Polar stratospheric clouds (PSCs) play important roles in stratospheric ozone depletion at high latitudes during winter and spring (e.g., the Antarctic ozone hole). PSC particles serve as sites for heterogeneous reactions that convert stable chlorine reservoir species to radicals that catalytically destroy ozone. Depletion of the ozone layer is due to strong polar fronts, the presence of polar stratospheric clouds and of chlorofluorocarbons. In this article, we will discuss the Role of the Polar Stratospheric Cloud in Ozone Depletion which will be helpful for UPSC exam preparation.

Stratospheric Clouds

What are Polar Stratospheric Clouds?

• Polar stratospheric clouds (PSCs), often called nacreous clouds from nacre or mother of pearl due to their iridescence, are clouds in the winter polar stratosphere.
• PSCs are wave clouds. They frequently occur downstream of mountain ranges, which can cause gravity waves in the lower stratosphere.
• As the waves progress, their sheet-like formations gradually undulate and extend.
• The clouds may also be accompanied by extremely strong surface winds, which could be a sign of or cause waves and winds in the stratosphere.

Formation of Polar Stratospheric Clouds

• PSCs form at temperatures around -85°C, which are colder than average lower stratosphere temperatures, and are made up of ice particles 10m in diameter.
• To produce the characteristic bright iridescent colours through diffraction and interference, the clouds must be composed of similar sized crystals.
• PSCs form at extremely high altitudes, between 15 and 25 kilometres (about 50,000 to 80,000 feet). Only at extremely low temperatures is enough condensation present to form clouds in the extremely dry air at these altitudes.
• Temperatures in the lower stratosphere can drop low enough in winter near the North or South Pole for PSCs to form.
• According to one theory, when orographic features are encountered, strong winds in the troposphere can cause mountain waves. These waves may propagate upward into the stratosphere, where the resulting lifting and cooling can produce PSCs.
• PSCs are more common near the South Pole, where temperatures are typically lower, but PSCs have recently been observed near the North Pole.

Types of Polar Stratospheric Clouds

PSCs are divided into two types based on their chemical composition, which can be measured using LIDAR. The technique also determines the cloud's height and ambient temperature.

Type I Clouds

• They are a source of polar ozone depletion because they contain water, nitric acid, and/or sulfuric acid. When temperatures fall to -78°C or lower, they form.
• Type Ia clouds are made up of large, aspherical particles made up of nitric acid trihydrate (NAT).
• Type Ib clouds contain small, non-depolarizing spherical particles of a liquid supercooled ternary solution (STS) of sulphuric acid, nitric acid, and water.
• Type Ic clouds are composed of metastable water-rich nitric acid in solid form.

Type II Clouds

• They are extremely rare in the Arctic and are made up entirely of water ice.
• They can only form at temperatures of -83°C or lower.
• Only Type II clouds are necessarily nacreous, whereas Type I clouds, like any other cloud, can be iridescent under certain conditions.

PSCs as a Cause of Ozone Depletion

• PSCs were long regarded as oddities with no real significance. However, Type I clouds are now recognised as sites of harmful stratospheric ozone destruction over Antarctica and the Arctic.
• Their surfaces serve as catalysts, converting less dangerous forms of man-made chlorine into active free radicals (for example ClO, chlorine monoxide).
• In a series of chain reactions, these radicals destroy many ozone molecules as spring sunlight returns.
• Cloud formation is doubly damaging because it removes gaseous nitric acid from the stratosphere, which would otherwise combine with ClO to form less reactive forms of chlorine.

Effect of Polar Stratospheric Clouds on Ozone Loss

• The chlorine released by the breakdown of CFCs exists initially as pure chlorine or chlorine monoxide (active chlorine/unstable), but these two forms react further to form the stable compounds chlorine nitrate and HCL (inactive chlorine).
• Because the stable compounds HCL and CLONO2 are chlorine reservoirs, chlorine must be liberated before it can participate in any reaction.
• There is a link between the ozone depletion cycle and the presence of polar stratospheric clouds (PSCs), which provided substrates for chemical reactions that liberated chlorine from its reservoirs.
• Normally, the reaction between HCl and CLONO2 is very slow, but it occurs at a faster rate when a suitable substrate is present, which is provided by stratospheric clouds at the poles.

HCl + Chlorine nitrate -> Molecular Chlorine

• As a result, molecular chlorine and nitric acid are formed. The atomic chlorine formed in the above reaction can be broken down to form molecular chlorine, allowing the ozone depletion reaction to continue.
• PSCs absorb reactive nitrogen as well as activate chlorine. Nitrogen oxides would combine with chlorine monoxide to form a reservoir of chlorine nitrate if they were present (CLONO2).
• Dimer Cl2O2 is formed when stratospheric chlorine monoxide reacts with itself.
• Sunlight easily dissociates this dimer, releasing free chlorine atoms that can then react further to destroy ozone.

