The Amazon Basin has long been thought of as a carbon sink which potentially mitigates some of the effects of rising CO₂ levels. But the scientific literature shows significant uncertainty when it comes to answering this question. To determine whether or not the Brazilian Amazon Basin (BAB) is a sink of carbon, I participated in the Balanço Atmosférico Regional de Carbono na Amazônia (BARCA) aircraft program which occurred in November 2008 and May 2009. The main goals of this experiment were to estimate the carbon budget for the BAB and to determine if regional aircraft experiments can provide strong constraints for the budget. BARCA encompassed the dry to wet and wet to dry transition seasons respectively and provided approximately 150 vertical profiles covering the BAB.

I estimated the carbon budget for the BAB by using a Lagrangian Particle Dispersion Model (STILT) to integrate satellite data, aircraft data, surface observations, and mesoscale meteorological fields to link bottom-up and top-down models to provide constraints and error bounds for regional CO₂ fluxes. I adapted and optimized a biosphere model (VPRM) to compute hourly a priori CO₂ fluxes for 7 tropical vegetation types and determined that shortwave radiation is the key driver that must be validated. I compared shortwave radiation from 3 mesoscale models to satellite-derived and in situ measurements and found large biases that affected the flux calculations. Finally, I applied the BARCA aircraft mixing ratios as a top-down constraint in an inversion that solved for parameters that control the flux calculation. The a posteriori estimates of the carbon budget of the BAB suggest a net source of CO₂ during the BARCA intensives.