In this chapter, we have explored the topic of stratospheric photochemistry. This included basic ozone creation and loss reactions known as the Chapman cycle. We then looked at ultraviolet radiation shielding by ozone. It is this shielding action which makes life possible on Earth's surface by blocking the biologically destructive ultraviolet light from the Sun. In the process of blocking UV radiation, ozone undergoes photolysis. This photolysis varies by altitude, latitude, season, and time of day, all of which we explored.
The next section dealt with catalytic loss cycles of ozone by various reactive chemical species. These species are grouped into families, including reactive hydrogen, nitrogen, chlorine, and bromine. We examined each of the catalytic cycles associated with these reactive compounds. Chlorine chemistry is responsible for Antarctic ozone loss. The source of this chlorine are the manmade chlorofluorocarbons (CFCs).
The last section dealt with the heterogeneous chemistry, which refers to chemical reactions occurring in multiple phases: gas, liquid, and solid state. Heterogeneous chemistry in the stratosphere is driven by the existence of Polar Stratospheric Clouds (PSCs) that form under very cold winter time conditions inside the polar vortex at the poles, though principally at the colder, more isolated South Pole. PSCs allow the heterogeneous reactions to occur that create chlorine species that are photolyzed to form the reactive ClO molecule and Cl atoms. They sequester or deactivate reactive nitrogen into nonreactive HNO3, which exists as a solid on the surfaces of the PSCs. When the PSC cloud particles settle out of the stratosphere, they carry the nitrogen with them. This leaves behind chlorine compounds which are photolyzed by solar radiation in the springtime, creating reactive chlorine species that destroy catalytically ozone. This occurs principally in the Antarctic, as conditions are more favorable there than in the Arctic.
We can calculate an expression for ozone loss due to ClO. By making a few reasonable assumptions, we can show that for preindustrial Clx values, the time scale for significant ozone loss was too long for an ozone hole to form over the Antarctic even in the presence of PSCs, while current Clx values, the time scale is on the order of 2 to 3 weeks. This is why the springtime Antarctic ozone hole likely did not exist in the past, and why it exists today.