From AMS Glossary
The most abundant single halocarbon, formula CH3Cl, found in the atmosphere, with a mixing ratio of about 600 parts per trillion (by volume) in the troposphere.
This compound is mostly of natural origin, as a result of production in the oceans. It provides the natural background amount of chlorine that was believed to be present in the preindustrial stratosphere and that will likely be present in the future following the phaseout of other chlorine source compounds (chlorofluorocarbons, etc.).