the sunlight reflected off the moon—if the moon is high enough and at least half full.

Most of the drive to Arrival Heights is along well-formed roads, but the last part is over a glacier, then a bumpy track through a rock field. On a good day it takes 15 minutes. If the snow is being blown around, or there are snowdrifts over the road, it can take at least twice that long.

During his first few months there, Bruce McGregor, the Scott Base sci­ence technician for 1990, saw the fierce side of Antarctic weather: week after week of blizzards. “Some days you couldn’t see half way down the bonnet—visibility was basically nil. I’d drive really slowly—five ks or less. If the next flag didn’t appear, I’d stop, or back up.”

On the day I drove out to Arrival Heights to see the Dobson in action the weather was grey and a 30-40 knot wind lashed snow low over the rocks and ice. A trail of route-mark­ing flags led to a couple of small box­like buildings (one American, one New Zealand) surrounded by a tan­gle of aerials and wires.

After clearing the snowdrifts from around the door, we went in. The first room was comfortable, though not palatial: a bed, armchairs, magazines on a small table, a bench with a mi­crowave, a large water flask, and what looked to me like enough food for several days.

The second, larger, room housed the banks of equipment which take the pulse of the upper atmosphere. Measurements are taken daily in summer; on average three times a week in winter.

It’s one thing to measure ozone from the ground, yet there is much more that can be learned about the gas by getting up amongst it.

For most people, the long flight to Antarctica in military planes is noisy and boring, to be endured rather than enjoyed. But these flights are a great opportunity to check up on the health of upper air layers. Making the most of this opportunity is a group from DSIR Physical Sciences.

On my flight, while the tightly packed passengers were reading, eat­ing packed lunches or trying to sleep (the three main options on these 7-11 hour marathons) scientist John Mak was working. Through a small hole in the aircraft’s body he was collect­ing large air samples and compress­ing them into stainless steel tanks. These would later be analysed for a variety of trace gases and natural ra­dioactive species, particularly meth­ane and carbon monoxide. Methane is one of the important “greenhouse gases”, and the measurements will help clarify our understanding of natural atmospheric processes and how these are being affected by pol­lutants.

While the thin air from outside the plane was being siphoned in and stuffed into tanks, a blue box on top of John’s jumble of gear was giving continuous readings of the ozone lev­els outside.

The US Navy’s ski-equipped Hercules aircraft that was ferrying us  to Antarctica flies in, or just above, the tropopause. This is the interface between the troposphere and the stratosphere. To get even higher, into the lower stratosphere where most of the ozone is found, you either need a special high-altitude plane, or you send a balloon up. The first approach has been used on many occasions now, notably in 1987, when Ameri­can ER2 and modified DC8 aircraft flew south into the ozone hole from the southern tip of Chile.

The balloon approach is being put to good effect by a group of US scien­tists at McMurdo station each spring. Every two days or so, from late Au­gust until early October when the ozone hole has reached maturity, one of a variety of balloons is launched. The smaller ones are basically the same as those launched every day by weather stations the world over, but with an ozone-measuring instrument and transmitting device tacked on. The largest balloons are impressively long, and look something like a plas­tic jellyfish when launched. A suc­cessful launch needs several people, plenty of open space and calm condi­tions.

I drove the three kilometres from Scott Base over to McMurdo to pho­tograph several of these launchings and to talk with scientists. The group of four from the University of Wyo­ming was led by Terry Deshler, a name that turns up often in the tor­rent of scientific papers published on ozone since 1985.

It would take the Wyoming group an hour or so to get the instruments ready, the balloon inflated with he­lium, and the whole set-up launched. The next few hours would then be spent making sure the information being transmitted back from the equipment was being received and stored correctly. Somewhere about 35km above the ground, the bal­loon—now 200 times its original vol­ume—would burst, terminating an­other valuable set of recordings. Since the instruments are costly, Terry hoped to recover as many of them as possible once helicopters started flying in October. So he fig­ured out as closely as he could where each payload landed, 50-150 km away.

