A phenomenon discovered in the mid-1980s that occurs in the winter–spring lower stratosphere over Antarctica. Through a sequence involving heterogeneous chemistry on polar stratospheric clouds and (intermittent) illumination by sunlight, much or all of the ozone in the lower stratosphere can be photochemically destroyed. Halogen species (chlorine and bromine) contained in fairly robust molecules are transformed via heterogeneous reactions into molecules that are easily photolyzed resulting in atomic or monoxide halogens that lead to chemical destruction of ozone. This phenomenon also occurs over the Arctic, although to a lesser extent because of a lower incidence of polar stratospheric clouds.