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Arctic ozone loss and Antarctic ozone hole

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Stratospheric Ozone and Ozone Hole

What is Ozone?

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Ozone is an allotrope of oxygen present in significant amounts in the stratosphere forming the earth’s ozone layer. It has three oxygen atoms bound to each other in a nonlinear fashion. 

It is a pale blue gas that is poisonous to human life even in small concentrations because of its very strong oxidizing action, much greater than that of oxygen.

What is stratospheric ozone?

In the stratosphere, when some of the oxygen molecules from the troposphere absorb the appropriate high energy (UV part) of the sunlight (energy) they break up into free oxygen atoms (O) which then combine with oxygen molecules (O2) to form ozone (O3). This stratospheric ozone acts to shield the earth against harmful UV radiation and forms the ozone layer in the stratosphere.

This stratospheric ozone acts as a sunshield for the life forms on earth, protecting us from the harmful UVB radiations and thereby also reducing the risks on the lives of plants and animals.   

What is tropospheric ozone?

Ozone is produced in the lower atmosphere as a result of chemical reactions between pollutants from the emissions from vehicle exhausts, gasolines etc. with the sunlight.  At ground levels of 10-15 Km it is found to be harmful to human health , plants and animals due to its strong oxidising efficiency. The tropospheric ozone acts as a greenhouse gas and is  capable of reacting with pollutants in the atmosphere forming the smog.
Being a greenhouse gas it has a powerful role to play in the realm of climate change. It causes global warming. It has a negative effect also on the yields of agricultural crops. 

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What is the ozone loss problem?

The harmful effects of ozone layer depletion on human beings are due to the action of these ultraviolet rays on eyes and skin, causing sunburn, cataract, skin cancer, etc. The UV-B rays cause direct damage to the genetic material or DNA of animal cells. Exposure of mammals to UV-B light has been shown to act on the immune system, enhancing the susceptibility of the body to infections and cancers.

In general exposure to high UV-B adversely affects the leaf area, net photosynthesis rate, transpiration rate, and water-use efficiency of plants. All-in-all overexposure to UV-B reduces the productivity and quality of the crop plants. This can have serious socio-economic consequences. According to a report of the United Nations Environment Programme (UNEP), in areas where substantial ozone depletion has occurred, a wide range of field studies indicate that increased UV-B radiation reduces terrestrial plant productivity by about 6 %.

All aquatic organisms are vulnerable to solar UV-B radiation at most stages of their lives, from sperm to adulthood. The detrimental effects of UVR are synergistically enhanced in the presence of other stress factors such as elevated temperatures, higher transparency, acidification, and other forms of pollution. In other words, the ecological impacts of climate change trigger complex interactions among different environmental variables and the enhanced UVR exposure due to the ozone hole often increase the severity of other adverse impacts besides causing adverse effects of its own.

Increased UV-B penetration resulting from the ozone hole accelerates photodissociation of key trace gases that control the troposphere chemistry and in turn other processes occurring in the troposphere. Among other things, this has implications on production as well as destruction of ozone (O3) and related oxidants such as hydrogen peroxide (H2O2 ), which are known to have adverse effects on human health, terrestrial plants, and outdoor materials. When atmospheric concentration of the hydroxyl radical (OH-) change, the atmospheric lifetimes of climatically important gases such as methane (CH4 ) and the CFC substitutes would change, too.
 

How an ozone hole is formed?

The ozone hole is not technically a “hole” where no ozone is present, but is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic that happens at the beginning of Southern Hemisphere spring (August–October). The ozone hole is caused by chemicals called CFCs, short for chlorofluorocarbons. CFCs escape into the atmosphere from refrigeration and propellant devices and processes. In the lower atmosphere, they are so stable that they persist for years, even decades. This long lifetime allows some of the CFCs to eventually reach the stratosphere. In the stratosphere, ultraviolet light breaks the bond holding chlorine atoms (Cl) to the CFC molecule. A free chlorine atom goes on to participate in a series of chemical reactions that both destroy ozone and return the free chlorine atom to the atmosphere unchanged, where it can destroy more and more ozone molecules.

