The year began with the memories of the plumes of smoke from the Amazon forest fires, the resurgence of the Ebola outbreak, and the raging Australian bushfires. Since then, the world has also experienced devastating floods in Indonesia, Taal volcano eruption, locust swarms in Africa, and continues to suffer the outbreak of SARS-CoV-2. Even though the disasters make everything look bleak around you, studies suggested that the quality of air and water improved during the coronavirus pandemic. Lockdowns across several countries have resulted in factories and roads shutting down, thus resulting in reduced emissions. Around the same time, it was observed that the largest Arctic ozone hole formed on the ozone layer above the North Pole. The hole was first detected in February and reached a maximum extension of around 1 million square kilometres. This is equivalent to the size of approximately 140 football fields!
Image Credits: www.theweathernetwork.com
What is Ozone, and where do we find it?
Ozone (O3), is a triatomic molecule of Oxygen, a pale blue and pungent-smelling gas. About ninety percent of the total ozone occurs in the stratosphere, the second lowest layer of the atmosphere. At the same time, it is sparsely observed in the troposphere, the lowest level of the atmosphere. To give you an idea, commercial airlines cruise at altitudes between 9 to 11.5 kilometres. The origin of ozone in the atmosphere can be attributed to both natural and manmade sources. Between 15 to 35 km above the ground, in the lower stratosphere, lies a region of high ozone concentration called the ozone layer. Ozone, when present in the stratosphere, is nicknamed as “Good Ozone” as it plays a crucial role in protecting all living organisms from the harmful ultraviolet (UV) radiation of the sun. On the contrary, when present in the troposphere, it is nicknamed as “Bad Ozone”, and is quite harmful as a primary ingredient in photochemical smog. It reduces the photosynthetic capacity of plants and can cause several respiratory diseases in humans.
Discovery of Ozone
In 1840, a German-Swiss scientist working in the University of Basel, Christian Friedrich Schönbein published his findings of a byproduct of his water electrolysis experiment. He reported that it rendered a distinct odour, similar to the one detected in the vicinity of a lightning storm. This observation raised the question about the presence of this newly discovered gas in the atmosphere. With the invention of Fabry–Pérot interferometer in 1899, the french physicist duo Henri Buisson and Charles Fabry discovered the presence of ozone in the atmosphere (the ozone layer) in 1913. An interesting fact to note is the abnormal temperature profile of the stratosphere. From our intuition and mountainside experience, we expect it to get colder as we gain elevation. Yet, by absorbing the incoming solar radiation, the ozone layer creates an increasing temperature profile.
In the stratosphere, ozone and essentially the ozone layer is mainly formed by a simple chemical reaction of photodissociation of oxygen molecules. When stratospheric diatomic Oxygen (O2) is exposed to short-wavelength Ultraviolet radiation (λ < 240 nm), it splits into two atomic oxygens (O). We all know that the atmosphere is mainly composed of two gases, namely, Nitrogen (N2) [about 78%] and Oxygen (O2) [about 21%]. Hence, the reactive atomic Oxygen collides with either of the two diatomic molecules. While N2 is an inert or non-reactive gas, O2 readily reacts with O to form ozone. This newly formed ozone is highly energetic and not so stable. As we can imagine, there is a high probability that this ozone bumps into the inert N2 and this collision results in the stabilising of the ozone molecule with the excess energy heating the air around.
The Ozone Hole
In 1973, a couple of scientists from the University of California, Irvine (UCI) began working on the stability of chlorine-containing organic volatile compounds called Chlorofluorocarbons or CFCs. He observed its destructive effects on the ozone. A decade later, in 1985, the Vienna Convention for the Protection of the Ozone Layer was signed and ratified by 20 major CFC-producing countries. With the discovery of an “ozone hole” just 1.5 years later, an international treaty was signed and ratified by 196 countries and the European Union in the city of Montreal in Canada on 16th September 1987 for the discontinuance of the production of ozone-depleting substances (ODs). Today, the emission of the ODs have reduced significantly, up to 2% of what it was before the signing of the treaty.
