Commentary. Ozone depletion : Its consequences and recovery measures

Transcription

1 Indian Journal of Radio & Space Physics Vol. 31, April 2002, pp Commentary Ozone depletion : Its consequences and recovery measures No nuclear bomb is required for the destruction of the planet earth and its habitats. The way anthropogenic activities are going on, the changes which are occurring due to human action in the interactive physical, chemical and biological processes that regulate the total earth system and the manners in which the unique environment of the earth that provides life are getting influenced by human aspiration are sufficient enough for the extinction of many species including man. The ecological hazards faced today by the planet earth have assumed alarming proportions. The Antarctic ozone phenomenon heightens the concern that chlorine and bromine containing chemicals such as chloro-fluoro carbons (CFCs) and nitrogen oxides can lead to a significant depletion of stratospheric ozone, and hence an increase in UV-B radiation. The consequences are very severe and pose a serious threat to the life on our planet. On the other hand, the alarming increases in green house gases and the related results in terms of global warming, sea-level rise, ocean current, vegetation and hydrological cycle, climate change, etc. are of great threat. The climate change that will occur over periods much shorter as compared to the adaptability of the existing ecosystems may lead to the disappearance of many living and non-living species. Most people, now-a-days, have a vague notion about human-induced changes in the global environment. The enormity(intensity) and gravity of these changes demands swift action from all corners and countries. The threat due to ozone depletion, the chemicals responsible for destruction of ozone layer and the initiatives that are being taken by different world bodies are highlighted here. Ozone layer and its importance The atmospheric envelope of the earth is divided into several regions, the lowest one being the troposphere. Virtually all human activities occur here. About 80% of the atmospheric mass is contained in this troposphere, which exhibits most of the day-today weather fluctuations. It extends up to about 10 km, a height more than the height of the Mt Everest. The next region is known as stratosphere extending from I 0 km to about 40 km. This is the region where most atmospheric ozone gets concentrated in a layer at a height of about km enveloping the earth. Ozone molecule is formed with three oxygen atoms. It is observed that out of 10 million air molecules, about 2 million are normal oxygen and the number of ozone molecules is hardly 3. Though the number looks very insignificant, it plays a vital role in the atmosphere. The ozone layer absorbs or traps certain wavelength (UV-B) of lethal lll ltraviolet radiation from the sun and acts as a protective shield for animals, plants and human beings. The ultraviolet radiation range is divided into three bands of wavelength I frequencies, namely, UV-A, having wavelength of nm, UY-B of wavelength of nm and UV-C of wavelength of nm. The UV-A is mainly responsible for cataract, while UV-B causes non-melanoma skin cancer and plays a major rok in the development of malignant melanoma. The physiological and developmental processes of plants are affected by this UV -B radiation. The indirect changes caused by this radiation like the changes in plant's form, changes in nutrients' distribution within the plants themselves, etc. are equally damaging. Because these changes can have important implications for plants competitive balance,, plant diseases and biogeochemical cycles. The UV-B has also a damaging effect on mari ne echo-systems. The foundation of aquatic food webs is formed with phytoplanktons. Their productivity is limited to the upper layer of water column where sufficient sunrays are available. Many phytoplanktons are capable of active movements that can enhance their prod ctivity and, therefore,, their survival. Exposure to the UV radiation bas been shown to affect the mobility of these phytoplanktons, resulting

