How the Ozone Layer Protects Life

An ordinary oxygen molecule contains two oxygen atoms. High-energy ultraviolet radiation can split some of these molecules into separate oxygen atoms. A free oxygen atom can then join another oxygen molecule to form ozone, which contains three oxygen atoms.

Ozone absorbs ultraviolet radiation and can split back into an oxygen molecule and a free oxygen atom. These reactions create a moving balance. Ozone is continually formed, destroyed and formed again rather than remaining as one permanent shield.

Most atmospheric ozone is found in the stratosphere, roughly above the weather-filled lower atmosphere. Its concentration is small compared with ordinary oxygen, but it is enough to absorb much of the most damaging ultraviolet radiation.

Chlorofluorocarbons, often called CFCs, were once widely used in refrigeration, aerosols and foam production. They are stable in the lower atmosphere, so they can persist long enough to reach the stratosphere.

There, ultraviolet radiation can release chlorine atoms from the molecules. A chlorine atom can react with ozone and later be regenerated, allowing it to destroy many ozone molecules through a catalytic cycle. Bromine-containing chemicals can cause similar damage.

The strongest seasonal ozone loss developed over Antarctica. Extremely cold conditions help form polar stratospheric clouds, while isolated winter air allows reactive chemicals to build up. When sunlight returns in spring, rapid reactions produce the region known as the ozone hole.

Less stratospheric ozone allows more ultraviolet B radiation to reach the surface. Higher exposure can increase skin cancer and eye damage and can affect crops, plankton and other living things.

Countries responded through the Montreal Protocol, which reduced the production and use of many ozone-depleting substances. Because these chemicals can remain in the atmosphere for decades, recovery is slow even after emissions fall.

Keep the main story clear:

• •Ultraviolet light helps create ozone from oxygen.
• •Ozone absorbs ultraviolet radiation.
• •CFCs can release reactive chlorine in the stratosphere.
• •Chlorine repeatedly destroys ozone molecules.
• •Reducing harmful chemicals allows gradual recovery.

Ozone depletion and global warming are often merged into one vague environmental worry, and they are separate issues with separate chemistry. The ozone layer is about ultraviolet light and the stratosphere; warming is about infrared radiation trapped by carbon dioxide and methane in the lower atmosphere. CFCs happen to be involved in both, because they are also powerful greenhouse gases, but fixing one does not fix the other.

The distinction matters because the ozone story is the success story. Global CFC production fell by more than 99 percent after the Montreal Protocol, the Antarctic hole has begun to shrink, and current projections put the layer back to its 1980 state around 2040 over most of the world and around 2066 over Antarctica. It is the clearest evidence that a worldwide atmospheric problem can be solved once the source is identified and the alternatives exist.

Most of what we know about the ozone layer comes from measuring it from below. In the 1920s the Oxford physicist Gordon Dobson built a spectrophotometer that compares sunlight at ultraviolet wavelengths ozone absorbs strongly with nearby wavelengths it barely absorbs. The ratio reveals how much ozone lies in the column of air overhead. Results are reported in Dobson units: if all the ozone above you were squeezed down to sea-level pressure at 0 degrees Celsius, 100 Dobson units would make a layer just one millimetre thick. A typical total of around 300 is therefore a film only about three millimetres deep.

Ground stations in Antarctica made these measurements year after year. In 1985 scientists from the British Antarctic Survey, Joe Farman, Brian Gardiner and Jonathan Shanklin, published records from Halley station showing that springtime ozone had fallen sharply since the 1970s. Satellite data were then re-examined and showed that the thinning covered a region wider than the continent itself. Today an ozone hole is usually defined as an area where the total drops below 220 Dobson units. That simple threshold is what lets scientists report each year whether the hole was larger or smaller than before.

The danger from CFCs was predicted more than a decade before the Antarctic hole was found. In 1974 the chemists Mario Molina and Sherwood Rowland, working at the University of California, Irvine, published a paper arguing that these supposedly harmless gases would drift up to the stratosphere, break apart in ultraviolet light and release chlorine that could destroy ozone. Manufacturers disputed the claim, and evidence took years to build. In 1995 Molina and Rowland shared the Nobel Prize in Chemistry with Paul Crutzen, who had shown how nitrogen oxides also affect ozone.

Yet the real damage did not look like the prediction. Early models expected slow, fairly even thinning across the globe, not a sudden springtime collapse over one continent, which is why the 1985 measurements came as such a shock. Within a couple of years, researchers including the atmospheric chemist Susan Solomon showed that reactions on the surfaces of polar stratospheric cloud particles convert chlorine from stable reservoir compounds into ones that sunlight breaks apart quickly. The Montreal Protocol was agreed in 1987, only two years after the hole was reported, and has since been ratified by every member of the United Nations.

Ozone at ground level is not released directly from exhaust pipes or chimneys. It forms when nitrogen oxides, produced mainly by vehicles and power stations, react with volatile organic compounds from fuels, solvents and even trees in the presence of sunlight. That is why ground-level ozone tends to peak on hot, sunny summer afternoons, often downwind of cities rather than in their centres. In busy streets, fresh exhaust contains nitric oxide, which reacts with ozone and removes some of it, so rural areas downwind can record higher ozone levels than the city that produced the ingredients.

The chemistry was worked out in the early 1950s by Arie Haagen-Smit, a Dutch-born chemist at the California Institute of Technology, who was investigating the brown smog that hung over Los Angeles. He showed that car exhaust and sunlight together produced ozone and other irritants, a finding that later shaped California's air quality rules and the spread of catalytic converters. Ground-level ozone irritates the lungs, worsens asthma and damages leaves and crops. It cannot simply drift up to repair the ozone layer, because it reacts and breaks down within a few weeks, long before it could reach the stratosphere in useful amounts.

The ozone layer protects life by absorbing harmful ultraviolet radiation in the stratosphere. Human-made chemicals disrupted its natural balance, but coordinated international action reduced the main sources of damage. Separate protective stratospheric ozone from polluted ground-level ozone, and the issue becomes much easier to understand.