Improved quantification of Chinese carbon fluxes using CO 2 /CO correlations in Asian outflow

We use observed CO 2 :CO correlations in Asian outflow from the TRACE‐P aircraft campaign (February–April 2001), together with a three‐dimensional global chemical transport model (GEOS‐CHEM), to constrain specific components of the east Asian CO 2 budget including, in particular, Chinese emissions. The CO 2 /CO emission ratio varies with the source of CO 2 (different combustion types versus the terrestrial biosphere) and provides a characteristic signature of source regions and source type. Observed CO 2 /CO correlation slopes in east Asian boundary layer outflow display distinct regional signatures ranging from 10–20 mol/mol (outflow from northeast China) to 80 mol/mol (over Japan). Model simulations using best a priori estimates of regional CO 2 and CO sources from Streets et al. [2003] (anthropogenic), the CASA model (biospheric), and Duncan et al. [2003] (biomass burning) overestimate CO 2 concentrations and CO 2 /CO slopes in the boundary layer outflow. Constraints from the CO 2 /CO slopes indicate that this must arise from an overestimate of the modeled regional net biospheric CO 2 flux. Our corrected best estimate of the net biospheric source of CO 2 from China for March–April 2001 is 3200 Gg C/d, which represents a 45% reduction of the net flux from the CASA model. Previous analyses of the TRACE‐P data had found that anthropogenic Chinese CO emissions must be ∼50% higher than in Streets et al. 's [2003] inventory. We find that such an adjustment improves the simulation of the CO 2 /CO slopes and that it likely represents both an underreporting of sector activity (domestic and industrial combustion) and an underestimate of CO emission factors. Increases in sector activity would imply increases in Chinese anthropogenic CO 2 emissions and would also imply a further reduction of the Chinese biospheric CO 2 source to reconcile simulated and observed CO 2 concentrations.