Interpreting the variability of space-borne CO₂ column-averaged volume mixing ratios over North America using a chemistry transport model

Abstract. We use the GEOS-Chem chemistry transport model to interpret the sources and sinks of CO₂ that determine variability of column-averaged volume mixing ratios (CVMRs), as observed by the SCIAMACHY satellite instrument, during the 2003 North American growing season. GEOS-Chem generally reproduces the magnitude and seasonal cycle of observed CO₂ surface VMRs across North America and is quantitatively consistent with column VMRs in later years. However, it cannot reproduce the magnitude or variability of FSI-WFM-DOAS SCIAMACHY CVMRs. 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 makes 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. These long lifetimes will improve the efficacy of observed CVMRs as surface CO₂ flux constraints.