Once in the stratosphere, CFCs face the prospect of disintegration. The high- energy ultraviolet light passing through the stratosphere attacks CFC molecules breaking them up and freeing their chlorine atoms. Each freed chlorine atom reacts with an ozone molecule (O3), forming a molecule each of oxygen (O2) and chlorine monoxide (ClO). ClO then combines with an atom of oxygen, resulting in the formation of an oxygen molecule (O2 ) and the generation of a new free chlorine atom (Cl):

Cl + O3 → ClO + O


ClO + O → Cl + O2

The net reaction is


O3 + O → O2 + O2

In this manner Cl atoms keep destroying ozone molecules even as they (Cl atoms) keep getting regenerated. The cycle goes on till a single chlorine atom has destroyed thousands of ozone molecules. Each cycle ends only when a reactive nitrogen or hydrogen compound is encountered which eventually takes the Cl atom out from this cycle and combines with it. Bromine atoms are even more harmful to ozone; each bromine atom destroys hundreds of times more ozone molecules than a chlorine atom does.
 

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Ozone over Antarctica. a, In 1985, Farman et al.1 reported that stratospheric ozone levels over the Halley and Faraday stations in Antarctica during the austral spring had declined greatly from previously steady values. The graph shows the Halley times series, extended to 2016. b, Subsequent satellite monitoring revealed that the area of ozone depletion — the ozone hole — extended over a vast region. This map shows a satellite ozone map for 10 September 2000, when ozone depletion was close to its maximum: blue indicates low ozone levels; red, high levels. The position of the Halley station is indicated. (adapted from  https://www.nature.com/articles/d41586-019-02837-5 )

Why are ozone holes found only in the Antarctic?

The Antarctic stratosphere is much colder than the Arctic’s. The lower temperature of the stratosphere above the Antarctic facilitates the formation of polar stratospheric clouds (PSCs) at much lower altitudes (even below 20 km from the surface of the earth). The PSCs play a major role in promoting the catalytic destruction of stratospheric ozone.


In normal situations absorption of sunlight by ozone causes an increase in temperature, with altitude, in the stratosphere. But when ozone is depleted, this rise in temperature is much lesser and the cooler air adds to the conditions that favour the formation of the PSCs further.

 

Stratospheric   air   in   the   polar   regions is relatively isolated from other stratospheric regions for  long  periods  in  the  winter  months.    The  isolation  comes  about because of strong winds that encircle the poles, forming a polar vortex, which prevents substantial motion of air into or out of the polar stratosphere. The vortex persists throughout the Antarctic winter, well into mid-spring. In contrast, the Arctic vortex disintegrates much faster and is gone before the arrival of the Arctic spring (March–April). The vortex in the Antarctic region is more stable compared to the Arctic region due to the lesser influence of the planetary waves from lower latitudes.

What is ozone loss saturation?

The most evident form of  ozone loss was discovered in 1985 by Farman et al., ever since then the concentration of ozone was decreasing and the ozone loss in the winters reached its maximum extent or saturation of loss due to the high levels of ODSs in the atmosphere by late 1990s. The saturation of ozone loss, i.e., the total or near-zero destruction of ozone in the lower stratospheric layers, mostly at 13–21 km, has reportedly begun in 1991

What is the status of the ozone hole now?

British Antarctic Survey Scientists, Jonathan Shanklin, Brian Gardiner, and Joseph Farman, discovered another hole in the ozone layer in 1984. This hole is a recurring hole that happens in the springtime in the Antarctic. Their study was published in May 1985 in the paper Nature. The study showed that the ozone had already dropped 10 percent below normal for January levels in Antarctica. By October levels were already down by 35 percent of the average for the 1960s. 

The concern over growing ozone depletion led to Montreal protocol in 1987, to ban the production of CFCs and other ozone depleting substances. The ban on emissions of ODSs resulted in a stabilization of atmospheric concentrations of CFCs in the late 1990s and led to a subsequent gradual decline—by approximately 1–1.5% per year—of the most important CFCs ever since. In conjunction, the thinning of the ozone layer during springtime over Antarctica stabilized in the late 1990s as well with springtime Antarctic average total ozone columns typically reduced by 50% compared to pre‐1980 pre ozone hole conditions. 

In 2018, the hole was documented as being 3 times the size of the United States coming in 13th in size ranking in 40 years from NASA. With all the air warming up, it is actually helping smaller holes. The hole is slowly but surely becoming smaller since the expansion in 1988.
The latest WMO /UN Environment Programme Scientific Assessment of Ozone Depletion, issued in 2018, concluded that the ozone layer is on the path of recovery and to potential return of the ozone values over Antarctica to pre-1980 levels by 2060. 

However, the recently discovered increase in CFC-11 emissions of ~ 13 Gg yr−1 may delay recovery. So far the impact on ozone is small, but if these emissions indicate production for foam use much more CFC-11 may be leaked in the future. Assuming such production over 10 years, the disappearance of the ozone hole will be delayed by a few years, although there are significant uncertainties.

