Chlorine in the stratosphere is believed to come primarily from cfcs rather than from natural sources such as volcanoes because:

Chlorine in the stratosphere is believed to come primarily from CFCs rather than from natural sources such as volcanoes because the rise in the concentration of chlorine present in the stratosphere coincides with the rise in the concentration of CFCs in the same period of time. Also, there are studies which measured samples of air from volcanoes which shows that the volcanoes have a very small contribution in the amount of chlorine found in the stratosphere.

he answer is because samples of air taken over erupting volcanoes shows that volcanoes contribute a small amount of chlorine in the stratosphere compared to CFCs. Volcanic eruptions account for a large instability of chlorine from land to the atmosphere on a yearly basis. This is in addition to chlorine that enters the atmosphere from sea spray, industrial processes and biological gases which are from CFCs. All of these inputs happen near or at the base of the atmosphere. Very little of the material emitted from volcanoes makes it up into the upper reaches of our atmosphere which is the stratosphere where it could touch the ozone layer. However, most of it is believed to be deposited lower down which is in the troposphere, where it then rained out back to the surface of the earth.