Ozone Depletion (Causes and Effects)

The ozone layer protects the Earth from the ultraviolet rays sent down by the sun. If the ozone layer is depleted by human action, the effects on the planet could be catastrophic.

Ozone is present in the stratosphere. The stratosphere reaches 30 miles above the Earth, and at the very top it contains ozone. The sun’s rays are absorbed by the ozone in the stratosphere and thus do not reach the Earth.

Ozone is a bluish gas that is formed by three atoms of oxygen. The form of oxygen that humans breathe in consists of two oxygen atoms, O₂. When found on the surface of the planet, ozone is considered a dangerous pollutant and is one substance responsible for producing the greenhouse effect.

The highest regions of the stratosphere contain about 90% of all ozone.

In recent years, the ozone layer has been the subject of much discussion. And rightly so, because the ozone layer protects both plant and animal life on the planet.

The fact that the ozone layer was being depleted was discovered in the mid-1980s. The main cause of this is the release of CFCs, chlorofluorocarbons.

Antarctica was an early victim of ozone destruction. A massive hole in the ozone layer right above Antarctica now threatens not only that continent, but many others that could be the victims of Antarctica's melting icecaps. In the future, the ozone problem will have to be solved so that the protective layer can be conserved.

How bad is ozone depletion?

Very bad, and getting worse every year! The problem is that most of the gases and vapours that cause ozone-depletion take a long time (10 - 15 years) to homogenise throughout the atmosphere, but they can last up to 100 years for CFCs and 300 years for halons (to remove roughly half of them). Fortunately, one substance that was used very massively as a solvent, 1,1,1-trichloroethane, has a half-life of only 12 years, so that, as it was banned in most major using countries in 1996, the levels of this compound in the air are already measurably improving.

It should be noted that the "ozone hole" is not really a hole but simply an important thinning of the ozone levels over the Antarctic in each southern spring. The cause is quite complex but is simply driven by a weather situation, in conjunction with the man-made chemicals in the stratosphere. On average, it has become worse each year since it was first discovered. 2003 was the worst yet, with 2001 a very close second. In 2002, a peculiar event happened, in that there were two "holes", each of slightly less individual importance. The 2004 "hole" was similar to the 2003 one, very slightly less in depletion but covering a larger area. Even with the application of the Montreal Protocol, it will become significantly worse before it gets better, probably peaking around 2040 - 2050 and then starting to improve. It is expected to heal itself by about the end of this century, provided the Protocol is followed to the letter.

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.

Figure 4.

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

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.

CAUSES:

Only a few factors combine to create the problem of ozone layer depletion. The production and emission of CFCs, chlorofluorocarbons, is by far the leading cause.

Many countries have called for the end of CFC production because only a few produce the chemical. However, those industries that do use CFCs do not want to discontinue usage of this highly valuable industrial chemical.

CFCs are used in industry in a variety of ways and have been amazingly useful in many products. Discovered in the 1930s by American chemist Thomas Midgley, CFCs came to be used in refrigerators, home insulation, plastic foam, and throwaway food containers.

Only later did people realize the disaster CFCs caused in the stratosphere. There, the chlorine atom is removed from the CFC and attracts one of the three oxygen atoms in the ozone molecule. The process continues, and a single chlorine atom can destroy over 100,000 molecules of ozone.

In 1974, Sherwood Rowland and Mario Molina followed the path of CFCs. Their research proved that CFCs were entering the atmosphere, and they concluded that 99% of all CFC molecules would end up in the stratosphere.

Only in 1984, when the ozone layer hole was discovered over Antarctica, was the proof truly conclusive. At that point, it was hard to question the destructive capabilities of CFCs.

Even if CFCs were banned, problems would remain. There would still be no way to remove the CFCs that are now present in the environment. Clearly though, something must be done to limit this international problem in the future.

Figure 1: Regions of the atmosphere. (larger image)

Figure 2: Total ozone values shown for high southern latitudes as measured by a NASA satellite instrument. (larger image)

Figure 3: Launch of an ozonesonde attached to a high-altitude balloon from South Pole, Antarctica. (larger image)

Figure 2: Altitude distribution of ozone in the atmosphere. The blue curve indicates a typical altitude profile of ozone showing no depletion. The high abundance of ozone near 15 km marks the center of the normal ozone layer. The red curve was obtained over South Pole, Antarctica, in winter. The ozone layer has been severely depleted at this location as indicated by the near zero values between 14 and 20 km. Similar depletion occurs over the entire Antarctic region in late winter/early spring, causing the Antarctic ";ozone hole" in satellite observations of Antarctic ozone. (larger image)