Re-evaluation of the lifetimes of the major CFCs and CH₃CCl₃ using atmospheric trends

Abstract. Since the Montreal Protocol on Substances that Deplete the Ozone Layer and its amendments came into effect, growth rates of the major ozone depleting substances (ODS), particularly CFC-11, -12 and -113 and CH₃CCl₃, have declined markedly, paving the way for global stratospheric ozone recovery. Emissions have now fallen to relatively low levels, therefore the rate at which this recovery occurs will depend largely on the atmospheric lifetime of these compounds. The first ODS measurements began in the early 1970s along with the first lifetime estimates calculated by considering their atmospheric trends. We now have global mole fraction records spanning multiple decades, prompting this lifetime re-evaluation. Using surface measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) and the National Oceanic and Atmospheric Administration Global Monitoring Division (NOAA GMD) from 1978 to 2011, we estimated the lifetime of CFC-11, CFC-12, CFC-113 and CH₃CCl₃ using a multi-species inverse method. A steady-state lifetime of 45 yr for CFC-11, currently recommended in the most recent World Meteorological Organisation (WMO) Scientific Assessments of Ozone Depletion, lies towards the lower uncertainty bound of our estimates, which are 54₄₈⁶¹ yr (1-sigma uncertainty) when AGAGE data were used and 52₄₅⁶¹ yr when the NOAA network data were used. Our derived lifetime for CFC-113 is significantly higher than the WMO estimates of 85 yr, being 109₉₉¹²¹ (AGAGE) and 109₉₇¹²⁴ (NOAA). New estimates of the steady-state lifetimes of CFC-12 and CH₃CCl₃ are consistent with the current WMO recommendations, being 111₉₅¹³² and 112₉₅¹³⁶ yr (CFC-12, AGAGE and NOAA respectively) and 5.04_4.92^5.20 and 5.04_4.87^5.23 yr (CH₃CCl₃, AGAGE and NOAA respectively).