Botany · Ch 8 — Environmental Issues
Ozone Depletion in the Stratosphere
Ozone Depletion in the Stratosphere
"Good" ozone versus "bad" ozone
Ozone formed near the ground, in the troposphere, is a harmful pollutant that damages plants and animals — this is the "bad" ozone studied earlier in chemistry. But ozone in the upper atmosphere, in a layer called the stratosphere, is entirely different in effect: this "good" ozone acts as a protective shield, absorbing dangerous ultraviolet (UV) radiation from the Sun before it reaches the surface. UV radiation is highly damaging to living organisms because DNA and proteins readily absorb it, and the high energy it carries can break the chemical bonds holding these molecules together. The total amount of ozone in a vertical column of air, from the ground to the top of the atmosphere, is measured in Dobson units (DU).
How ozone is made and lost — and why the balance has been disrupted
Ozone in the stratosphere is continually being both formed (by the action of UV radiation on ordinary molecular oxygen) and broken down again, and under natural conditions these two processes stay roughly balanced. This balance has been disturbed by chlorofluorocarbons (CFCs), industrial chemicals widely used as refrigerants. Once released, CFCs drift upward from the lower atmosphere into the stratosphere, where UV radiation breaks them apart, releasing free chlorine atoms. Each chlorine atom can break down ozone into ordinary molecular oxygen while itself acting only as a catalyst — it is not consumed in the reaction, so a single chlorine atom can go on destroying ozone molecules again and again. This means that any CFCs added to the stratosphere have a long-lasting, cumulative effect on ozone levels. Although ozone depletion is a global phenomenon, it is most pronounced over Antarctica, where it has produced a large area of severely thinned ozone commonly called the ozone hole.
- cover: 16.8 Ozone hole over Antarctica — a false-colour satellite image looking straight down over the South Pole shows concentric bands of colour, from deep purple/violet at the centre over Antarctica (the thinnest ozone) through blue, green/yellow, to orange/red at the outer edge (thicker ozone); the caption notes the hole forms each year between late August and early October.
Effects of ozone depletion on health …
What this figure shows. A false-colour satellite image of Antarctica viewed from directly above the South Pole, shaded in concentric bands from the centre outward: a large deep-purple/violet region in the middle (over the Antarctic landmass, whose dark outline is faintly visible beneath it) representing the thinnest ozone, surrounded successively by rings of blue, then green/yellow, then orange/red at the outer edge, representing progressively thicker ozone. Caption text beneath reads: 'Ozone hole is the area above Antarctica, shown in purple colour, where the ozone layer is the thinnest. Ozone thickness is given in Dobson unit (see carefully the scale shown in colour violet to red). The ozone hole over Antarctica develops each year between late August and early October. Courtesy: …