Reconstructed histories of the annual mean atmospheric mole fractions for the halocarbons CFC-11, CFC-12, CFC-113, and carbon tetrachloride

Annual mean mixing ratios for the halocarbons CFC-11 (CCl3F), CFC-12 (CCl2F 2), CFC-113 (CClF2CCl2F), and carbon tetrachloride (CCl4) have been determi ned from their first year of industrial production through 1998. From the l ate 1970s (in the case of CFC-11 and CFC-12) or early 1980s tin the case of CFC-113 and carbon tetrachloride) the reported mixing ratios have been det ermined from experimental observations made by the Atmospheric Lifetime Exp eriment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gas es Experiment program. For years prior to these times we have used estimate s of industrial emissions and atmospheric lifetimes to calculate historic c oncentrations. The likely error bounds of the annual mean values are also r eported here. Errors in the annual mean mixing ratio may primarily be a res ult of incorrect industrial emissions data, an incorrect atmospheric lifeti me,or uncertainty in the ALE/GAGE/AGAGE observations. Each of these possibl e sources of error has been considered separately. These results show that atmospheric concentrations for each of these compounds have experienced a r apid rise in the early part of their production. Tt is only within the past decade that rise rates have decreased sharply and (except in the case of C FC-12) in the past few years that atmospheric concentrations have begun to decrease. The uncertainties in the reconstructed histories are a similar pr oportion for each of the chlorofluorocarbons (<4% for most of the history). However, uncertainty in the history of carbon tetrachloride is much greate r (up to 12%, and this is mainly the result of poor knowledge of CCl4 emiss ions.