Re-evaluation of the lifetimes of the major CFCs and CH[subscript 3]CCl[subscript 3] 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 CH[subscript 3]CCl[subscript 3], 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 CH[subscript 3]CCl[subscript 3] 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 54[61 over 48] yr (1-sigma uncertainty) when AGAGE data were used and 52[61 over 45] 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 109[121 over 99] (AGAGE) and 109[124 over 97] (NOAA). New estimates of the steady-state lifetimes of CFC-12 and CH[subscript 3]CCl[subscript 3] are consistent with the current WMO recommendations, being 111[132 over 95] and 112[136 over 95] yr (CFC-12, AGAGE and NOAA respectively) and 5.04[5.20 over 4.92] and 5.04[5.23 over 4.87] yr (CH[subscript 3]CCl[subscript 3], AGAGE and NOAA respectively).