Ozone Depletion and its Impacts

We wish to learn:
 

• What is the difference between tropospheric and stratospheric ozone?
• Where, to what extent, and why is ozone depletion occurring?
• What is the cause of the polar ozone hole?
• What are the impacts of ozone depletion?

Introduction

Some Definitions:

TROPOSPHERE (TOO MUCH Ozone!!):

The troposphere is the lowest layer of the atmosphere, extending from the ground to roughly 10 – 17 km altitude.  The vertical extent of the troposphere varies with latitude and season.  With the most intense heating (and subsequent convection) occurring at the Earth’s equator year-round, it is not unusual to find the troposphere extending to ~ 17 km.  Near the winter pole, the air is cold and dense and the troposphere may only extend to 9 or 10 km.    The temperature in the troposphere decreases with height.  That is, as you go higher in altitude, the temperature decreases.  If you consider an air parcel that rises in the troposphere, it encounter air parcels having temperatures colder that itself.  Since hot air (less dense air) rises, it will continue to rise.  The troposphere is a zone of rapid vertical mixing in addition to horizontal winds.

TROPOPAUSE:

The tropopause separates the troposphere and the stratosphere.

STRATOSPHERE (TOO LITTLE Ozone!!):

The stratosphere is the layer above the troposphere.  It extends from the tropopause to ~ 45 – 55 km altitude.  Unlike the troposphere, temperature increases with height in the stratosphere.  This means that a parcel of air that gets displaced upward will encounter airparcels that are warmer.  The colder, denser air parcel will sink back to its initial position and prevents vertical mixing.  The stratosphere is thus a region of high horizontal winds but no vertical mixing, so it is horizontally “stratified” or layered.

Fate of compounds in the atmospshere

Chemical compounds that are emitted naturally or anthropogenically at the Earth’s surface can remain in the atmosphere for long periods of time if they are not reactive (chemically or photolytically), water soluble, or sticky (prone to dry deposition).  Most greenhouse gases are long-lived, which means that they react slowly (e.g., CH4) or only partially water soluble (e.g., CO2), or are only vulnerable to breakup by solar radiation in the stratosphere (e.g., N2O and CFCs).  (Of course, a gas must also be a powerful absorber of infrared radiation to be a greenhouse gas, so even though N2 is long-lived, it is not a greenhouse gas because it is not an efficient absorber.)

However, many gases that are emitted from the Earth’s surface are short-lived in the atmosphere because they are highly reactive, highly water-soluble, or very sticky.  For example, there are numerous compounds that are quickly oxidized by reaction with OH or other oxidants or that are readily broken up by sunlight (i.e., photodissociated) (e.g, NO2).  And there are many other species that are very water-soluble (such as acidic compounds like nitric and sulfuric acid) or very sticky and are removed from the atmosphere by contact with surfaces or vegetation (like OH and nitric acid).

Tropospheric Ozone 

Important issues associated with tropospheric ozone and global change:
 

Stratospheric Ozone

Ozone – primarily in the stratosphere - also plays a very important role in protecting organisms at the Earth’s surface from UV-B radiation.   Changes in stratospheric ozone levels can thus affect human and ecosystem health as well as the chemistry of the troposphere.
 

From the above discussion, we can see that ozone protects us from UV light and it is a greenhouse gas in its own right. We next focus on the chemistry of ozone - how is it produced and how is it destroyed?

Stratospheric Ozone Abundance

Ozone occurs in a layer, centered at around 30 km altitude, reaching a peak abundance of ~10 parts per million. Even at the peak of the ozone layer, however, it is still very much a trace constituent - two orders of magnitude down from CO2 and 5 or 6 orders down from O2 and N2. If we were to take all the ozone in a column overhead and bring it down to sea level (room temperature and pressure) it would occupy a layer of only 3 mm in thickness!

It is interesting to notice how different the ozone distribution is from most of the other gases in the stratosphere. Ozone occurs in a layer, while the other gases have simple exponential drop-offs with altitude.

Why does stratospheric ozone exist is a layer? To answer this question, we need to understand the production mechanism for ozone.

Ozone Production

Ozone is a deep blue, explosive, and poisonous gas. It is made in the atmosphere by the action of sunlight on molecular oxygen. In the stratosphere, UV light is available that can split up ordinary molecular oxygen into two atomic oxygen atoms.

