The Atmospheric Ozone Layer

The stratospheric ozone layer exists at altitudes between about 10 and 40km depending on latitude, just above the tropopause. Its existence is crucial for life on earth as we know it, because the ozone layer controls the absorption of a portion of the deadly ultraviolet (UV) rays from the sun. UV-A rays, including wavelengths between 320 and 400nm, are not affected by ozone. UV-C rays between 200 and 280nm, are absorbed by the other atmospheric constituents besides ozone. It is the UV-B rays, between 280 and 320nm, absorbed only by ozone, that are of the greatest concern.

Any loss or destruction of the stratospheric ozone layer could mean greater amount of UV-B radiation would reach the earth, creating among other problems, an increase in skin cancer (melanoma) in humans. As UV-B rays increase, the possibility of interferences with the normal life cycles of animals and plants would become more of a reality, with the eventual possibility of death. Stratospheric ozone has been used for several decades as a tracer for stratospheric circulation.

Initial measurements were made by ozonesondes attached to high altitude balloons, by chemical-sondes or optical devices, which easured ozone concentrations through the depletion of UV light. However, the need to measure ozone concentrations from the surface at regular intervals, led to the development of the Dobson spectrophotometer in the 1960s. The British Antarctic Survey has the responsibility to routinely monitor stratospheric ozone levels over the Antarctic stations at Halley Bay (76S 27W) and at Argentine Islands (65S 64W).

Analysis of ozone measurements in 1984 by a team led by John Farnam, made the startling discovery that spring values of total ozone during the 1980-1984 period had fallen dramatically compared to the arlier period between 1957-73. This decrease had only occurred for about six weeks in the Southern Hemisphere spring and had begun in the spring of 1979. This discovery placed the British scientists into the limelight of world publicity, for it revived a somewhat sagging public interest in the potential destruction of the stratospheric ozone layer by anthropogenic trace gases, particularly nitrogen species and chlorofluorocarbons.

Ozone concentrations peak around an altitude of 30km in the tropics and around 15-20km over the polar regions. The ozone formed over the tropics is distributed oleward through the stratospheric circulation, particularly in the upper stratosphere where the airflow is the strongest and most meridional. Since the level of peak ozone is considerably higher in altitude in the tropics, ozone descends as it moves toward the poles, where because of very low photochemical destruction, it accumulates, particularly in the winter hemisphere (see fig. 1).

Some ozone eventually enters the troposphere over the poles. Seasonal variations are much stronger in the polar regions reaching 50% of the annual mean in the Arctic. In spring, Northern Hemisphere transport of ozone toward the poles builds to a maximum (40-80N), associated with the maximum altitude difference in the major ozone regions of the tropics and the poles. The polar flux of ozone ceases as the westerly circulation dominant in winter is replaced by easterlies over the tropics. In the Southern Hemisphere the spring maximum occurs near 60S, one to two months after the maximum in the subtropics.

Throughout the summer, photochemical reactions reach a maximum in the lower tropical stratosphere and ozone concentrations fall. Autumn circulations are the eakest, with the latitudinal gradient between the poles and the equator virtually disappearing. Ozone concentrations throughout most of the stratosphere reach a minimum. As the circumpolar vortex expands for winter, the strength of circulation increases rapidly, ozone transport from the tropics also increases strongly, and meridional circulation and variability peak in the winter months.

Anthropogenic influences on the stratospheric ozone layer Figure 2, establishes the basic natural formation and destruction processes associated with stratospheric ozone. However, several other gases which have ong lifetimes in the troposphere, eventually arrive in the stratosphere through normal atmospheric circulation patterns and may interfere with or destroy the natural ozone cycle. The trace gases of most importance are hydrogen species (particularly OH and CH4), nitrogen species (NO, N2O and NO2) and chlorine species.

The gases not only react directly with ozone or odd oxygen atoms, but also may combine in several different ways in chain processes to interfere with the ozone cycle. Figure 2, presents examples of these reactions. The lifetime of these trace gases is crucial to the chemistry of the stratospheric ozone layer. Figure 3 illustrates the photochemical lifetime of the major trace gases affecting the ozone layer according to altitude. Many of these major gases have lifetimes of less than a month in the stratosphere compared to more than 100 years in the troposphere.

Hydrogen species The influence of OH, HO2 and of CH4 on the stratospheric ozone layer tends to be less important than the other major trace gases, except in the upper stratopshere. The major indirect influence of the hydrogen species in the mid to lower stratosphere is through their catalytic properties, enhancing nitrogen and hlorine species reactions. Nitrogen species There is not much information available about seasonal and annual Nox species in the stratosphere compared to ozone. NO and NO2 concentrations in winter are considerably lower than in summer in both hemispheres.

In the early 1970s there was major concern that Nox emissions from supersonic aircrafts would create a major depletion of the ozone layer. Considerable ozone reductions (16%) were expected in the Northern Hemisphere, where most of the supersonic transports would be flying, but stratospheric circulation patterns would ensure at least an % reduction in ozone over the Southern Hemisphere. Fortunately for the globe, the massive fleets of supersonic transports never eventuated. The Concorde was barred from landing at many airports for noise and other environmental reasons and now flies only limited routes, mainly from Great Britain and France.

