The Antarctic Ozone Hole + The Polar Vortex + Polar Stratospheric Clouds (PSCs) AND Chemistry of Antarctic Ozone Depletion

What is the Antarctic ozone hole

A major seasonal decrease in ozone concentrations over Antarctica.

When does the Antarctic ozone hole typically become apparent

During the Antarctic spring.

Why is the Antarctic ozone hole associated with cold temperatures

Extremely cold polar winter conditions promote the formation of polar stratospheric clouds and reactions that activate chlorine.

When was the largest Antarctic ozone hole shown in the lecture recorded

September 2006.

Why does the ozone hole appear in the polar spring rather than throughout the entire year

Chlorine-containing compounds accumulate during winter and are activated by sunlight when spring begins.

What happens to ozone concentrations during Antarctic ozone depletion

Ozone concentrations can become extremely low, especially between approximately 15–20 km.

What is the polar vortex

A circulation of strong winds that forms around the polar region and traps air over Antarctica.

When are Antarctic stratospheric temperatures extremely cold

During the polar winter, approximately June to September.

How cold can the lower Antarctic stratosphere become during polar winter

Below approximately −90°C.

Why does the Antarctic stratosphere become extremely cold during polar winter

The absence of sunlight reduces exothermic photochemical reactions that would otherwise warm the region.

What happens to air during the Antarctic polar winter

It cools and descends toward Earth.

What do the winds of the polar vortex do

They trap air over Antarctica and limit mixing with surrounding air.

Why is the polar vortex important for ozone depletion

It isolates the cold polar air mass, allowing conditions that promote ozone depletion to develop.

What are polar stratospheric clouds (PSCs)

Clouds formed when extremely cold temperatures cause water vapour and other substances in the stratosphere to condense.

At what altitude do PSCs occur

Approximately 10–25 km, where ozone depletion occurs.

What are the two types of polar stratospheric clouds

Type I and Type II.

What are Type I PSCs made of

Water (H₂O), nitric acid (HNO₃), and sulfuric acid (H₂SO₄).

At what temperature do Type I PSCs form

At or below approximately 195 K.

What are Type II PSCs made of

Solid water ice (H₂O), possibly with some nitric acid.

At what temperature do Type II PSCs form

At or below approximately 188 K.

How do PSCs contribute to ozone depletion

They provide surfaces where chlorine reservoir species can react to form active chlorine-containing compounds.

Why are reactions on PSCs important

Reactions that are very slow in the gas phase can occur much more quickly on cloud surfaces.

What is a heterogeneous reaction

A reaction that occurs between substances in different phases or on a surface.

How do PSCs activate chlorine reservoir species

They provide surfaces for reactions that convert reservoir compounds into active chlorine-containing compounds.

What happens when chlorine nitrate reacts with water on PSCs

HOCl and HNO₃ are formed.

What is the reaction between water and chlorine nitrate on PSCs

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

What happens when hydrogen chloride reacts with chlorine nitrate on PSCs

Cl₂ and HNO₃ are formed.

What is the reaction between HCl and ClONO₂ on PSCs

HCl(aq) + ClONO₂(g) → Cl₂(g) + HNO₃(s).

What happens to molecular chlorine during the polar winter

It accumulates until sunlight returns in the spring.

What happens when sunlight returns to Antarctica in October

Cl₂ and HOCl undergo photolysis and release reactive chlorine radicals.

What is the photolysis reaction of chlorine gas

Cl₂ + hν → 2Cl•.

What is the photolysis reaction of hypochlorous acid

HOCl + hν → Cl• + OH•.

How does atomic chlorine destroy ozone during Antarctic spring

It participates in Mechanism II, converting ozone into molecular oxygen.

What is the net reaction for chlorine-driven Mechanism II

2O₃ + hν → 3O₂.

What is the role of chlorine monoxide (ClO•) during Antarctic ozone depletion

It participates in the catalytic cycle that regenerates chlorine radicals and destroys ozone.

What is chlorine dimer (ClOOCl)

A compound formed when two chlorine monoxide radicals combine in the chlorine-catalyzed ozone-destruction cycle.

What happens to ClOOCl when it absorbs sunlight

It undergoes photolysis, helping regenerate reactive chlorine radicals.

What happens to trapped nitric acid during Antarctic spring

It can undergo photolysis to produce NO₂• and OH•.

What is the photolysis reaction of nitric acid

HNO₃ + hν → NO₂• + OH•.

What happens when temperatures rise in spring and summer

PSCs dissipate and the conditions supporting rapid ozone depletion weaken.

What happens to reactive chlorine after PSCs disappear

Chlorine radicals can be converted back into reservoir species such as HCl and ClONO₂.

When does Antarctic ozone depletion generally persist

Through spring and into summer, until temperatures rise and PSCs dissipate.

How does Arctic ozone depletion compare with Antarctic ozone depletion

Arctic ozone depletion can occur, but the conditions are generally less consistently cold than in Antarctica.

Why is Arctic ozone depletion more variable

Arctic stratospheric temperatures and atmospheric circulation vary from year to year.

What conditions are needed for substantial polar ozone depletion

Very cold stratospheric temperatures, PSC formation, and sunlight to activate chlorine radicals.