Contrary to what the name suggests, an ozone hole does not technically refer to a “hole” in the atmosphere. It is instead a zone in the ozone layer that is hugely depleted in ozone. There is a class of compounds called ozone-depleting substances (ODs) like Chlorofluorocarbons (CFCs), Hydrochlorofluorocarbons (HCFCs), halons, and halocarbon refrigerants and propellants. When the ozone in the stratosphere reacts with these substances, the ozone hole is created. These halogen compounds undergo photodissociation to give X radical (X = Cl, Br), which spontaneously reacts with the ozone and catalyses the oxygen gas forming reaction. It is a cyclic process, and the chlorine atom is regenerated at the end. Chlorine has a residence time of about two years. Thus, a single atom of chlorine can destroy ozone up to 2 years unless locked in stable reservoirs like Hydrogen Chloride (HCl) or Chlorine Nitrate (ClONO2).
Image Credits: learner.org
We have all experienced the blast of warm/cold air as we step inside a mall or a shopping complex. We realise that this ‘air curtain’ prevents the hot air and the pollutants outside from entering the premises, and ensure that the premises are kept cool. During the winters, the polar regions experience a severe winter and complete darkness. The atmospheric conditions above it become unusual. Working by the same principle, strong swirling whirlpools of stratospheric winds called the polar vortex isolates the air in the polar region. This isolation results in the retention of all the reactive chlorine or bromine compounds that get released into, or formed in the stratosphere, and facilitates the rapid destruction of ozone, creating the ozone hole.
Generally, clouds are formed when moisture-laden warm air rises, and the moisture content of the air increases. When the air is saturated with water vapour, it condenses into minute water droplets and forms the clouds. Yet, as an exceptional case of extreme chillness, the polar vortex forms a unique type of cloud known as the polar stratospheric cloud (PSC), despite the air being dry and thin. On the surface of these clouds, some unusual chemical reactions unique to this region take place.
These reactions involve the uptaking of the HCl gas onto the ice surface. HCl gas is then converted into a solid as it forms hydrogen bonds with the water molecule. Further reactions of solid HCl with gaseous ClONO2, HOCl, and N2O5 results in the conversion of the inert form of chlorine in the reservoirs to readily reactive chlorine gas (Cl2). This process releases chlorine back into the system, paving the way to the catalytic destruction of ozone, leading to the formation of the ozone hole.
As the air gets warmer, the polar vortex loses its strength, and this weakens the isolation of the air. The mixing of this cold and pollutant-rich polar vortex with the warmer air from lower latitudes results in the dissolution of polar stratospheric clouds. The ‘special reactions’ mentioned above can no longer take place. With HNO3 being resupplied to the vortex (and not consumed), the reactive chlorine is also consumed by forming the reservoir compounds. Further, the mixing also results in the dispersion of the reactive chlorine compounds, thus preventing ozone depletion. At the same time, the increased sunlight aids the ozone-forming reactions, and the hole begins to mend itself. This process is repeated every year.
Image Credits: www.theweathernetwork.com
So, what makes this ozone hole unique if this is just another yearly phenomenon?
Generally, the ozone hole above the Arctic is not as depleted as its southern counterpart to raise the alarm. The winters are much warmer in the north, with temperatures that are much variable as compared to the regularly plummeting temperatures of the south (Antarctic Circle). Thus, the polar vortex is not as powerful, and consequently, the formation of PSCs is also less likely. This year, the ozone hole over the Arctic was depleted more than ever recorded, the most recent notable depletion in 2011. Yet, contrary to public beliefs, this was not due to the reduced pollution levels associated with the global lockdown. Rather unusual freezing temperatures concentrated the polar vortex above the Arctic region for much longer than is typical.
An Arctic ozone hole of such a vast size could expose all the living beings in Europe, North Asia, and Canada to harmful UV rays. Even though the ozone hole, which was open for about a month, was finally shut again, it may happen again. Atmospheric scientists firmly believe that the closing of the hole is also attributed to the polar vortex and has no relation with the lockdown. Further, they have also informed that no novel trends can be estimated from this event. With the increasing effects of global warming on the atmosphere, the likelihood of such an event soon is inevitable if carbon emissions aren’t reduced tremendously around the world.
This article is an extended and updated version of the one written as a part of the Scicatalyst Newsletter Vol. VII - May 2020. Illustrator: Atreyasai. Editors: Ananya Dash, Rachita Dash, and Mandira Choppella.