Concern over Nox emissions has been overshadowed by the potential problems associated with the chlorofluorocarbons. Chlorofluorocarbon species In 1974, Molina and Rowland first suggested that anthropogenic emissions of chlorofluorocarbons (CFCs) could be depleting stratospheric ozone through the emoval of odd oxygen by the chlorine atom. CFCs released from aerosol spray cans, refrigerants, foam insulation and foam packaging containers, increased concentrations of Cl compounds in the troposphere considerably. CFCs are not soluble in water and thus are not washed out of the troposphere.

There are no biological reactions that will allow their removal. The result is very long tropospheric residence times and the inevitable transport into the stratosphere through normal atmospheric circulation. The chlorine atom, released from a CFC, reacts with ozone to form ClO and O2. Since ClO reacts with ozone six times faster than any of the nitrogen species (Rowland and Molina, 1975), it becomes the dominant mechanism to destroy stratospheric ozone. As a result, a lone Cl atom can be responsible for destroying several hundred thousand ozone molecules.

Based on recent results, reductions of ozone for 5-9% are possible with locational changes 4% in the tropics, 9% in the temperate zones and 14% in the polar regions. Recent discoveries such as that by Farnam (1985) lead most experts to believe that important destruction of the stratospheric ozone layer is not far off. The Polar “Holes” – The Antarctic With the help of the Dobson spectrophotometer, Farnam (1985) was able ….. to establish that the total ozone concentrations over the bases in Antarctica had been falling during the October-November period since 1979.

The trend of ozone loss during this time varied from year to year, but over the six year period showed an overall decrease. Verification from other bases in Antarctica came soon afterward (Table 4-Komlyr, 1988). Further verification came from the Nimbus satellite, from which the scientists were able to produce graphic colour- nhanced photographs of the depletion of ozone over Antarctica. The media began using the phrase “Antarctic Ozone Hole” to describe this phenomenon and unfortunately its importance has been expanded out of proportion to the global total ozone situation.

By definition, the “hole” represents a depletion of ozone concentrations over Antarctica, not an empty space in atmosphere. Atmospheric scientists were at first puzzled about the cause of the ozone hole. Three theories were suggested. The first was that there was a connection with the 11 year sunspot cycle. When a large number of sunspots occur, there is onsiderable NOx produced in the upper atmosphere which could interact with the ozone by reactions shown in table 2.

The second was that during the period when the sun was rising, there could be dynamic interactions between the troposphere and the stratosphere with an upwelling of ozone-poor air into the stratosphere from below. Such upwelling should also include many tropospheric trace gases not normally found in abundance in the stratosphere. Third, the ozone hole could be caused by chemical reactions, particularly reactive Cl, somehow released from reservoir molecules which were transported to Antarctica by the stratospheric irculation from source regions much further North.

Detailed investigations of these theories were made by the United States National Academy of Sciences (N. A. S. ) in 1988. The theory suggesting sunspot influences was discounted because there was minimal NO2 measured in the upper stratosphere over Antarctica, and in the main area of expected ozone loss, above 25km, ozone concentrations remained relatively high during the lifetime of the hole. The second theory, suggesting convective upwelling from the troposphere, was also eliminated as a possibility, since trace gas concentrations normally ound in the troposphere were not measured in the stratospheric ozone hole.

This left the third possibility, Cl chemistry, which the N. A. S. report suggested, occurred under a unique set of meteorological circumstances At the end of the Southern Hemisphere winter, as the sun is beginning to appear over Antarctica, the circumpolar vortex circulation in the lower stratosphere is at its strongest. Extremely stable and durable at this time of year (September and October), the vortex blocks any incursions of warmer air from the mid- latitudes and allows an extensive drop in temperature inside, over the continent.

Within the depths of the hole, important chemical reactions which deplete the ozone concentration are taking place. In order for the chemical reaction theory to work, there must be an overabundance of ClO in the Antarctic stratosphere between 12 and 25km and a diminished concentration of NOx series, which might interfere with Cl attacks on ozone. Concentrations of NOx species decrease toward the hole centre and ClO concentrations are 100 to 500 times higher than observed outside the hole. In 1987, the increases in ClO occurred across a very sharp boundary layer, fluctuating between about 67 and 75S.

Over a latitude span of about 1, ClO increased from less than 100 pptv to over 200 pptv, depending on altitude. Ozone averaged 256DU. This area of steep change marked the chemical boundary of the hole. Spatial distributions of ClO and ozone showed a marked negative correlation inside the hole. Whereas ozone decreased by about 60% crossing the boundary, ClO increased by greater than a factor of 10. This result provides strong circumstantial evidence that the link between ozone loss and chlorine over Antarctica is real. There is still much to be learned about what causes the Antarctic ozone hole.

Questions regarding changes in ClO at various latitudes, changes in concentrations in molecules from day to night, the progressive deepening of the ozone hole through the 1980s, and several other details remain unanswered. Colder stratospheric temperatures within the hole are likely to create thicker, longer lasting clouds which enhance processes for ozone removal, but details are not yet clear. Day-to-day variations in ozone within the hole have not yet been properly explained, and there is some question whether the ozone hole will continue its depth and persistence in future years.

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