<P> Chlorofluorocarbons (CFCs) and other halogenated ozone depleting substances (ODS) are mainly responsible for man - made chemical ozone depletion . The total amount of effective halogens (chlorine and bromine) in the stratosphere can be calculated and are known as the equivalent effective stratospheric chlorine (EESC). </P> <P> CFCs were invented by Thomas Midgley, Jr. in the 1930s . They were used in air conditioning and cooling units, as aerosol spray propellants prior to the 1970s, and in the cleaning processes of delicate electronic equipment . They also occur as by - products of some chemical processes . No significant natural sources have ever been identified for these compounds--their presence in the atmosphere is due almost entirely to human manufacture . As mentioned above, when such ozone - depleting chemicals reach the stratosphere, they are dissociated by ultraviolet light to release chlorine atoms . The chlorine atoms act as a catalyst, and each can break down tens of thousands of ozone molecules before being removed from the stratosphere . Given the longevity of CFC molecules, recovery times are measured in decades . It is calculated that a CFC molecule takes an average of about five to seven years to go from the ground level up to the upper atmosphere, and it can stay there for about a century, destroying up to one hundred thousand ozone molecules during that time . </P> <P> 1, 1, 1 - Trichloro - 2, 2, 2 - trifluoroethane, also known as CFC - 113a, is one of four man - made chemicals newly discovered in the atmosphere by a team at the University of East Anglia . CFC - 113a is the only known CFC whose abundance in the atmosphere is still growing . Its source remains a mystery, but illegal manufacturing is suspected by some . CFC - 113a seems to have been accumulating unabated since 1960 . Between 2010 and 2012, emissions of the gas jumped by 45 percent . </P> <P> Scientists have attributed ozone depletion to the increase of man - made (anthropogenic) halogen compounds from CFCs by combining observational data with computer models . These complex chemistry transport models (e.g. SLIMCAT, CLaMS--Chemical Lagrangian Model of the Stratosphere) work by combining measurements of chemicals and meteorological fields with chemical reaction rate constants . They identify key chemical reactions and transport processes that bring CFC photolysis products into contact with ozone . </P>

What is the natural process that is causing the depletion of the ozone layer