This article is from the Ozone Depletion: Stratospheric Chlorine and Bromine FAQ, by Robert Parson email@example.com with numerous contributions by others.
~80% from CFC's and related manmade organic chlorine compounds,
such as carbon tetrachloride and methyl chloroform
~15-20% from methyl chloride (CH3Cl), most of which is natural.
A few % from inorganic sources, such as volcanic eruptions.
[Russell et al. 1996] [WMO 1991, 1994] [Solomon] [AASE]
[Rowland 1989,1991] [Wayne]
These estimates are based upon >20 years' worth of measurements of
organic and inorganic chlorine-containing compounds in the earth's
troposphere and stratosphere. Particularly informative is the
dependence of these compounds' concentrations on altitude and
their increase with time. The evidence is summarized in section 2
of this FAQ.