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Why did the ozone hole form over Antarctica during polar spring?

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Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

The Antarctic ozone hole forms during the polar spring due to a specific sequence of atmospheric conditions and chemical reactions involving chlorine and bromine.1

During the dark polar winter, the circumpolar vortex isolates the air over Antarctica, causing temperatures to drop low enough to form polar stratospheric clouds (PSCs). Chemical reactions occurring on the surface of these cloud particles convert stable, less-reactive chlorine compounds into highly reactive forms, such as molecular chlorine, which accumulate throughout the polar night.1

When the sun returns in the early spring, sunlight triggers the photodissociation of this accumulated molecular chlorine into single chlorine atoms. These atoms then engage in a catalytic cycle that rapidly strips oxygen atoms from ozone molecules, leading to the severe ozone depletion observed between September and November. This destruction persists until the polar vortex breaks up, allowing the ozone-depleted air to mix with air from outside the region.12