                                              O2 + UV photon --> O + O

Now, atomic oxygen is a very reactive species - so much so that it is very hard to make in the laboratory - it immediately combines with something else. In the stratosphere, atomic oxygen can quickly combine with molecular oxygen (in the presence of a third body) to yield the almost equally reactive other allotrope of oxygen: ozone or O3.

                                      O + O2 + third body --> O3 + third body

The combination of these two reactions, mediated by sunlight, converts molecular oxygen into ozone. Thus ozone is continually being created in the stratosphere by the combination of molecular oxygen and sunlight.
 

Ozone Layering
 

We can now explain why ozone is created in a layer in the stratosphere. Figure 1 illustrates the altitude dependence of the ozone production rate. The two ingredients for stratospheric ozone production are molecular oxygen and UV sunlight. On the topside of the layer, production is limited by the availability of molecular oxygen, which drops off exponentially with altitude. On the bottomside of the layer, production is limited by the availability of UV sunlight (which gets rapidly absorbed by ozone itself). The net effect of these two factors is to produce the characteristic layer for ozone.

Figure 1. Production of the Ozone Layer  in the Stratosphere.

Ozone Loss

Ozone is lost through the following pair of reactions:

                                               O3 + UV photon --> O2 + O

                                                             O + O3 --> 2O2

The first of these two reactions serves to regenerate atomic oxygen for the second reaction which converts the ozone back to molecular oxygen. This second reaction is very slow. It can be enormously accelerated, however, by catalytic reactions (see below). In the absence of such catalytic reactions, ozone can survive for 1-10 years in the stratosphere.
 

Catalytic Destruction of Ozone by Chorine from CFC's

Catalysis refers to the acceleration of a particular chemical reaction by a catalyst, a substance that is not destroyed in the reaction, enabling it to continue having the same accelerating effect time and time again.

Rapid catalytic destruction of ozone is best explained by reference to the famous example of CFC's (also known as freons) in the stratosphere.

Chlorofluorocarbons (CFC's) were developed to be colorless, odorless, non-staining, chemically inert, non-toxic, non-flammable, and to have certain other properties that make them excellent refridgerants, solvents, propellants for aerosol cans, and foam-blowing agents. These same properties make them essentially inert in the troposphere.

In the stratosphere, however, the CFC's can be broken apart into more reactive fragments under the action of UV light. When this splitting occurs, free chlorine is liberated which can catalytically destroy ozone. The process occurs in two steps:
 

Notice that the net effect of this pair of fast reactions is to turn two ozone molecules into three normal molecules of oxygen. The (catalyst) atomic chlorine is recovered in the second reaction, making it available to start over. In fact, each chlorine atom can destroy hundreds of thousands of ozone molecules!

These two steps turn a very unreactive chemical into a devastatingly effective destoyer of ozone. Whenever free chlorine atoms exist in the stratosphere, ozone is quickly depleted. Other species (such as bromine and fluorine) can also act as ozone-destroying catalysts.

Given this chemistry, it is useful to consider a typical life history of CFC's in the atmosphere:

1. Spray starch aerosol can is emptied in Ann Arbor
2. The CFC is rapidly dispersed until it is uniformly distributed throughout the troposphere. It takes about a year to mix across into the southern hemisphere as well, carried by weather patterns
3. After a few years, some of the CFC leaks into the stratosphere. At a sufficiently high altitude (~30 km), the available UV light can photolyze the CFC, liberating chlorine.
4. Each atom of chlorine participates in the catalytic destruction of thousands of molecules of ozone.
5. Eventually the chlorine atom reacts with methane to produce HCl, a molecule of hydrochoric acid.
6. Some of the HCl reacts with OH to liberate Cl again, but a small fraction of it mixes down into the troposphere where it can dissolve in rainwater and be lost to the atmosphere through precipitation.
7. The time scale for this process is ~100 years!

Theoretical models have been developed to predict future changes in ozone abundance. Figure 2  shows the results of one such projection into the future. The Montreal Protocol was signed in 1987 and has since been strengthened. It commits to phase out production of the CFC's (first invented in 1930) by the turn of the century. Without the Montreal Protocol, we would be looking at a disastrous reduction in ozone levels.

