The Atmospheric Ozone Layer Essay, Research Paper

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

measured 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 (76?S 27?W)

and at Argentine Islands (65?S 64?W). 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

earlier 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

poleward 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 (40-80?N), 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 60?S, 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

weakest, 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

long 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

chlorine 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

8% 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

removal 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-

enhanced 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

considerable 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

circulation 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

found 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 75?S. 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.

The Arctic

The discovery of the Antarctic ozone hole raised the possibility that a similar

hole could exist over the Arctic. Early results from a series of measurements in

the winter of 1988-89 suggests that ozone loss over the Arctic exists, but not

to the degree of that over the Antarctic

Trends in global total ozone

The publicity surrounding the discovery of and research activity in the

Antarctic ozone hole has unfortunately tended to obscure a potentially far

greater problem, decreases in total ozone concentrations across the globe. The

loss of ozone above the tropics and mid-latitudes, and the resultant increase in

harmful UV radiation could be disastrous to the earth’s population if the

changes were major. Since the late 1970s, there has been a slow but steady

decrease in global total ozone, even if the major losses over Antarctica is not

included. The trend is on the order of -2.7% per year in all seasons with the

greater losses occurring in the Northern Hemisphere autumn and winter (greater

than 3%) and the least in the Northern Hemisphere summer (1.6%).

Surface impacts and political decisions

The impacts of a depleted ozone layer on surface organisms depend on their

location to increased UV-B radiation. As a rough estimate, many experts suggest

that the percentage increase in UV-B radiation affecting surface organisms would

be about twice the percentage loss in stratospheric ozone from anthropogenic

causes. The most immediate effect on human beings would be an increase in

various skin cancers and skin cancers are increasing. Increases in the evidence

of cataracts and interference with the human immunity system are other possible

influences. A more serious potential long-term threat is the damage to cell DNA

and the genetic structure in not only human beings but in other animals, plants

and organisms.

With the discovery of the Antarctic ozone hole and the resultant world-wide

interest, publicity and concern, a historic meeting occurred in Montreal, Canada

in September 1987. For the first time ever, 57 countries and organisations met

to make a specific decision to limit the emissions of a series of pollutants

which were likely to create major environmental problems affecting the globe in

the future. The eventual document adopted on September 16, 1987 and entitled

“The Montreal Protocol”, was signed immediately by 24 countries and since has

been ratified by several more.

REFERENCES:

1. Jonathan Weiner, “Plant Earth”, New York, Bantam Books, 1986

2. “Atmospheric Ozone, Global Ozone Research and Monitoring Project” (Vol. 16,

Geneva 1985 International Organisation of Meteorology)

3. Lydia Dotto and Harold Sciff, “The Ozone War”, Garden City, N.Y., Doubleday,

1978

4. John Gribbin, “The Hole in the Sky”, N.Y., Bantam Books, 1988

5. James G. Titus, “Effect of Changes in Stratospheric Ozone and Global Climate”

Vol. 2, United Nations Environmental Programme

6. G. Levi, 1988, “Ozone depletion at the Poles”, Physics Today

7. P. Bowman, 1988, “Global trends in total Ozone”, Science

8. Hans U. Dutsch, “Vertical Ozone Distribution”, International Centre for

Atmospheric Research, Boulder, Colorado