Conclusion

Polar stratospheric clouds contain nitric and sulphuric acid which when combined with chlorofluorocarbons lead to the formation of chlorine atoms. Hence, the formation of polar stratospheric clouds with an increase of chlorofluorocarbons destruct the ozone layer and harms the planet earth which should be reduced by limiting the emissions of chlorofluorocarbon.

FAQs

MCQs

1. What is the main component of Type II polar stratospheric clouds?

A) Nitric acid
B) Water ice
C) Sulfuric acid
D) Methane

2. How do polar stratospheric clouds aid in ozone depletion?

A) By reflecting sunlight
B) By promoting the release of active chlorine
C) By increasing atmospheric pressure
D) By trapping greenhouse gases

3. At what temperature do polar stratospheric clouds typically form?

A) Below -50°C
B) Below -78°C
C) Below 0°C
D) Below -100°C

4. Which international treaty aimed to curb ozone depletion?

A) Kyoto Protocol
B) Paris Agreement
C) Montreal Protocol
D) Geneva Convention

5. What role does the ozone layer play for life on Earth?

A) Increases global warming
B) Blocks harmful ultraviolet radiation
C) Decreases atmospheric pressure
D) Promotes carbon dioxide absorption

GS Mains Questions and Model Answers

Q1: Explain the formation and role of polar stratospheric clouds in the depletion of the ozone layer. How do these clouds contribute to this environmental issue?

Answer: Polar stratospheric clouds (PSCs) form at high altitudes in the stratosphere during the polar winter when temperatures fall below -78°C. They are composed of water, nitric acid, and sulfuric acid. PSCs play a significant role in ozone depletion by providing a surface for chemical reactions that convert inactive chlorine and bromine compounds into reactive forms. When sunlight returns to the polar regions, these activated compounds catalyze the destruction of ozone molecules, leading to the formation of the ozone hole. This process has been particularly evident over Antarctica, where PSCs contribute significantly to seasonal ozone depletion.

Q2: Analyze the importance of the Montreal Protocol in addressing ozone depletion. What impact has this treaty had on the recovery of the ozone layer?

Answer: The Montreal Protocol, adopted in 1987, was a landmark international treaty aimed at phasing out the production and use of ozone-depleting substances (ODS), such as chlorofluorocarbons (CFCs) and halons. By mandating the reduction of these harmful chemicals, the protocol has significantly contributed to the gradual recovery of the ozone layer. Studies show that the levels of ODS in the atmosphere have declined, resulting in a decrease in the size and severity of the ozone hole, especially over Antarctica. The treaty's success has been attributed to its legally binding nature, comprehensive monitoring, and global cooperation. The Montreal Protocol is considered one of the most successful environmental agreements, demonstrating that coordinated international action can effectively address environmental challenges.

Q3: Discuss the environmental and health impacts of ozone layer depletion. How do polar stratospheric clouds amplify these issues?

Answer: Ozone layer depletion has significant environmental and health impacts. The ozone layer acts as a shield, absorbing harmful ultraviolet (UV) radiation from the Sun. Depletion leads to increased UV exposure, which can cause skin cancer, cataracts, and immune system suppression in humans. It also affects ecosystems, damaging phytoplankton in oceans and impacting food chains. Polar stratospheric clouds (PSCs) amplify ozone depletion by facilitating chemical reactions that release active chlorine and bromine, which destroy ozone molecules. This is particularly severe in polar regions, contributing to the formation of the ozone hole and leading to increased UV exposure during the spring and summer months when sunlight returns.

Previous Year Questions on Polar Stratospheric Clouds and Ozone Depletion

1. UPSC CSE Prelims 2021:

Question: Which of the following is a major contributor to ozone depletion in the stratosphere?

A) Increase in nitrogen levels
B) Release of active chlorine due to PSCs
C) High levels of carbon dioxide
D) Formation of water vapor clouds

Answer: (B)

Explanation: Polar stratospheric clouds provide a surface for reactions that release active chlorine, which plays a major role in ozone depletion.

2. UPSC CSE Mains 2020 (GS Paper 3):

Question: "Explain the role of international treaties like the Montreal Protocol in mitigating ozone depletion. What challenges remain in ensuring the complete recovery of the ozone layer?"

Answer: The Montreal Protocol was pivotal in mitigating ozone depletion by phasing out the production and use of ozone-depleting substances (ODS). Its success is evident in the reduced presence of ODS in the atmosphere and signs of recovery in the ozone layer, particularly over the polar regions. Challenges that remain include the illegal production of certain ODS, the potential release of these substances from old equipment, and the slow pace of recovery due to the long atmospheric lifespan of some chemicals. Continuous international cooperation and stricter enforcement of regulations are necessary to ensure the complete recovery of the ozone layer.