An Italian group at McMurdo is using yet another approach to meas­ure high altitude ozone. These scientists fire rapid pulses of green laser light up into the sky and then, through a telescope aligned with the laser beam, measure the light re­flected back. The intensity of reflec­tions from particles in the atmos­phere tells them how many particles there are, and the time it takes for the light to return indicates their height. This work enables them to build up a picture of the otherwise invisible “polar stratospheric clouds” (PSCs) that are the key to the ozone destruc­tion in polar regions.
The discovery of the Antarctic ozone hole gives us an intrigu­ing insight into how science works. It is a story combining pains­taking detective work and high ad­venture.

Scientists working for the British Antarctic Survey at Halley Bay, on the other side of the continent from Scott Base, had been measuring ozone levels since 1956. In the late 1970s they noticed a slow but steady decline in springtime ozone levels. For a long time they didn’t believe their own data, especially as no-one else was reporting the same deple­tion, particularly the ozone-measur­ing satellites. In the early 1980s they installed a second Dobson, newly recalibrated, and for a few more sea­sons ran the two instruments to­gether.

After recalibrating the instru­ments several times and checking and rechecking their measurements, they finally published their findings in the international science journal Nature. That was in 1985. The ozone hole has never been far from newspa­per headlines since.

Terry Deshler continues the story: “Once they had published, the scien­tists working with the TOMS (Total Ozone Mapping Spectrometer) satel­lite at NASA asked, ‘Why didn’t we pick this up?’ and went back through their data and procedures. They found that in the late 1970s they had indeed recorded lower values than expected.

“Assuming something was wrong with their instruments, they had modified the data analysis part of their program and chucked out the low values. Fortunately, they saved all their raw data. After reprocessing it they said, ‘Sure enough, there it was, a great big hole right over Antarctica’.” But why? Scientific know­ledge at the time said ozone just couldn’t disappear that fast and at such low temperatures.

The high adventure part of the story was literally high—up to 21km high. As part of a $US10 million ozone research push in 1987, three pilots flew 12 high-altitude solo mis­sions from the southern tip of Chile, south into the ozone hole and back. Their planes were Lockheed ER2 high altitude aircraft—civilian ver­sions of the U2 spy plane.

Flying into the stratosphere, the pilots were astonished to find them­selves in translucent clouds of tiny ice crystals. Temperatures dropped to -90°C, cold enough to cause wor­ries about the fuel freezing. At 18km, winds of up to 150 knots knocked the planes around, and during landings 40-knot wind gusts caused even more heart-stopping moments for the pi­lots of these heavy-winged planes.

The ER2 and DC8 flights were a resounding success, yielding a major breakthrough. As the planes flew into the ozone hole, ozone levels dropped just as chlorine monoxide levels shot up. Here was the “smoking gun” the chemists were looking for.

But chlorine from where? Chlo­rine is such a reactive element that the only known way it could reach the stratosphere was in very stable man-made molecules called chlorofluorocarbons (CFCs).

When CFCs were invented in the 1930s (by Thomas Midgley Jr, who also discovered the benefits of adding lead to petrol) they were hailed as an industrial breakthrough. Their great usefulness lay in their chemical in­ertness: at normal temperatures they do not react with other materials. In the decades that followed, CFCs were widely used as coolants in refrigera­tors and air conditioning units, as solvents for cleaning electronic cir­cuit boards, as propellants for aero­sol sprays and as the gases used to make plastic foam products.

Now the very molecules that seemed to be such an ideal solution for industrial applications have be­come a spectre that haunts atmos­pheric chemists and climatologists. The problem is, there is no known “sink” for CFCs; no natural process which binds them into the ocean or the soil—no way of taking them out of circulation. Says Terry Deshler: “They just bounce around and bounce around and eventually, be­cause they are not destroyed, make their way up into the ozone layer.” This diffusion process can take an astonishing 30 to 60 years.

Once in the stratosphere, CFCs en­counter something new: high-energy, short-wavelength ultraviolet that breaks them up, releasing free chlo­rine atoms. This free chlorine imme­diately attacks ozone, forming the normal two-atom oxygen we breathe, plus chlorine monoxide. Chlorine monoxide goes through a number of subsequent reactions which regener­ate free chlorine, which is then avail­able to destroy more ozone. It has been estimated that a single chlorine atom is capable of destroying up to 100,000 ozone molecules before be­ing finally neutralised.

