Methyl Chloride - CH₃Cl, this compound supplies chlorine to the stratosphere by occasional volcanic eruptions and by tropospheric to stratospheric transport. Methyl chloride is also produced by seaweed. The natural chlorine content of the stratosphere as a consequence of these sources is about 0.6 ppbv.
[Graedel, T. E. and Paul Crutzen. Atmospheric Change: An Earth System Prospective. W.H. Freeman and Company: New York, 1996: 143.] 
Atmospheric Chemistry Glossary