Variations of the concentrations and stable isotope ratios of atmospheric CO2 contain information about sources and sinks at the underlying surface. We have investigated mesoscale variations of atmospheric CO2 over a heterogeneous landscape of forests, pastures, and large rivers during the Santarém Mesoscale Campaign (SMC) during August, 2001. We simulated the variations of surface fluxes and atmospheric concentrations of CO2 using the CSU Regional Atmospheric Modeling System (RAMS) on a multiply-nested grid which included a 1-km inner grid centered on the Flona Tapajos. Surface fluxes of CO2 were prescribed in the model using idealized diurnal cycles over forested and pasture vegetation, and over surface water using a value suggested by in-situ measurements in the Amazôn River. Land vegetation cover was prescribed using AVHRR NDVI data. Atmospheric winds and structure and boundary-layer depth were compared to observations made by radiosondes at Belterra and by SODAR at Santarém.

Mesoscale circulations were simulated in the vicinity of both the Amazôn and Tapajos Rivers on most days, with magnitudes of 1-2 m s-1 near the surface. These "riverbreeze" circulations were also present in observations made in the field. Simulated CO2 concentrations were perturbed by over 10 ppm in the immediate vicinity of the rivers, with the strongest effect in the early morning. By midafternoon, the effect of the river evasion fluxes on simulated concentrations was mixed through a deeper layer and influenced by the riverbreeze, but still easily measurable.