Chlorine is eventually taken out of the system by reactions which lock it up as either chlorine nitrate or hydrochloric acid—relatively inac­tive compounds which are known as “reservoir species”. Were it not for these reactions, ozone loss through­out the world would by now have reached catastrophic proportions. As it is, the depletion has been gradual—with the dramatic exception of Ant­arctica and, to a lesser extent, the Arctic region.

The rapid ozone loss in Antarctica took scientists by surprise. If ozone destruction by chlorine took place only slowly in warmer parts of the world, how could it happen so quickly when it is very cold? The normal rule is that chemical reac­tions slow down as the temperature falls.

As it turned out, it was precisely because of the cold that ozone could be destroyed so fast. During winter, Antarctica becomes the coldest place in the world, not only on the surface, but also up in the stratosphere. A wall of wind (caused by inflowing winds being deflected eastwards by the earth’s rotation) roars round and round the edge of the continent, ef­fectively isolating a pool of air in the middle, which gets colder and colder. This still, cold air is the “polar vor­tex”.

Within the vortex, at temperatures of -80°C or colder, polar stratospheric clouds form, composed mainly of ice particles. They are not clouds in the normal sense of being easily visible—they are too diffuse for that. Look up into the sky when PSCs are known to be overhead and all you see is clear, blue sky. If you send a camera up with a balloon, however, and take photographs towards the sun but with the sun just below the horizon, you may see a layer of blue light against the inky blackness of space­PSCs.

These nearly invisible clouds are the main reason why ozone destruc­tion happens in Antarctica. They ab­sorb active nitrogen from the atmos­phere and provide a reaction surface upon which the two reservoir spe­cies, chlorine nitrate and hydrochlo­ric acid, can combine together to re­lease active chlorine, the ozone at­tacker. The other product of the reac­tion, nitric acid, binds with the ice particles, and for this reason PSCs are sometimes called “nitric acid clouds”.

So, for Antarctic ozone destruc­tion to occur you need a special brew of ingredients including the chlorine-containing reservoir molecules, PSCs, and a container for it all to happen in. The container is the cold, isolated pool of air—the polar vortex. Terry Deshler calls it the “witches’ cauldron”. Add the sunlight of spring and…presto! No ozone. Almost none, anyway. Between 12-20 kilometres above Antarctica, up to 98 per cent of the ozone is destroyed each spring. The destruction above and below this layer is less, but it still adds up to the elimination of about half the ozone over Antarctica. The “hole” itself covers some 25 million square kilo­metres (the size of the United States) and is as deep as Mt Everest is tall.

By November the stratosphere is starting to warm up. The PSCs evapo­rate, nitrogen concentrations in­crease, chlorine gets locked back up for another year, and ozone destruc­tion stops. As the cauldron breaks apart, ozone-rich air flows in from warmer latitudes and ozone levels return to normal.

Not much ozone is created over Antarc­tica, though—the sunlight there is too weak to break oxygen molecules apart effi­ciently. Most ozone is made above the equator, then moved towards the poles by normal upper atmos­phere air flows.

In the Arctic, the pattern of ozone de­pletion matches that at the Antarctic, though the total ozone loss is much less, around 20 per cent. This is because the air above the Arc­tic never gets as cold as in Antarctica, and there is more air movement around the North Pole.

The effect of ozone depletion on the planet and its inhabitants has become a major environmental con­cern in recent years. As the ozone screen deteriorates, it lets more ultra­violet radiation penetrate to the earth’s surface. A one per cent de­crease in atmospheric ozone level al­lows about two per cent more ultra­violet through.

Problems predicted from in­creased ultraviolet radiation include increases in skin cancers and eye cataracts, reduced yields from crops, problems with our own immune sys­tems and changes in plankton pro­duction.

New Zealanders, living so close to Antarctica, have viewed the situa­tion with some alarm, particularly in 1987, when it seemed that an ozone hole was developing over the coun­try. For about three weeks in early summer, scientists in Australia and New Zealand noticed ozone levels to be about 10 per cent lower than ex­pected. Using data from the TOMS satellite and “trajectory analysis” (a computer program which identifies where air masses have come from) they were able to trace the ozone-depleted air back to Antarctica.

The scientific paper announcing this, co-authored by Andrew Matthews, a physicist with DSIR’s Physical Science unit in Lauder, Otago, caused some of the biggest ripples in the ozone pond since the original 1985 paper.

