Making and losing ozone
Ultraviolet (UV) light has a shorter wavelength and more energy than visible light. Nitrogen, oxygen and atomic oxygen absorb UV shorter than 240 nm; ozone absorbs UV from 240 to 310 nm. Ozone, O₃, forms in two steps. Step 1, photodissociation: UV shorter than 240 nm breaks an oxygen molecule, O₂ + UV → O + O. Step 2: an oxygen atom joins an oxygen molecule, O + O₂ → O₃ (this step can go backwards). Ozone depletion is the thinning of the layer, mainly by chlorine and bromine compounds such as CFCs from industry. More UV then reaches the ground, which can damage the DNA of skin cells (and may lead to skin cancers), weaken immune cells and harm the lens of the eye (cataract). The “ozone hole” is not a hole in the sky; it is a region of much thinner ozone over Antarctica. It is not the same thing as global warming. The Montreal Protocol banned many ozone-depleting chemicals (1987), and the layer has begun to recover. Everyday help: avoid products that contain CFCs or halons, and in strong sun use shade, clothes, a hat and sunglasses; never look at the Sun. Note the other side of ozone: high up in the stratosphere it shields us, but in the troposphere ozone forms from nitrogen oxides and hydrocarbons in sunlight, helps make smog, and harms leaves, crops, plastic and rubber.
| Wavelength | What happens |
|---|---|
| 200 nm | breaks O₂ |
| 280 nm | absorbed by ozone |
| 350 nm | not absorbed, reaches the ground |
Bead or paper-clip model, with a grown-up. Use 2 beads of one colour joined by a clip for O₂. Pull the two beads apart: these are two O atoms (step 1). Join one loose bead to another O₂ pair to make a three-bead chain O₃ (step 2). Count the beads each time; the number never changes. Then make a sun-safety plan with five items (shade, clothes, hat, sunglasses, sunscreen as the label says), without any advice on tanning.