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Why is ozone helpful high in the stratosphere but harmful near the ground?

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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.

Ozone ( $O_{3}$ ) is a vital component of the Earth's atmosphere, but its impact depends entirely on its location.

The Stratosphere: A Protective Shield

In the stratosphere, ozone occurs naturally and is essential for life on Earth. This ozone layer acts as a protective blanket that absorbs the Sun’s harmful ultraviolet radiation. By screening out these lethal rays, the ozone layer prevents significant injury to living organisms and has historically facilitated the migration of life from the oceans to land.312

The Ground: A Harmful Pollutant

Near the ground, in the troposphere, ozone is considered a major air pollutant and a primary ingredient in photochemical smog. It is not emitted directly but forms through chemical reactions between pollutants like nitrogen oxides and hydrocarbons in the presence of strong sunlight.2415

Because ozone is a powerful oxidizing agent, it is highly toxic and irritating to living tissue. Exposure to ground-level ozone can cause irritation of the eyes and mucous membranes, reduce respiratory capability, and exacerbate conditions like asthma.254