Ozone production and transport over the Amazon Basin during the dry-to-wet and wet-to-dry transition seasons

The Regional Carbon Balance in Amazonia (BARCA) campaign provided the first Amazon Basin-wide aircraft measurements of O3 during both the dry-to-wet (November and December 2008) and wet-to-dry (May 2009) transition seasons. Extremely low background values (<20 ppb) were observed to the west and north of Manaus in both seasons and in all regions during the wet-to-dry transition. On the other hand, elevated O3 levels (40–60 ppb) were seen during the dry-to-wet transition to the east and south of Manaus, where biomass burning emissions of O3 precursors were present. Chemistry simulations with the CCATT-BRAMS and WRF-Chem models are within the error bars of the observed O3 profiles in the boundary layer (0–3 km a.s.l.) in polluted conditions. However, the models overestimate O3 in the boundary layer in clean conditions, despite lacking the predominant NO source from soil. In addition, O3 simulated by the models was either within the error bars or lower than BARCA observations in mid-levels (3–5 km a.s.l.), indicating that the models do not represent the free troposphere – boundary layer gradient in O3. Total tropospheric O3 retrieved from OMI/MLS was higher than that simulated by the models, suggesting that the satellite observations are dominated by the middle troposphere and long-range processes and are not a~good indication of O3 conditions in the PBL. Additional simulations with WRF-Chem showed that the model O3 production is very sensitive to both the O3 deposition velocities, which were about one half of observed values, and the NOx emissions. These results have implications for the monitoring and prediction of increases in O3 production in the Amazon Basin as the regional population grows.