We use observed CO₂: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 CO₂ budget including, in particular, Chinese emissions. The CO₂/CO emission ratio varies with the source of CO₂ (different combustion types versus the terrestrial biosphere) and provides a characteristic signature of source regions and source type. Observed CO₂/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 CO₂ and CO sources from Streets et al. [2003] (anthropogenic), the CASA model (biospheric), and Duncan et al. [2003] (biomass burning) overestimate CO₂ concentrations and CO₂/CO slopes in the boundary layer outflow. Constraints from the CO₂/CO slopes indicate that this must arise from an overestimate of the modeled regional net biospheric CO₂ flux. Our corrected best estimate of the net biospheric source of CO₂ 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 CO₂/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 CO₂ emissions and would also imply a further reduction of the Chinese biospheric CO₂ source to reconcile simulated and observed CO₂ concentrations.