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.
Constraints on regional carbon fluxes from CO2:CO correlations in Asian outflow
The left-hand panel shows a range of regional CO2/CO emissions ratios, (plotted as slopes) based on bottom-up emissions inventories. The CO2/CO emission ratio varies with the source of CO2 (e.g., differences in combustion efficiency vs. terrestrial biospheric flux) and provides a characteristic signature of source regions and source type. The right-hand panel shows CO2:CO correlations in Asian outflow measured on DC-8 Flight 16 (March 29, 2001) of the TRACE-P campaign out of Japan. Distinct correlations were observed, typical of the majority of TRACE-P flights. Highest CO2:CO slopes were observed over Japan (where CO emissions are regulated) and lower slopes characterized Chinese boundary layer outflow. The analysis of Suntharalingam et al. [2004] employs these observed CO2:CO correlations from the TRACE-P aircraft campaign together with a 3-D global chemical transport model (GEOS-CHEM), to constrain specific components of the East Asian CO2. Model simulations using best a priori estimates of regional CO2 and CO sources overestimate CO2 concentrations and CO2:CO slopes in Asian boundary layer outflow. Constraints from observed CO2:CO slopes indicate that this must arise from an overestimate of the modeled Chinese biospheric CO2 flux. Our corrected best estimate of the net biospheric source of CO2 from China for March-April 2001 is 3200 Gg C/day, which represents a 45% reduction of the a priori flux.
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