MARCH 2005: The Impact of Reduced Carbon Oxidation on the Atmospheric CO2 Distribution: Implications for Inverse Analyses

Spatial distribution of the chemical pump concentration
adjustment at the surface (annual mean). Units are ppm.

Recent inverse estimates of regional CO2 fluxes have assumed that the CO2 source from atmospheric oxidation of reduced carbon (CO, CH4, and non- methane volatile organic compounds) is released at the surface rather than distributed globally in the atmosphere. This produces a bias in the estimates of surface fluxes. We used a 3D atmospheric chemistry model (GEOS-CHEM) to evaluate the magnitude of this effect in modeled CO2 concentrations and flux estimates (the "chemical pump effect"). We find that resolving the 3-D structure of the atmospheric CO2 source, as opposed to emitting this reduced carbon as CO2 at the surface, yields a decrease in the modeled annual mean surface interhemispheric gradient (N- S) of 0.21 ppm. As seen in the above figure, larger adjustments (up to -0.6 ppm) are apparent on a regional basis in and downwind of regions of high reduced carbon emissions.

The interhemispheric difference of the concentration residuals (mean across all models) in the annual mean TransCom3 inversion of Gurney et al. [2002] was 2.3 ppm. This north-south difference implied a northern continental carbon sink of 2.2 Pg C per year (mean across all models). The chemical pump effect would decrease this interhemispheric difference of residuals by about 10%, and imply correspondingly lower northern continental carbon uptake in inverse analyses. We used TransCom3 simulations from three transport models to evaluate the implications for annual mean inversion estimates. The main impact was a systematic decrease in estimates of northern continental land uptake (i.e., by 0.22 to 0.26 Pg C per year). Our results, discussed in Suntharalingam et al. [2005], highlight the need for a realistic description of reduced carbon emission and oxidation processes in deriving inversion estimates of CO2 surface fluxes.