The Antarctic Ozone Hole
 

Figure 3.

The famous Antarctic Ozone Hole was discovered by British scientists who made systematic obseravtions of ozone using a simple ground instrument - the Dobson Meter. They published this famous figure that illustrated the downward trend of total ozone over Halley Bay, Antarctica in the month of October (austral Spring).. These measurements of Farman et al., provided a wake-up call to the atmospheric science community. They were quickly verified by satellite observations and several campaigns were organized to find out what was happening in this region and during this particular time of the year. The Farman et al., paper, published in 1985 showed a dramatic decrease in ozone. The decline from year to year has continued, more-or-less to this day.

The figure shown below (figure 4)  illustrate the satellite view of the same phenomena for the years leading up to the present. There are now several satellite missions that are dedicated to unraveling the chemistry and dynamics of ozone. These include the Total Ozone Mapping Satellite, TOMS and the Upper Atmosphere Research Satellite, UARS.

The hole deepens and becomes enlarged from year to year, as well as deeper although not monotonically.

The Antarctic Ozone Hole is now well understood. Briefly what happens can be summarized as follows:
 

Figure 5.

The Antarctic Ozone hole is limited in space and time, occurring at the time of year when the Sun first appears above the horizon after the long polar night.  During Polar Winter, a polar vortex forms and the polar air mass in the stratosphere becomes separated from other air masses. The temperature drops and drops, ultimately leading to the stratospheric air trapped in the vortex becoming very cold - in fact the coldest air to be found in any part of the Earth's stratosphere. In this cold vortex, polar stratospheric ice crystal clouds form. Gas phase HCl dissolves in the surfaces or clings to the surfaces of the clouds. The CFC's react with the HCl ice, converting relatively unreactive chlorine to the more active species, Cl2, ClONO2, and HOCl.  At sunrise, in October, the chlorine-bearing compounds are photolyzed, releasing the highly reactive Cl atoms that attack ozone.  Ozone densities drop rapidly, only to recover when the polar vortex breaks up, mixing warmer air in and releasing the ozone-depleted air to move away from the polar region.  The ozone loss is felt globally!

Northern Hemisphere Ozone

The Northern hemisphere is not immune from Ozone Holes. In the north, the stratospheric polar vortex is not as well formed as in the south. This is because of the larger contrast between land and water in the northern hemisphere. The existence of land masses tends to break up the symmetry of the polar vortex in the north. However, the same processes operate as in the south and satellite data show the effect occurring in March (Springtime in the northern hemisphere).

Sooner or later, we will see colder than usual northern polar stratospheric temperatures in the early Spring and heavily populated areas will be warned of unusually low ozone levels. Since ozone depleting compounds will be in the atmosphere for many tens of years, we have to live with these effects. Ultimately, chlorine compounds will cleanse themselves from the stratosphere and the Earth's ozone shield will return to normal - for our grandchildren's children.

For a movie showing the latest in Northern Hemisphere ozone hole formation, click here. For a movie showing the 1997 hole formation, click here

Potential Effects of Depleted Ozone

Of primary concern are the enhanced levels of UV radiation that reach the Earth's surface for a depletion in stratospheric ozone. It is customary to break up the UV spectrum into two parts:

UV-A: 400 - 320 nm
UV-B: 320 - 290 nm

Relatively little is known or understood about the consequences of enhanced UV-B levels. We do know, however, that a 1% decrease in ozone abundance causes a ~2% increase in UV-B.  Increased UV-B exposure at the Earth’s surface can impact human, agriculture and forest growth, marine ecosystems, biogeochemical cycles, and materials.  Table 1  summarizes some of the potential effects of UV-B increases.
 

Effects on Human Health
 

Our best understanding of potential effects is in the area of skin cancers, for which detailed epidemiological records and studies exist. It is known, for example, that more than 90% of non-melanoma skin cancers are related to UV-B exposure. A 2% increase in UV-B is linked with a 2-5% increase in basal-cell cancer cases and a 4-10% increase in squamous-cell cancer cases. In 1990, there were ~500,000 cases of basal-cell cancer in the U.S. and ~100,000 cases of squamous-cell cancer. A 1% depletion of ozone would cause an increase in skin cancer cases of ~20,000 per year.  To put this rather alarming figure in context, it is necessary to discuss briefly the geographical prevalence of skin cancer in the U.S.

