Question 16: Has the Montreal Protocol been successful in reducing ozone-depleting substances in the atmosphere? Deniz Ural

YES! ODS* decrease after Montreal Protocol (MP). ODS reduction depends on how rapidly an ODS is used and released to the atmosphere after being produced the lifetime (τ) for the removal of the ODS from the atmosphere. Short τ  faster removal and no storage EESC**: Measure of success of MP. Measure of potential Ozone depletion in the stratosphere that can be calculated from atmospheric surface abundance of ODS and natural chlorine and bromine containing gasses. How to calculate: measurements, past values, projections Long term trend: 1950-1990: steady increase, after MP: slow down of increase and started to decrease. Back to 1980 values will take several decades. * ODS: Ozone depleting substance ** EESC: Effective Equivalent Stratospheric Chlorine

CFC: τ = 45-100 years. Ended in 1996 (developed countries) and 2010 (developing countries). Halons: Bromine containing ODS. τ = 65 years. Ended in 1994 and 2010. Methyl Chloroform: τ = 5 years. No storage. Ended in 1996 and 2015. HCFC substitute gases: Lesser threat to Ozone. Increasing trend. Phase out in 2020 and 2030. Carbon tetrachloride: Phased out in 1996 and 2010. Less rapid decrease then expected.  either larger than reported emissions or τ is longer than estimated. Methyl chloride methyl bromide: distinct among halogen source gases because substantial fractions of their emissions are associated with natural processes.