Susan Solomon’s work led to confirmation that chlorine-containing compounds can deplete ozone. In particular, she explained why this depletion was focused over the poles. This graphic looks at how ozone depletion happens. Below, we’ll look in more detail at Solomon’s contributions.
The molecule ozone gives the ozone layer its name. Ozone is composed of three oxygen atoms – one more than oxygen gas. The ozone layer is 20-30 kilometres above Earth’s surface and absorbs a large amount of the ultraviolet radiation from the Sun. This is crucial for life on Earth; the ultraviolet radiation screened out by the ozone layer would otherwise cause genetic damage.
Chemists considered the reactions that create and destroy ozone as far back as in the 1930s. However, only in the 1970s did we start to consider the effect humans might have on the ozone layer. In particular, scientists identified man-made chlorofluorocarbons (CFCs) as possible ozone depleters.
CFCs are compounds often known by their collective brand name, Freon. They were widely used as refrigerants, propellants in aerosols, and solvents. Though used in part because of their low reactivity, scientists worried that they might react with ozone high in the atmosphere.
These fears seemed to be confirmed in the early 1980s. Observations showed that ozone levels in the stratosphere above Antarctica were up to 35% lower compared to levels in the 1960s. The drop suggested that something had caused an alarming change – but if it was CFCs, why was it focused above Antarctica? Surely, if these man-made gases were to blame, we’d expect to see drops in ozone levels across the globe?
This is where Solomon came in. Previously, she had been investigating how CFCs could lead to ozone depletion. Though CFCs are usually very unreactive, when they reach the stratosphere the molecules are broken apart by UV radiation. This produces chlorine atoms with unpaired electrons, termed chlorine radicals.
A radical is extremely reactive. Chlorine radicals produced by the breakdown of CFCs rapidly react with ozone molecules. This reaction produces an oxygen molecule and a chlorine oxide radical (ClO). The chlorine oxide radical reacts with another ozone molecule, forming two oxygen molecules and regenerating a chlorine radical – which can then react with more ozone. This chain reaction can cycle repeatedly, leading to significant depletion of ozone in the atmosphere.
Solomon theorised that polar stratospheric clouds might explain why ozone depletion was greater over the poles. She predicted that the solid surface provided by the ice particles in these clouds could help speed up the reactions.
To put this theory to the test, Solomon led expeditions to Antarctica to gather direct evidence. If CFCs were responsible, then there should be high levels of chlorine oxide radicals where there was also ozone depletion. During these expeditions, Solomon and her team were able to gather enough evidence to show that this was the case.
This was huge: the first direct evidence that CFCs were responsible for the developing hole in the ozone layer. Solomon’s work, along with the theoretical work of other scientists which had preceded it, led to the Montreal Protocol. This international treaty, created in 1987, protects the ozone layer by phasing out man-made chemicals known to deplete it – including CFCs.
Work on CFCs and ozone depletion led to Nobel Prizes for scientists who had worked on the original theories. For her part, Solomon received the National Medal of Science from the U.S. government, and also has a glacier in Antarctica named after her. She continued working on ozone-related science – most recently publishing a paper showing the positive effects on the Montreal Protocol on the Antarctic ozone layer.
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References & Further Reading
- Stratospheric ozone depletion – a review of concepts and history – S Solomon
- Ozone depletion – uncovering the hidden hazard of hairspray – University of California, Berkeley
- Emergence of healing in the Antarctic ozone layer – S Solomon
- Susan Solomon, saving the atmospheric ozone – DiscovHer
- Susan Solomon – Chemical Heritage Foundation