2 COMMENTARY Ill in the reduction of their survival rates. This radiation has also been found to cause damage to the early developmental stages of fish, shrimps, crabs, amphibians, etc. The most serious effects are their decreased reproductive capacity. The UV-C radiation kills the very building block of their life (DNA, RNA). The net result is a serious disturbance in the aquatic food web and thereby the marine echosystem. The tremendous impact of all these phenomena may finally affect the economic and social scenario in our country. Traditional livelihoods like those of fi shermen and modern coastal industries like prawn cultivations, sea-food processing industries can lose a bulk of their revenue. The adverse effects of this radiation are also found in case of nonliving species like synthetic polymers, naturally occurring biopolymers and other materials of commercial interest. Therefore, any increase in the solar UV-B will accelerate their breakdown processes. This lethal radiation is prevented from reaching the earth only by the ozone layer present in the stratosphere which ultimately protects the living species and nonliving materials. The ozone molecules in the stratosphere are formed and destroyed constantly by natural phenomenon and layer's concentration remains relatively stable. Sometimes, ozone concentration gets affected with sunspot activity. Ozone level that is destroyed naturally gets recovered also naturally. Now the problem is that the man-made sources such as nitrogen oxides (NO) and CFCs are destroying the stratospheric ozone much faster than nature can replenish it. As a result the ozone layer is getting thiner. Reduction of ozone levels will lead to higher level of UV-B reaching the earth's surface. Less ozone means less protection and more exposure to UV-B. Thus the living and non-living species on the earth are becoming more vulnerable to the damaging effects of this lethal radiation today. Human contribution to the destruction of atmospheric ozone Be it ozone depletion or global warming, there are some aspects of human interference that are responsible for this overall change. Ever since the industrial revolution, the economic activities have increased many folds. The increase in production and hence consumption produces ever larger amounts of discharge which must be absorbed by the earth's echo-system, forcing environmental changes at a global scale. The most important and dominant factor is probably the human aspiration. The universal human drive to improve material standards of well being is a fundamental factor in global change, like ozone depletion, global warming and its associated devastating effects. While consumption and production have increased globally, this incn!ase has been found to occur mostly in developed countries. Those living in less developed or developing countries have legitimate aspirations for increased levels of affluence which means further increases in production and consumption, thereby environmental stress. Even those who are living in already developed countries are in no way satisfied. They demand more economic growth. Thus, it is a human race for this affluence. As for example, today we cannot do away with a fridge. There is a need of air-conditioning, fire extinguisher, cars, foam blowing, etc. in houses, offices and industries. For all these gadgets, the most popular synthetic chemicals that are being widely used are CFCs. It is marketed under the popular brand name Freon or Genetron. In 1930,Dr Thomas Midgely of American Chemical Society discovered this chemical compound. For the last 60 years, CFC has been considered to be a miracle substance, as it is colourless, odourless, non-toxic, non-corrosive, non-flamable and inexpensive to produce. This substance is used as solvent also. Other chlorine-containing compounds include methyl chloroform, a solvent, and carbon tetrachloride, an industrial chemical. Halons, extremely effective fire extinguishing agents and methyl bromide, an effective soil fumigant, contain bromine. All of these have atmospheric lifetimes long enough to allow them to be transported by winds into the stratosphere. Because they release chlorine or bromine when they break down, they damage the protective ozone layer. In early 1970s researchers began to investigate the effects of various chemicals on the ozone layer, particularly CFCs, which contain chlorine. They also examined the potential impacts of other chlorine sources. Chlorine from swimming pools, industrial plants, sea-salt, and volcanoes does not reach the stratosphere. Chlorine compounds from these sources readily combine with water and repeated measurement show that they rain out of the

