It's
carbon released from the chlorofluorocarbons (CFC, a.k.a. freon) that reacts with ozone (O3, as opposed to normal O2 oxygen). If freon stayed together, it wouldn't get into the ozone layer, but it doesn't stay together.
For a rebuttal of this theory, see http://www.thenewamerican.com/tna/1994/vo10no23.htm