Volcanoes don't inject significant chlorine into the stratosphere but they do increase small particles, which increase the amount of polar stratospheric clouds with which the human - made chlorine reacts. (sciencedaily.com)
The chemical reactions that break down ozone are particularly intense within cold, acidified clouds called polar stratospheric clouds. (sciencemag.org)
The stable air that causes the ultra-cold conditions where polar stratospheric clouds form in Antarctica is much less likely. (greenbiz.com)