3 11 2 INDIAN J RADIO & SPACE PHYS, APRIL 2002 troposphere very quickly. In contrast, CFCs are very stable and do not dissolve in rain. Thus there are no natural processes that remove the CFCs from the lower atmosphere. Over the time, winds drive the CFCs into the stratosphere. The CFCs are so stable that only exposure to strong UV radiation breaks them down. When thi s happens, the CFC molecule releases atomic chlorine. One chlorine atom can destroy more than 100,000 ozone molecules. The net effect is to destroy ozone faster than it is naturally created. Large fires and certain types of marine life produce one stable form of chlorine that does not reach stratosphere. However, numerous experiments have shown that CFCs and other widely used chemicals produce roughly 85% of the chlorine in the stratosphere, while natural sources contribute only 15 % of it. Sometimes volcanic eruption like that of Mt Pinatubo in 1991 can have indirect effect on ozone levels. But this type of effects is short-lived. The most significant example of ozone depletion is the Antarctic ozone hole that has been occurring every year in Antarctic Spring.The thinning of ozone layer over Antarctica was first discovered in early 1980s.While the ozone did not completely disappear in this area, it was so thin that scientists started calling it as ozone hole. Rather than being a hole through the layer, the ozone hole is a large area of the stratosphere with extremely low amounts of ozone. The ozone hole is defined as the area having less than 220 Dobson unit (DU) of ozone in the overhead column. Ozone is conventionally measured in DU (1 DU = x molecules/cm 2 ). Ozone levels are found to fall by over 60% of that normally found over Antarctica. The Antarctic ozone hole now measures about 9 million square miles, which is nearly the size of North America. The scientific study of the atmosphere over Antarctica can provide valuable inputs for predicting the long-term climate changes over our country. Over the years many scientific expeditions to this continent have provided wealth of ozone data using different types of instruments. It is not only over Antarctica, researchers have shown that ozone depletion occurs over North America, Europe, Asia, and much of Africa, Australia and South America. Over US, ozone levels have been found to fall by about 5-10%, depending on the season. Thus, ozone depletion has become a global issue and not just a problem at the South Pole. The researchers who discovered the thinning of ozone in earth's atmosphere and the devastating consequences of its depletion, were Dr. F. Sherwood Rowland and Mario Molina of the University of California. In 1974, they established a relationship between the CFCs and the thinning of ozone layer, a pioneering work in the field that brought them the 1995 Nobel Prize in Chemistry along with Paul J Crutzen. Montreal pmtocol The global threat of this depleted ozone layer had worried all nations of the world. In September 1987, forty such nations got together at Montreal, Canada, to decide the future course of action to combat the threat. They signed an agreement deciding on how to reduce and limit the use of ozone destroying chemicals in the coming years. This agreement is known as the Montreal Protocol. Basically, the Protocol concerns about the total elimination of CFCs, the major man-made chemical that destroys the protective ozone layer of the stratosphere, within 15 years from its inception. It was discussed that, since the lifetime of CFCs is years, even if it is stopped now, the CFC loading in the atmosphere will still continue to increase. Therefore, the intention was to cut off the production of CFCs as quickly as possible and thus stop loading. As per the Protocol, after amendment, the situation was that the developed countries would cut off the production and use of most of the CFCs by 2000 and the developing countries would do it. by 20 l 0 AD. Since 1987, over 160 nations have signed this landmark environmental treaty. By agreeing to the terms of the Montreal Protocol, signatory nations, including the United States, committed to take actions to protect the ozone layer, hoping to reverse the damage, in the long run, that had been done by the use of ozone depleting substances. The US Environmental Protection Agency (EPA) is more concerned about the management of air quality and atmospheric protection issues. As part of the USA's commitment to implementing the Montreal Protocol, the U.S. Congress amended America's Clean Air Act by adding provision for protection of the ozone layer.

