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 ozone (O-3) 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 O-3 levels (40-60 ppb) were seen during the dry-to-wet transition to the east and south of Manaus, where biomass burning emissions of O-3 precursors were present. Chemistry simulations with the CCATT-BRAMS and WRF-Chem models are within the error bars of the observed O-3 profiles in the boundary layer (0-3 km a.s.l.) in polluted conditions. However, the models overestimate O-3 in the boundary layer in clean conditions, despite lacking the predominant NO source from soil. In addition, O-3 simulated by the models was either within the error bars or lower than BARCA observations in mid-levels (3-5 km a.s.l.), and lower than total tropospheric O-3 retrieved from the OMI/MLS instruments, which is primarily comprised of middle troposphere O-3 and thus reflects long-range transport processes. Therefore, the models do a relatively poor job of representing the free troposphere-boundary layer gradient in O-3 compared with aircraft and satellite observations, which could be due to missing long-range and convective transport of O-3 at mid-levels. Additional simulations with WRF-Chem showed that the model O-3 production is very sensitive to both the O-3 deposition velocities and the NOx emissions, which were both about one-half of observed values. These results indicate the necessity of more realistic model representations of emissions, deposition, and convective processes for accurate monitoring and prediction of increases in O-3 production in the Amazon Basin as the regional population grows.