The 16th of September is the International Day for the Preservation of the Ozone Layer. For the international community this is an opportunity to reflect on how the situation is evolving, and to see whether the measures that were introduced to stop ozone layer depletion are being effective or not.
Ozone O3: dangerous and useful at the same time
Molecular oxygen is normally present in the atmosphere as O2; it has, however, another less common and less stable form – ozone, O3. O3’s main characteristic is that, being less stable, it is also very reactive. Ozone can be a dangerous or useful molecule, depending on where it is located.
When ozone is present in the lower atmosphere, for instance in the air we breathe in cities, it can be very harmful for human health. O3 can cause problems to the respiratory systems, such as irritations and allergies. For this reason, ozone is considered a pollutant and its concentration in the air must be constantly monitored and kept under control.
Ozone, however, is an essential compound when it is present in the upper atmosphere.
The ozone layer is present mainly in the stratosphere – between 15 and 50 km above the Earth’s surface.
Up there, the ozone acts as a UV filter: in fact, due to its instability, it reacts with the UV light coming from the sun. In this way, ozone stops this radiation from reaching the earth. UV radiations are very dangerous, for humans and for all living species. The presence of ozone in the upper atmosphere, therefore, is vital, for life on our planet.
Ozone Layer depletion
Due to human activity, however, the ozone layer is depleted. Over Antarctica the effect was particularly enhanced, with the formation of a hole in the ozone layer above the landmass.
This depletion was due to the emission into the atmosphere of some gases, mainly the Chloro-Fluoro-Carbons (CFCs). These compounds are widely used as refrigerants, due to their stability and very low boiling points.
Once emitted into the atmosphere, the CFCs react with the ozone with the following mechanism: the UV light breaks the chemical bond between the chlorine and the carbon, forming a chlorine free radical – a very unstable species. This reacts with the ozone and makes the layer thinner and thinner.
The Montreal protocol
To avoid the worsening of the situation, and to prevent the expansion of the ozone hole, an agreement was achieved at international level. On the 16th of September 1987, the Montreal protocol was approved. According to this protocol, CFCs had to be progressively phased out from use and replaced by less harmful substances.
HCFCs: better but not ideal
Hydro-Chloro-Fluoro-Carbons (HCFCs) were substituted for CFCs . These molecules contain much less chlorine than the CFCs; hence, their Ozone Depletion Potential (ODP) is much smaller – between 2 and 10 %, depending on the species.
However, HCFCs still have an effect on the ozone, and this can cause long-term damage to the environment. Furthermore, some HCFCs are also greenhouse gases; a complete classification of the Global Warming Potential (GWP) for these molecules was compiled by the Environmental Protection Agency.
For all these reasons, HCFCs should also be phased out; this target should be achieved by 2020. They should be replaced by Hydro-Fluoro-Carbons (HFCs); as they are chlorine-free molecules, their ODP is 0, but HFCs have an even higher GWP than HCFCs.
The situation today
The International Day for the Preservation of the Ozone Layer marks one more year since the approval of the Montreal protocol; several events were organized for this day all over the world, to involve the general public and to sensitize public opinion to this issue.
24 years on, the results are mixed. On the 16th of September 2011, the extension of the ozone hole was about 25 million square kilometers. The area of the hole, therefore, has decreased since the record reached in the year 2006 of almost 30 million square kilometers. Although this is surely a positive result, more needs to be done to reduce further the size of the hole, as it is still very big, and potentially very dangerous.
More controversy about HCFCs and HFCs
In addition to this, other issues have to be taken into account, when evaluating the whole problem.
As mentioned above, for instance, there is strong criticism of the use of HFCs to substitute for the HCFCs, as many of them are very powerful greenhouse gases, with very high GWP. Therefore, there is the fear they may harm the environment in a different way.
Furthermore, the production of some HCFCs can cause problem. One of the most common HCFCs used today is the chloro-difluoro-methane HCFC-22. A by-product in its synthesis is the trifluoro-methane HFC-23, a gas with very high GWP. Recent data showed that the emissions of HFC-23 were very high in many European countries, much higher than the official data published.
All this shows that despite all the efforts put in to reducing ozone depletion up to now, the situation still needs to be considerably improved in the future.
United Nations Environmental Programme. The International Day for the Preservation of the Ozone Layer. Accessed September 2011.
The World Data Center for Remote Sensing of the Atmosphere. Monitoring the Antarctic Ozone Hole. Accessed September 2011.
C.A. Keller, D. Brukker, S. Henne, M.K. Vollmer, S. O’Doherty, S. Reimann: Evidence for under-reported western European emissions of the potent greenhouse gas HFC-23. Geophysical Research Letters, 38, L15808, 2011.
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