Top-down Validation of Global and East Asian Emissions of Tetrafluoromethane and Hexaflurorethane

CF4 (tetrafluoromethane, PFC-14) and C2F6 (hexafluororethane, PFC-116) are powerful greenhouse gases (100-year Global Warming Potentials of 7,390 and 12,200, respectively), and are mostly emitted during aluminum manufacture and semiconductor production. Global emissions of these species were recently reported by Mühle et al. (2010) using in situ measurements of the Advanced Global Atmospheric Gases Experiment (AGAGE) combined with a global 12-box inversion model. The study found a discrepancy of up to ~6 Gg/yr for CF4 in recent years between their measurement derived (top-down) and industry-reported (bottom-up) emissions, and speculated that underestimated emissions from aluminum manufacture in China could be one of the sources of this discrepancy. In this study, we use in situ measurements taken at Cape Grim (Tasmania, Australia) and Gosan (Jeju Island, Korea) within AGAGE to estimate C2F6/CF4 emission ratios (ER) from aluminum manufacture (in Australia) and semiconductor production (in Korea), deriving ERs of 0.11 (by mass) and 0.42 (by mass), respectively. Assuming that all emissions of CF4 and C2F6 occur from just these two sources, industry-specific emissions of these compounds can be calculated from the following simple formulas: ECF4,global = ECF4,Al + ECF4,SC (1) EC2F6,global = (ECF4,Al)*ERAl + (ECF4,SC)*ERSC (2) whereby ECF4,global and EC2F6,global are global emissions of CF4 and C2F6 reported in Mühle et al. (2010), and RAl and RSC are the emission ratios of C2F6/CF4 derived for the aluminum industry and the semiconductor industry, respectively, thus solving for the global CF4 emissions from the aluminum industry (ECF4,Al) and semiconductor industry (ECF4,SC), respectively. Analysis of global CF4 emissions from 1990-2008 show that bottom-up emissions reported by the International Aluminum Institute (IAI, 2010) are in good agreement with top-down emissions derived for the aluminum industry until 2005, after which time the industry reports are underestimated by ~2.8 Gg. Furthermore, CF4 emissions from China's aluminum industry in 2008 are calculated using both the method described above (substituting the global emissions for Chinese emissions) and an interspecies correlation method, using Chinese emissions reported by Kim et al. (2010) and Saito et al. (2010). Results based on our methods suggest that China’s emissions may be underestimated in industry reports (IAI, 2010; 1.29 Gg/yr), and indicate that China may be a dominant source of discrepancy in the global budgets of CF4 emitted from aluminum production. A direct inversion of atmospheric measurements at Gosan to solve for Chinese emissions of CF4 is currently in progress, and should help reduce the uncertainties in our findings.