4 COMMENTARY 11 3 Under the Clean Air Act, EPA has created several regulatory programmes to address numerous issues, including: (i) Ending the production of ozone-depleting substances (ODS). (ii) Ensuring that refrigerants and halon fireextinguishing agents are recycled properly. (iii) Identifying safe and effective alternatives to ozone-depleting substances. (iv) Banning the release of ozone-depleting refrigerants during the service, maintenance, and disposal of air conditioners and other refrigeration equipment. Because of their relatively high ozone depleting potential, several man-made compounds including chlorofluorocarbons (CFCs), carbon tetrachloride (CCI 4 ), methyl chloroform (CH 3 CCl 3 ), and halons were targeted first for phase-out. The EPA is developing additional regulations under its ozone protection programme for the continued protection of the environment and public health. To pursue the ongoing amendments to the Montreal Protocol and other treaties, the EPA works with other US Govt. agencies as well as with International governments. The refinements to the Protocol and other treati~s are based on ongoing scientific assessments of ozone depletion, which are coordinated by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO), with cooperation from EPA and other agencies around the globe. As far as India is concerned, it uses about 7000 tonnes of CFCs and halons taken together. The present capacity of Indian production of CFCs is around 20,000 tonnes per year. The per capita annual consumption of CFCs is only 10 gm in India. The per capita ceiling set down, under Montreal Protocol, is high enough for India to have even more than 20,000 tonnes of CFCs annually. How to combat the situation In order to tackle the problem, persistent efforts are to be made to find a better alternative to CFCs, the widely used chemicals. The various CFCs such as CFC-11 (Trichlorofluoromethane), CFC-12 (Dichlorodifluoromethane), CFC-13 (Chlorotrifluoromethane) and CFC-14 (Dichlorotetrafluoroethane) have ozone depleting potential of 1.0 with their lifetimes 45, 100, 640 and 300 years, respectively. Whereas, the hajons, the bromine contammg compounds, are having more ozone depleting potential (ODP). For example, Halon 1301 (Bromotrifluoromethane) has an ODP of 10 and a lifetime of 65 years. In contrast, HCFCs have been found to be one of the better alternatives of CFCs. Due to the presence of hydrogen atoms their adverse impact on the ozone layer is greatly diminished. It has less than Ill O'h of ODP of CFCs. For example, HCFC-21( Dichlorofluoromethane) is having an ODP of 0.04 and a lifetime of 2 years; HCFC-22 (Monochlorodifluoromethane) has an ODP of and lifetime 11.8 years; HCFC-131 (Trichlorofluoroethane) has ODP of ; and HCFC-133a (Monochlorotrifluoroethane) has ODP in the range of Another alternative to CFCs is found to be hydrofluorocarbon (HFC). Though the developed countries had to stop the production of CFCs with immediate effect and a complete stoppage by 2000 after the Montreal Protocol (1987) and go for alternatives, the developing countries were allowed to continue with the production of CFCs till Ultimately all the countries will have to discard the CFCs. India is under the process of developing the alternatives of CFCs. The National Chemical Laboratory, Pune, and the Indian Institute of Chemical Technology, Hyderabad, the two constituent institutes of the Council of Scientific and Industrial Research, are on the job and are producing HCFC-22 which is being used in some types of air conditioning. Another alternative,hcfc-134, is under study. How do the CFCs deplete ozone? There is no doubt that CFCs are a potent destroyer of ozone (0 3 ), but they themselves are inert. As they go up to the levels high enough for strong ultraviolet (UV) radiation from the sun, they get dissociated by the UV -radiation and highly reactive chlorine (Cl) or chlorine oxide (ClO) radicals are produced. It is this reactive species that cause the breakdown of ozone molecule and lead to its depletion in the stratosphere. Recent findings (i) The springtime Antarctic ozone hole continues unabated. The extent of ozone depletion has remained essentially unchanged since the early 1990s. There

