Ozone Depletion

Introduction to Ozone

• Ozone is an allotrope of oxygen.

• Ozone molecule is made up of three oxygen atoms.

• Traces of ozone in the Troposphere are harmful to plants , animals and human beings but the ozone layer present in the stratosphere shields the life on earth against almost all solar UV radiations.

• Ozone absorbs UV radiation and then emits IR radiation.

• Ozone found in the stratosphere is formed due to the effect of UV radiation on dioxygen (O2) molecules.
• The UV radiation breaks down the molecular oxygen into separate oxygen (O) atoms.
• These oxygen atoms react with the molecular oxygen to create ozone molecules.

• Ozone is unstable thermodynamically and breaks down into molecular oxygen.

• As a result, there is a dynamic balance between the production and decomposition of ozone molecules.

Types of Ozone

1. Stratospheric Ozone

• Aboul90 per cent of the total ozone in the atmosphere is located in the stratosphere.

• The unit of measurement of ozone in the atmospheric column is Dobson Unit (DU).

Production-

• Ozone is produced in the stratosphere by ionization or photodissociation of oxygen in the presence of UV radiation from the Sun.

• Inspite of splitting of oxygen molecule, oxygen remains abundant in the stratosphere by reconversion of ozone into oxygen.

• Thus, natural production and destruction of ozone occur in the stratosphere.

    O2 (g) →O(g) + O(g)

    O(g) + O2 (g) →O3 (g)

• In recent years, there have been reports of the depletion of this protective ozone layer because of the presence of certain chemicals in the stratosphere.

2. Tropospheric Ozone

• Small quantity of ozone occurs also in the troposphere and near the earth's surface.

• Ozone is injected down into the troposphere from the stratosphere in the region of tropopause break in the middle latitude.

• Small quantities of 0, can be also locally produced by flushes of atmospheric electricity in thunder storm or through any electric spark.

• Ozone is a greenhouse gas in the troposphere and hence contributes to global warming and it is also cause of photochemical smog.

Measurement of atmospheric ozone

• Dobson Spectrophotometers are used to measure the atmospheric ozone and it is represented by Dobson unit and it is the thickness of the ozone column (compressed at Standard Temperature and Pressure (STP)) in Millicent meters.

• India has also a network of Dobson instruments.

Distribution of Ozone

A. Equator vs Polar

• The total column ozone is minimum in equatorial latitudes compared to high latitudes.

• This apparent paradox was explained by Dobson who concluded that ozone, after production in the equatorial stratosphere, is transported towards polar latitudes by atmospheric meridional (south to north) circulation processes which carry ozone molecules toward poles.

• Thus total column ozone is maximum in sub-polar regions

B. Seasonally

• Seasonally maximum total column ozone is found in the spring season and minimum in the summer in middle and high latitudes.

• The seasonal variation of ozone in the low latitudes is Very small. For India the maximum amplitude of ozone on annual basis is observed at Srinagar.

C. Vertically

• Ozone is mostly, confined within a layer of the atmosphere extending from 15-35 km in the stratosphere.

• Its concentration is rather small in the troposphere.

• Surface ozone also varies diurnally as it is produced by photochemical action.

Depletion of Ozone

There are some chemicals which are responsible for depletion of ozone layer in our atmosphere

1. Chlorofluorocarbons (CFCs-CFC-11 and CFC-12)

• Non-toxic, non-flammable, and non-carcinogenic Chlorofluorocarbons (CFCs), commonly referred to as Freon, have been developed for various industrial and household purposes.

• Chlorofluorocarbons are not "washed" back to Earth by rain or destroyed in reactions with other chemicals.

• They simply do not break down in the lower atmosphere.

• CFCs are instead transported into the stratosphere where they are eventually broken down by ultraviolet (UV) rays from the Sun, releasing free chlorine.

• The chlorine becomes actively involved in the process of destruction of ozone.

• CFCs molecules are made up of chlorine, fluorine and carbon.

Uses-

• They are used as refrigerants, propellants in aerosol sprays,foaming agents in plastic manufacturing, fire extinguishing agents, solvents for cleaning electronic and metallic components, for freezing foods etc .

Time-

• 40-150 years.

Escape-

• Chlorofluorocarbons (CFCs) enter the atmosphere gradually as they evaporate from their sources.

• Discarded refrigerators are one source from which CFCs can escape into the atmosphere.

• CFCs are thermally stable, which means they can survive in the troposphere.

Cl + O3 = ClO + O2

ClO + O = Cl + O2

• The process of converting ozone into oxygen releases the chlorine atom, which can then repeat the cycle up to 100,000 times. As a result, the level of ozone is diminished.

2. Nitrogen Oxides-

Source-

• The sources of nitrogen oxides are mainly explosions of thermonuclear weapons , industrial emissions and agricultural fertilizers.

Escape -

• Under anaerobic conditions, nitrates undergo denitrification and release nitrous oxide (N2O) from solids. On the other hand, under aerobic conditions, ammonia undergoes nitrification and also releases N2O.

Other-

• Halons and HBFCs, which are bromine-containing compounds, are responsible for releasing bromine into the atmosphere.

• Sulphuric acid particles play a role in preventing the formation of chlorine reservoirs by freeing chlorine from molecular reservoirs and converting reactive nitrogen into inert forms.

The Ozone Hole

During the 1980s, researchers in Antarctica who specialized in atmospheric science documented the reduction of the ozone layer, which became known as the "ozone hole," located above the South Pole.

• It was found that a unique set of conditions was responsible for the ozone hole.

Process of ozone hole formation

A. In Summer season -

• When nitrogen dioxide and methane come into contact with chlorine monoxide and chlorine atoms, they undergo a chemical reaction that leads to the formation of chlorine sinks.

