Atmospheric Trends and Radiative Forcings of CF^sub 4^ and C^sub 2^F^sub 6^ Inferred from Firn Air

The atmospheric histories of two potent greenhouse gases, tetrafluoromethane (CF...) and hexafluoroethane (...), have been reconstructed for the 20th century based on firn air measurements from both hemispheres. The reconstructed atmospheric trends show that the mixing ratios of both CF... and ... have increased during the 20th century by factors of ...2 and ...10, respectively. Initially, the increasing mixing ratios coincided with the rise in primary aluminum production. However, a slower atmospheric growth rate for CF... appears to be evident during the 1990s, which supports recent aluminum industry reports of reduced CF... emissions. This work illustrates the changing relationship between CF... and ... that is likely to be largely the result of both reduced emissions from the aluminum industry and faster growing emissions of ... from the semiconductor industry. Measurements of ... in the older firn air indicate a natural background mixing ratio of