Atmos. Chem. Phys. Discuss., 8, 7339-7371, 2008
www.atmos-chem-phys-discuss.net/8/7339/2008/
doi:10.5194/acpd-8-7339-2008
© Author(s) 2008. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Interpreting the variability of CO2 columns over North America using a chemistry transport model: application to SCIAMACHY data
P. I. Palmer1, M. P. Barkley1, and P. S. Monks2
1School of GeoSciences, University of Edinburgh, UK
2Department of Chemistry, University of Leicester, UK

Abstract. We use the GEOS-Chem chemistry transport model to interpret variability of CO2 columns and associated column-averaged volume mixing ratios (CVMRs) observed by the SCIAMACHY satellite instrument during the 2003 North American growing season, accounting for the instrument averaging kernel. Model and observed columns, largely determined by surface topography, averaged on a 2°×2.5° grid, are in excellent agreement (model bias=3%, r>0.9), as expected. Model and observed CVMRs, determined by scaling column CO2 by surface pressure data, are on average within 3% but are only weakly correlated, reflecting a large positive model bias (10–15 ppmv) at 50–70° N during midsummer at the peak of biospheric uptake. GEOS-Chem generally reproduces the magnitude and seasonal cycle of observed CO2 surface VMRs across North America. During midsummer we find that model CVMRs and surface VMRs converge, reflecting the instrument vertical sensitivity and the strong influence of the land biosphere on lower tropospheric CO2 columns. We use model tagged tracers to show that local fluxes largely determine CVMR variability over North America, with the largest individual CVMR contributions (1.1%) from the land biosphere. Fuel sources are relatively constant while biomass burning make a significant contribution only during midsummer. We also show that non-local sources contribute significantly to total CVMRs over North America, with the boreal Asian land biosphere contributing close to 1% in midsummer at high latitudes. We used the monthly-mean Jacobian matrix for North America to illustrate that: 1) North American CVMRs represent a superposition of many weak flux signatures, but differences in flux distributions should permit independent flux estimation; and 2) the atmospheric e-folding lifetimes for many of these flux signatures are 3–4 months, beyond which time they are too well-mixed to interpret.

Citation: Palmer, P. I., Barkley, M. P., and Monks, P. S.: Interpreting the variability of CO2 columns over North America using a chemistry transport model: application to SCIAMACHY data, Atmos. Chem. Phys. Discuss., 8, 7339-7371, doi:10.5194/acpd-8-7339-2008, 2008.
 
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