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

Abstract

Annual mean mixing ratios for the halocarbons CFC‐11 (CCl3F), CFC‐12 (CCl2F2), CFC‐113 (CClF2CCl2F), and carbon tetrachloride (CCl4) have been determined from their first year of industrial production through 1998. From the late 1970s (in the case of CFC‐11 and CFC‐12) or early 1980s (in the case of CFC‐113 and carbon tetrachloride) the reported mixing ratios have been determined from experimental observations made by the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gases Experiment program. For years prior to these times we have used estimates of industrial emissions and atmospheric lifetimes to calculate historic concentrations. The likely error bounds of the annual mean values are also reported here. Errors in the annual mean mixing ratio may primarily be a result of incorrect industrial emissions data, an incorrect atmospheric lifetime, or uncertainty in the ALE/GAGE/AGAGE observations. Each of these possible sources of error has been considered separately. These results show that atmospheric concentrations for each of these compounds have experienced a rapid rise in the early part of their production. It is only within the past decade that rise rates have decreased sharply and (except in the case of CFC‐12) in the past few years that atmospheric concentrations have begun to decrease. The uncertainties in the reconstructed histories are a similar proportion for each of the chlorofluorocarbons (<4% for most of the history). However, uncertainty in the history of carbon tetrachloride is much greater (up to 12%), and this is mainly the result of poor knowledge of CCl4 emissions.