5 114 INDIAN J RADIO & SPACE PHYS, APRJL 2002 occurs a near-complete destruction of ozone within the Antarctic lower stratosphere during springtime. (i i) The observed abundances of the substitutes for the CFCs are increasing. The abundances of HCFCs and HFCs are increasing as a result of a continuation of earlier uses and of their use as substitutes for the CFCs. In 1996, the HCFCs contributed about 5% to the tropospheric chlorine from the long-lived gases. This addition from the substitutes neutralizes some of the decline in tropospheric chlorine, but it is still about I 0 times less than that from the total tropospheric chlorine growth rate throughout the 1980s. (iii) The total combined abundance of ozonedepleting compounds in the lower atmosphere peaked in about 1994 and is now slowly declining. Total chlorine is declining, but total bromine is still increasing. The peak total tropospheric chlorine abundance was 3. 7 ± 0.1 ppb between mid-1992 and mid The decreasing abundance of total chlorine is due mainly to reduced emissions of methyl chloroform. Chlorine from the major CFCs is still increasing. The abundances of most of the halons continue to increase, but the rate has slowed down in recent years. (iv) The link between the long-term build-up of chlorine and the decline of ozone in the upper stratosphere has been firmly established. Model predictions based on the observed build-up of chlorine in the upper stratosphere indicate a depletion of ozone that is in good quantitative agreement with the altitude and latitude dependence of the measured ozone decline during the past several decades, which peaks at about 7 % per decade near 40 km at midlatitudes in both the hemispheres. (v) The observed total column ozone losses from 1979 to the period are about 5.4, 2.8, and 5.0 %, respectively, for northern mid-latitudes in winter/spring, northern mid-latitudes in summer/fall, and southern mid-latitudes year round. (vi) The late-winter/spring ozone values in the Arctic are unusually low. The possibility of such depletions was predicted in the 1989 Assessment. Elevated stratospheric halogen abundances over the next decade or so imply that the Arctic will continue to be vulnerable to large ozone losses. (vii) Stratospheric ozone losses have caused a cooling of the lower stratosphere (about 0.6 C per decade over ). The lower stratosphere that is cooler results in Jess infrared radiation reaching the surface/troposphere system, causing global-average negetive radiative forcing of the climate system. (viii) The amplitude of the annual cycle of ozone at middle to high latitudes has decreased by approximately 15% in the last decades, because larger declines have occurred during the season of maximum ozone values. For northern mid-latitudes, the downward trend is largest near 40 and 15 km (approximately 7% per decade) and is smallest at 30 km (2% per decade). The bulk of ozone column decline is found to lie between the tropopause and 25 km. (ix) Mos of the mid-latitude ozone column decline during the last two decades arose because of its depletion in the lower stratosphere. This region is influenced by local chemical ozone loss that is enhanced by volcanic aerosol, and by transport from other regions. It was well established that the halogens are the primary cause of the vertical, latitudinal, and seasonal characteristics of the depletion of mid-latitude ozone. The expected low ozone amounts in the mid-latitude lower stratosphere following the Mt. Pinatubo volcanic eruption further strengthened the connection between ozone destruction and anthropogenic chlorine. A question may arise as to why we cannot, in India, adopt the results and findings of developed countries directly into our policy programmes regarding CFCs. The anthropogenic and other activities that decide the degree of ozone destruction may not be the same for all countries. Hence a thorough investigation, in terms of ozone study, is an important task lying ahead for our scientists. Can ozone be brought back to its original levei? The future of recovery If one asks as to whether it is possible to create more ozone to fill up the defici t, the answer is both 'yes' and 'no'. It is 'yes', if we act with full obedience to the Montreal Protocol and its subsequent Amendments and Adjustments, and ' no', if we do not pay attention to it. In the absence of other changes, stratospheric ozone abundances should rise in the future as the halogen loading falls in response to regulation. However, the future behaviour of ozone will also be affected by the changing atmospheric abundances of methane (CH 4 ), nitrous oxide (N 2 0),

6 COMMENTARY 115 water vapor (H 2 0), sulphate aerosol, and changing climate. Thus, for a given halogen loading in the future, the atmospheric ozone abundance may not be the same as found in the past for that same halogen loading. Future global ozone abundances are predicted to recover only slowly toward their 1980 values. The return toward 1980 ozone values depends sensitively on the emission scenarios used. Understanding the methane trend is an important priority for understanding the future ozone recovery. The detection of the onset of ozone recovery from halogen-induced depletion should be possible earlier in the Antarctic than in the Arctic or globally, because there is less variability in the ozone loss in the Antarctic. Estimates of the timing of the detection of the onset of ozone recovery are uncertain. However, it is clear that unambiguous detection of the beginning of recovery