It was the first hard evidence that the ozone hole was not just an Ant­arctic phenomenon—it also affected the middle latitudes. Until then a number of countries, particularly northern hemisphere ones, had been saying, “Who cares if penguins get skin cancer? We’ve got more urgent problems here at home. CFCs can wait.” Such attitudes were suddenly much harder to justify.

The paper also led to a few mis­conceptions. The Meteorological Service now sometimes fields inquir­ies from Europeans along the lines of: “Is it safe to visit New Zealand? We’ve heard about your ozone hole.” The fact is that since those few weeks in 1987 there have not been any de­pressed ozone levels over New Zea­land that could be positively traced to the Antarctic hole. On the other hand, no-one can say it has not hap­pened, but unless a very cohesive pocket of ozone-poor Antarctic air reaches this far north and coin­cides with natu­rally low local lev­els at the time, then it isn’t noticed amongst the large and often sudden natural fluctua­tions.

Andrew Matthews likens the 1987 situation to a big surf rolling, with some waves coming a long way up the beach, pushing ozone-poor air as far north as Aus­tralasia. In 1988 the Antarctic ozone hole was not as deep as 1987, and although 1989 and 1990 both had deep holes again, “the waves broke more gently on the shore and didn’t reach us”. In “low surf years”, the mixing of ozone-poor and ozone-rich air takes place closer to Antarctica and we don’t see any large changes over New Zealand.

However, there is definitely less ozone over New Zealand than there used to be. With high annual varia­tions (there can be 50 per cent more ozone in spring than there is in au­tumn) it is hard to detect changes of only half a percent per year, which is about the rate our ozone levels are dropping, but it is clear that in the southern hemisphere there is a gra­dation of ozone loss from the equator (where levels have dropped 1-2 per cent over the last 10 years) to New Zealand latitudes (about 5 per cent), to Antarctica (8-10 per cent over the last 10 years).

The annual Antarctic holes are al­most certainly the reason for the greater rate of ozone depletion in the mid- and high latitudes.

The outlook is worrying, and there may be worse to come. Recently, sci­entists have been asking whether it would be possible for an ozone hole to develop anywhere other than at the poles. The answer, disturbingly, is yes.

The requirements for rapid ozone depletion in the lower stratosphere are threefold: a suitable reaction sur­face, a decrease in active nitrogen and the release of chlorine from its  inactive chemical reservoirs. All three conditions are met in, of all places, a major volcanic eruption, thrusting sulphur gases into the stratosphere. Dr Tom Clarkson, head of the New Zealand Meteorological Service’s ozone research group, be­lieves such an event would have a catastrophic effect on the ozone shield in mid-latitudes.

“A Krakatoa-sized volcano would destroy the ozone layer for months or even years. The implications are much more severe than with the Antarctic hole.”

Cataclysms aside, the obvious so­lution to the ozone problem is to reduce the amount of chlorine getting into the stratosphere. Over the last three years, governments have been trying to agree on a global plan for eliminating CFCs. What they have come up with is the Montreal Proto­col.

In its original form, the Protocol proposed a 50 per cent phase-out of CFCs by the year 2000. That was in 1987. It quickly became apparent, however, that to save the ozone layer action was needed on a much faster timetable. In June 1990 a revised Pro­tocol was put forward, requiring all signatories to reduce consumption of the five principal CFCs to 50 per cent of their 1986 levels by 1995, 15 per cent by 1997 and zero by the year 2000.

So far, 70 nations have signed the Protocol, and CFC use is dropping (helped along by strong public opin­ion favouring the use of “ozone friendly” products). In New Zealand, CFC consumption has fallen from 2000 tonnes in 1986 to 600 tonnes in 1990.

Despite these encouraging signs, many environmentalists believe the action will be “too little, too late”. The amount of chlorine in the strato­sphere is increasing at a rate of 4 per cent per year, and, because of the long-lived nature of CFC molecules, will continue to do so until early in the 21st century. At the moment the concentration is approaching 4 parts per billion; it will peak well above this, and will not fall back to the level at which the first Antarctic hole ap­peared (2ppb) until at least 2050. As New Scientist magazine reported, “We are condemned to 60 years of the unknown.”

Pass the suntan lotion, Bruce.

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