Evaluation of SF6, C2Cl4, and CO to approximate fossil fuel CO2 in the Northern Hemisphere using a chemistry transport model

The distribution of the fossil fuel component in atmospheric CO₂ cannot be measured directly at a cheap cost. Could anthropogenic tracers with source patterns similar to fossil fuel CO₂ then be used for that purpose? Here we present and evaluate a methodology using surrogate tracers, CO, SF₆, and C₂Cl₄, to deduce fossil fuel CO₂. A three-dimensional atmospheric chemistry transport model is used to simulate the relationship between each tracer and fossil fuel CO₂. In summertime the regression slopes between fossil fuel CO₂ and surrogate tracers show large spatial variations for chemically active tracers (CO and C₂Cl₄), although C₂Cl₄ presents less scatter than CO. At two tall tower sites in the United States (WLEF, Wisconsin, and WITN, North Carolina), we found that in summertime the C₂Cl₄ (CO) versus fossil CO₂ slope is on average up to 15% (25%) higher than in winter. We show that for C₂Cl₄ this seasonal variation is due to OH oxidation. For CO the seasonal variation is due to both chemistry and mixing with nonanthropogenic CO sources. In wintertime the three surrogate tracers SF₆, C₂Cl₄, and CO are about equally as good indicators of the presence of fossil CO₂. However, our model strongly underestimates the variability of SF₆ at both towers, probably because of unaccounted for emissions. Hence poor knowledge of emission distribution hampers the use of SF₆ as a surrogate tracer. From a practical point of view we recommend the use of C₂Cl₄ as a proxy of fossil CO₂. We also recommend the use of tracers to separate fossil CO₂. Despite the fact that the uncertainty on the regression slope is on the order of 30%, the tracer approach is likely to have less bias than when letting one model with one inventory emission map calculate the fossil CO₂ distribution.