will be delayed beyond the maximum loading of stratospheric halogens. The results from more than two decades of research have provided a progressively better understanding of the interaction of human activities and the chemistry and physics of the global atmosphere. Global observations have shown that the combined abundance of anthropogenic chlorine-containing and bromine-containing ozone-depleting substances in the lower atmosphere peaked in 1994 and has now started declining. If this trend continues, it is expected that the ozone concentration would come to its pre-1980s level by The stratospheric abundance of halogenated ozone-depleting substances is expected to return to its pre-1980 level of 2 ppb chlorine equivalent over the next 50 years. The atmospheric abundances of global and Antarctic ozone will start to slowly recover within coming decades toward their pre-1980 levels once the stratospheric abundances of ozone-depleting (halogen) gases start decreasing. But, if the Protocol and its Amendments and Adjustments are not respected with full compliance, the abundance of ozone-depleting gases in 2050 would be at least 17 ppb of equivalent effective chlorine, which is about 5 times larger than today's value. Moreover, the ozone depletion would be at least 50 % at mid-latitudes in the northern Hemisphere and 70 % at mid-latitudes in the southern Hemisphere, about 10 times larger than that of today. The surface UV -B radiation would be at least double at mid-latitudes in the northern Hemisphere and quadruple at mid-latitudes in the southern Hemisphere compared with an unperturbed atmosphere. Moreover, all of the above impacts would have continued to grow in the years beyond It is important to note that, while the provisions of the original Montreal Protocol in 1987 would have lowered the above growth rates, recovery would have been impossible without the Amendments and Adjustments (London, 1990; Copenhagen, 1992; and Vienna, 1995). The ozone layer is currently in its most vulnerable state. Total stratospheric loading of ozone-depleting substances was expected to maximize before the year All other things being equal, the current ozone losses are about (i) 6% at northern Hemisphere midlatitudes in winter/spring; (ii) about 3% at northern Hemisphere mid-latitudes in summer/fall; (iii) about 5% at southern Hemisphere mid-latitudes on a yearround basis; (iv) about 50% in the Antarctic spring; and (v) about 15% in the Arctic spring. The recovery of the ozone layer will be affected if we fail to comply with the international agreements of the Montreal Protocol. For example, illegal production of ktonnes per year of CFC-12 and CFC-113 for the next years would increase the equivalent effective chlorine loading above the 1980 abundance. Changes in ozone affect the earth's climate, and changes in climate and meteorological conditions affect the ozone layer, because the ozone depletion and climate change phenomena share a number of common physical and chemical processes. Conclusions On the basis of past emissions of ozone-depleting substances as well as on the basis of the projection of the maximum allowances under the Montreal Protocol, the maximum ozone depletion is estimated to lie within the current decade or the next two decades. But the identification and the evidence for the recovery of the ozone layer lie still further ahead. The falloff of total chlorine and bromine abundances in the stratosphere in this century will be much slower than the rate of increase observed in past decades. This is because of the slow rate at which natural processes remove these compounds from the stratosphere. The most vulnerable period for ozone depletion will be extended into the coming decades. Detection of the beginning of the recovery of the

7 116 INDIAN.I RADIO & SPACE PHYS, APRIL 2002 ozone layer could be achievable early, in this century, if decreasing chlorine and bromine abundances were the only factor. However, potential future increases or decreases in other gases important in ozone chemistry (such as nitrous oxide, methane, and water vapour global ) and climate change will influence the recovery of the ozone layer. When combined with the natural variability of the ozone layer, these factors imply that unambiguous detection of the beginning of the recovery of the ozone layer is expected to be well after the maximum stratospheric loading of ozonedepleting gases. Ultimately, it is the human perception of global environmental changes, which will be dominated by the attitude and assessment of the current challenges and opportunities. References 1 URL: fa ct.html, WMO/UNEP: Scientific Assessment of Ozone Depletion: Kotamarthi V R. Rodri gue z J M. Ko M K W., Tromp T K & Sze ND "Tritluoroacetic acid from the degradation of HCFCs and HFCs: A three-dimensional modeling study," J. Geophys. Research, DrN C Monda! Editor, IJRSP