• These sinks are crucial in preventing significant depletion of the ozone layer.

B. In Winter season -

• Antarctica is home to a unique type of cloud known as polar stratospheric clouds.

• These clouds play a crucial role in the formation of hypochlorous acid, which is important for the ozone depletion process.

• Chlorine nitrate is also formed in the polar stratospheric clouds and is hydrolyzed to produce hypochlorous acid.

• This reaction only takes place under certain conditions, including the presence of polar stratospheric clouds.

• The formation and behavior of these clouds are still being studied by scientists to better understand their impact on atmospheric processes.

• It also reacts with hydrogen chloride produced to give molecular chlorine.

ClO (g) + NO2 (g) → ClONO2 (g)

Cl (g) + CH4 (g) → CH3 (g) + HCl(g)

ClONO2 (g) + H2O (g) → HOCl (g) + HNO3(g)

ClONO2 (g) + HCl (g) → Cl2 (g) + HNO3 (g)

• When sunlight returns to the Antarctica in the spring, the sun’s warmth breaks up the clouds and HOCl and Cl2 are photolysed by sunlight.

HOCl (g) →(hν)  OH (g) + Cl(g)

Cl2 (g) →(hν)  2 Cl (g)

• The chlorine radicals thus formed, initiate the chain reaction for ozone depletion as described earlier.

Relation between Polar stratosphere clouds and Ozone depletion

• The presence of polar stratospheric clouds (PSCs) is correlated with the cycle of ozone depletion. These clouds, consisting of ice particles, provide substrates for chemical reactions that release chlorine from its reservoirs.

• Usually, the reaction between HCl and ClONO2 is very slow, but this reaction occurs at a faster rate in the presence of a suitable substrate which is provided by the stratospheric clouds at the poles.

Why ozone depletion is dominating in Antarctic

It is because of 2 reasons

1. Dominance of Polar stratospheric clouds due to low temperature of Antarctic stratosphere.

2. longevity of the Antarctic vortex which further enhances formation of polar stratospheric cloud and restrict mixing of fresh ozone from mid latitude to Antarctic.

Antarctic vortex -

• The vortex is a ring of rapidly circulating air that confines the ozone depletion in the Antarctic region.

Process

• In the stratosphere, an increase in altitude is associated with a characteristic increase in temperature due to the absorption of sunlight by ozone.

• Depletion of ozone can cause the air to become cooler, which can contribute to the formation of Polar Stratospheric Clouds (PSCs) and stabilization of the vortex.

• The vortex remains in the Antarctic region throughout the polar winter and into mid-spring.

• In contrast, the vortex in the Arctic disintegrates by the time the polar spring (March-April) arrives.

• In June Antarctic winter begins, the vortex develops and the temperature falls enough for the clouds to form.

• In July and August, polar stratospheric clouds (PSCs) cause denitrification and dehydration in the stratosphere.

• This process involves the precipitation of hydrochloric acid and chlorine nitrate, which react on cloud surfaces to release chlorine.

• During this time, winter temperatures drop to their lowest point.

• In September, the austral spring begins and sunlight returns to the center of the vortex.

• As temperatures increase, PSCs disappear.

• During October lowest levels of ozone are reached.

Effects of Depletion of the Ozone Layer

A. Effects on Human and Animal Health

• The potential rise in solar UV-B radiation can significantly impact human health, leading to an increased risk of eye diseases, skin cancer, and infectious diseases.

• UV-B radiation can adversely affect the immune system causing a number of infectious diseases.

B. Effects on Terrestrial Plants

• In forests and grasslands increased UV-B radiation is likely to result in changes in species composition (mutation) thus altering the bio-diversity in different ecosystems.

• The plant community could be indirectly influenced by UV-B, leading to alterations in plant structure, secondary metabolism, and other related factors.

C. Effects on Aquatic Ecosystems

• Aquatic ecosystems can suffer from reduced productivity due to increased UV exposure.

• In tropical and subtropical regions, high UV exposure can impact the distribution of phytoplankton, which are essential for aquatic food webs.

• UV-B radiation can harm the early development stages of various aquatic animals such as fish, shrimp, crab, and amphibians.

D. Effects on Bio-geo-chemical Cycles

• Increased solar UV radiation could affect terrestrial and aquatic bio-geo-chemical cycles thus altering both sources and sinks of greenhouse and important trace gases, e.g. carbon dioxide (CO2), carbon monoxide (CO), carbonyl sulphide (COS), etc.

• These changes would contribute to biosphere-atmosphere feedbacks responsible for the atmosphere build-up of these gases.

E. Effects on Air Quality

• The increase in UV-B radiation and reduction of stratospheric ozone levels can lead to a rise in photo dissociation rates of crucial trace gases that regulate the chemical reactivity of the troposphere.

• This can increase both production and destruction of ozone and related oxidants such as hydrogen peroxide which are known to have adverse effects on human health, terrestrial plants and outdoor materials.

• Modifications in the hydroxyl radical (OH) concentrations within the atmosphere can lead to alterations in the atmospheric lifetimes of significant gases like methane and substitutes for chlorofluorocarbons (CFCs).

F. Effects on Materials

• An increased level of solar UV radiation is known to have adverse effects on synthetic polymers, naturally occurring biopolymers and some other materials of commercial interest.

• The lifespan of these materials is limited due to the acceleration of photo degradation rates by UV-B radiation.

G. Effects on Climate Change

• Due to depletion of ozone overall temperature equilibrium of the atmosphere gets impacted and triggers climate change.

Important one liner on Ozone Depletion.