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Ozone (O3) Hole and Healing…are we there yet?

Posted on June 7th, 2016 by in Chemical Manufacturing Excellence

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Most everybody has heard about the hole in the Ozone Layer: halogenated gases cause large seasonal ozone losses in Polar Regions. It may seem an old subject but it is still with us. A generation ago the Hole was front-page news.

Its relegation to scientific journals and websites, by no means, should be taken as problem resolved. Mankind must continue to eye the Ozone Layer vigilantly, if she is to continue to live and prosper on this planet. Here is an update on where things stand with respect to the Ozone Layer. It is actually a good story about the efforts of chemical industry to stem environmental disaster as well as global cooperation. Global warming is another story but that’s for another post(s)…..

Of course, we have to start with how things used to be.

The Ozone Layer is the Earth’s natural sunscreen that protects humans from melanoma (an aggressive form of cancer), plants and animals by filtering out harmful UV-B radiation. Its role in plant and animal life is vital. Scientists from British Antarctic Survey began monitoring ozone in 1957. In 1985 scientists discovered that ozone values over Halley and Faraday research stations in Antarctica, had been steadily dropping when the sun reappeared each spring. Something in the stratosphere (about 20km above Earth) was destroying ozone.

Paul Crutzen, Mario Molina and Sherwood Rowland raised concern about the effect of man-made chemicals, especially chlorofluorocarbons, on the Ozone Layer as early as the 1970’s. Their pioneering work was recognized in 1995 by the awarding of the Nobel Prize in Chemistry. The discovery of the annual depletion of ozone above the Antarctica was first announced in a paper by Joe Farman, Brian Gardiner and Jonathan Shanklin, in the journal Nature in May 1985[1]. Later, NASA scientists re-analyzed their satellite data and found that the whole of Antarctica was affected.

Ozone is formed throughout the atmosphere in multistep chemical processes that require sunlight. In the stratosphere, the process begins with an oxygen molecule (O2) being broken apart by ultraviolet radiation from the Sun. The combination of an atom (O) and a molecule (O2) of oxygen produce O3. In the lower atmosphere (troposphere), ozone is formed by different chemical reactions that involve naturally occurring gases and those from pollution sources. The tropospheric ozone does not significantly contribute to the concentration of stratospheric ozone[2]. Actually direct contact with ozone is harmful to plants, animals and humans. Ground level, “bad,” ozone forms when nitrogen oxide gases from vehicles and industrial emissions react with volatile organic chemicals (like volatile paint thinners).

Ozone degrades through several steps in the stratosphere starting with the emission of chlorine and bromine source gases emanating primarily from human activities. Those gases are called ozone-depleting substances (ODSs). The other steps in the action of ODS include their accumulation, transport, conversion, chemical reaction in stratosphere, and final removal. Ozone depletion only stops when reactive halogen gases are removed by rain and snow in the troposphere and land on the ground[3]. A factor called Ozone Depletion Potential (ODP) was accepted to measure the effect of the fluorocarbons on ozone depletion (proposed by Prof Donald J. Wuebbles in 1983). ODP of CFC 11 (CFCl3) was assumed to have a value of 1; ODPs of other substances were determined relative to CFC 11.

Chlorofluorocarbons (CFC) compounds were proven to be the culprits. They rise over time and reach the stratosphere. During the rise, ultraviolet light (UV) degrades CFCs splitting off chlorine or bromine (Halons) atoms. For example, CFC 11 (CFCl3) degrades by UV light exposure yielding Cl atoms. Cl reacts with atomic oxygen resulting in the highly reactive ClO that participates in a catalytic O3 degradation cycle. Until the chlorine atom reacts with another atom, or is removed in another way, a single atom of Cl can continue to consume ozone. The reaction paths for bromine is similar to chlorine except that bromine, as an ODS, is 50 times more potent than chlorine[4].

Countries of the world met in the 1980’s and reached an agreement in 1987, called Montreal Protocol on Substances that Deplete the Ozone Layer, to reduce/eliminate harmful fluorocarbons. It was agreed to phase out the CFCs and replace them with harmless chemicals as quickly as possible. Considering the applications of fluorocarbons gases and liquids, it would be virtually impossible to maintain the present living standards without them thus ruling out elimination. Those applications include refrigerants, fire retardant, explosion prevention, aerosols (asthma inhalers), solvents, etchant (semicon chips), foam blowing and a host of other uses. Montreal Protocol has survived the test of times. Indeed it has been updated several times since its inception.

Manufacturers of fluorocarbons went to work quickly to implement the Montreal Protocol. The task to replace CFCs was immense and thus divided into intermediate and final phases. Just imagine the magnitude of work to exchange the gas in every new and old refrigerator and in home and auto air conditioning systems in the world! In the intermediate phase CFCs were replaced by hydrofluorocarbons (HCFCs), intended to be replaced later by hydrofluorocarbons (HFCs) (Table 1). HCFCs are significantly less harmful to ozone because of having hydrogen in their chemical structures. The ODP of these fluorocarbons was in the following order: ODPCFC< ODPHCFC <ODPHFC.

Table 1 Examples of the three Main Commercial Classes of Fluorocarbons

Table

The worst CFCs (CFC-11, 12, 113, 114 and 115) were phased out by 1996. Phase-out of a few CFCs and Halons took until 2010. CFCs were replaced by the far less active HCFCs in an intermediate step. HCFCs are scheduled for phase-out by 2020 in the developed countries and by 2040 in the developing countries.

Are we there yet? Well, not quite but there is good news.

An excellent visual presentation of changes in the Antarctic ozone concentration between 1979 and 2013 can found in a short NASA video

Long-term observations reveal that the Ozone Layer has been strengthening following the international agreements to protect this vital layer of the atmosphere. According to the ozone sensor on Europe’s MetOp weather satellite, the hole over Antarctica in 2012 was the smallest in the prior decade.

The Ozone Layer is far from full recovery. The United States EPA states: continued declines in ODS emissions are expected to result in an almost complete recovery of the Ozone Layer by the middle of the 21st century. The long time scale for this recovery is due to the slow rate of ODS removal from the atmosphere by natural processes.

So, fasten your seat belts. We may have another 150 years to go before we are home again.

[1] Farman, J. C.; Gardiner, B. G.; Shanklin, J. D. (1985). “Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction”, Nature 315 (6016): 207

[2] D. W. Fahey, M. I. Hegglin, Twenty Questions and Answers About the Ozone Layer: 2010 Update, Scientific Assessment of Ozone Depletion, World Meteorological Organ Global Ozone Res and Monitoring Project – Rep No. 52, www.esrl.noaa.gov, 2010

[3] D. W. Fahey, M. I. Hegglin, Twenty Questions and Answers About the Ozone Layer: 2010 Update, Scientific Assessment of Ozone Depletion, World Meteorological Organ Global Ozone Res and Monitoring Project – Rep No. 52, www.esrl.noaa.gov, 2010

[4] Ozone Layer-3: Mechanisms of Ozone Production and Destruction, Department of Earth & Climate Sciences, SF State University, May 2016


 

All opinions shared in this post are the author’s own.

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