Figure 6.

Figure 6  illustrates the rate of skin cancer as a function of latitude. While the data has some scatter, the trend is clear. A decrease of ~110 in latitude results in an increase of a factor of 2 in skin cancer occurrence. This occurs because the UV-B exposure increases towards the equator (~ a factor of 50 from pole to equator).   An increase of ozone of 1% gives an increase of ~20,000 cases of skin cancer per year.  This is equivalent to a southward shift in the average latitude of the U.S. population by only ~12 miles.
 

Actual ozone depletions at the latitude of the U.S. are ~1-3% already - primarily caused by chlorine catalytic chemistry.   Figure 7 illustrates the global ozone reductions estimated from satellite data as a function of latitude and season. The depletions occur at all latitudes and seasons, but are most dramatic in the southern polar region in austral springtime (October). This depletion is the famous Antarctic Ozone Hole, as discussed above.  We can see from this figure that we live in an ozone depleted world already. People living in the southern hemisphere are already well aware of this. Children growing up in Teirra del Fuego and New Zealand are very conscious of the need to wear hats in the midday sun. Daily news reports quote the ozone levels so that people can adjust their exposure to sunlight accordingly.

Figure 7.

Effects on Plants

UVB radiation affects plant physiological and developmental processes and can affect plant growth. Indirect changes, such as in the manner in which nutrients are distributed within the plan, the timing of developmental phases and secondary metabolism and plant form, may be as or more important than the directly damaging effects of UVB.

Effects on Marine Ecosystems

Phytoplankton are the foundation of aquatic food webs, and their productivity is limited to the upper layer of the water column in which there is sufficient sunlight to support net productivity. Exposure to solar UVB radiation affects phytoplankton orientation mechanisms and motility and lowers survival rates for these organisms.  UVB radiation has also been found to damage early developmental stages of fish, shrimp, crab, amphibians and other animals.

Effects on Biogeochemical Cycles

Increases in solar UV radiation might affect terrestrial and aquatic biogeochemical cycles, which could affect sources and sinks of greenhouse and a number of other trace gases e.g., carbon dioxide (CO2), carbon monoxide (CO), carbonyl sulfide (COS) and possibly ozone. Such changes would contribute to interactions between the atmosphere and biosphere that attenuate or reinforce the atmospheric buildup of these gases.

Effects on Materials

Although a number of materials are now somewhat protected from UVB by special additives, synthetic polymers, naturally occurring biopolymers, and other materials of commercial interest are adversely affected by solar UV radiation.  Increases in solar UVB levels will therefore accelerate their breakdown and limit their useful life outdoors.
 

Mitigation Strategies

Following the publication of the Farman et al. Findings in 1985, a series of ground-based and airborne measurements campaigns were conducted to develop an understanding of the chemistry and dynamics associated with the Antarctic Ozone Hole.  This understanding lead to the Montreal Protocol on Substances that Deplete the Ozone Layer in October 1987.  It required a freeze on the annual use of CFCs as early as 1990 with decreases leading to a 50% reduction by the year 2000.  In 1990, the Montreal Protocol was amended to take into account the severe losses during the ozone hole events and the downward trends in global ozone.  The participating countries substantially strengthened the protocol, calling for accelerated reductions in emissions, and requiring complete phase out of CFCs and other major ozone-depleting substances by 2000.   The Montreal Protocol was further amended in 1992, calling for the complete phase out of CFCs, etc, by 1996.

Of Note

In 1996, Molina, Rowland and Crutzen received the first Nobel Prize (for Chemistry) to ever be awarded in Atmospheric Sciences. These scientists were instrumental in predicting the problem and in developing the needed scientific case for governmental action.
 

• Environmental Effects  Of Ozone Depletion, 1994 Assessment, UNEP Stratospheric Ozone and Human Health Project
• Earth Under Siege:  From Air Pollution to Global Change, Richard Turco, Oxford University Press, 1997.
• Atmospheric Chemistry and Global Change, edited by G. P. Brasseur, J. J. Orlando, and G. S Tyndall, Oxford University Press, 1999.