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Copyright holder: Copernicus Atmosphere Monitoring Service (CAMS).

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 What is the status of global stratospheric ozone?

The substantial ozone decline observed since the 1960s ended in the late 1990s. Since then, ozone levels have remained low, but have not declined further. Now general ozone increases and a slow recovery of the ozone layer is expected. The clearest signs of increasing ozone, so far, are seen in the upper stratosphere and for total ozone columns above Antarctica in spring. These two regions had also seen the largest ozone depletions in the past. Total column ozone at most latitudes, however, does not show clear increases yet. With the expected continued decline in equivalent effective stratospheric chlorine (EESC) due to the Montreal Protocol, ozone ceased its decline and began its slow healing process (also referred to here as recovery) near the turn of the 21st century. 

Healing is expected to become increasingly detectable in observations and has been a prime focus of ozone research. Multiple studies have shown signs of Antarctic ozone recovery. Total column measurements of ozone between the Earth's surface and the top of the atmosphere indicate that the ozone layer has stopped declining across the globe, but no clear increase has been observed at latitudes between 60° S and 60° N outside the polar regions (60–90°). Evidence from multiple satellite measurements show that ozone in the lower stratosphere between 60° S and 60° N has indeed continued to decline since 1998. 

One can find that, even though upper stratospheric ozone is recovering, the continuing downward trend in the lower stratosphere prevails, resulting in a downward trend in stratospheric column ozone between 60° S and 60° N.  In the Arctic the growing climatic changes leading to colder winters is said to increase the ozone loss amount and is also possible that in future it might even lead to Arctic ozone hole.

As a global average concentration, it’s expected that ozone levels will return to their 1960 levels around mid-century. Antarctica, where ozone depletion has been most severe due to very low temperatures is expected to recover much more slowly. 

What do we do?

Actions required globally to continue the recovery of the ozone layer are:
○    Ensuring that existing restrictions on ozone-depleting substances are properly implemented and global use of ozone-depleting substances continue to be reduced.
○    Ensuring that banks of ozone-depleting substances (both in storage and contained in existing equipment) are dealt with in an environmentally friendly manner and are replaced with climate-friendly alternatives.
○    Ensuring that permitted uses of ozone-depleting substances are not diverted to illegal uses.
○    Reducing use of ozone-depleting substances in applications that are not considered as consumption under the Montreal Protocol.
○    Ensuring that no new chemicals or technologies emerge that could pose new threats to the ozone layer (e.g. very short-lived substances).
○    Conduct regular inspection and maintenance of air-conditioning and refrigeration appliances to prevent and minimize refrigerant leakage.
○    For existing air-conditioning and refrigeration appliances that operate on HCFCs or CFCs, the refrigerant should be recovered or recycled whenever an overhaul of equipment is to be carried out. Replacing or retrofitting such equipment to operate on non-HCFCs refrigerants should also be considered. 
○    When motor vehicle air-conditioners need servicing, make sure that the refrigerants are properly recovered and recycled instead of being vented to the atmosphere

SIGNIFICANCE

First proposed by Molina and Rowland in early 1970s (Rowland and Molina won Nobel prize for Chemistry in 1995 along with Paul Crutzen), Stratospheric ozone hole in the Antarctic region due to chemical destruction involving halogen compounds is very well known.

Ozone hole has resulted in significant changes in southern hemispheric climate in the past decades (Gillett and Thompson, 2003; Thompson et al., 2011). It resulted in the enactment of an international treaty, popularly known as Montreal Protocol, adopted on September 16, 1987 in Montreal. It aimed to protect the ozone layer by controlling the global emissions of ozone depleting substances (ODS), known as chlorofluorocarbons (CFC).

Due to the decrease in atmospheric loading of ODSs which has peaked around 2000 (Velders et al., 2007), the Antarctic ozone hole is expected to be on the mend since 2000. A number of studies have already reported the early signs of the healing of ozone hole (Solomon et al., 2016; Kuttippurath et al., 2017). However, there has not been any study reporting the healing in saturation layers. Ozone hole saturation layers are the atmospheric layers in the lower stratosphere (13-21 km from the sea level) where the complete destruction of ozone occurs.

A few of the studies have suggested the beginning of ozone saturation in 1991 (Jiang et al., 1996; Yang et al., 2008) but there have been references of the same occuring at McMurdo in 1987 (Hofmann et al., 1993; Gardiner et al., 1988).