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

摘要

The Regional Carbon Balance in Amazonia (BARCA) campaign provided the firstAmazon Basin-wide aircraft measurements of ozone (O₃) during both thedry-to-wet (November and December 2008) and wet-to-dry (May 2009) transitionseasons. Extremely low background values (< 20 ppb) were observedto the west and north of Manaus in both seasons and in all regions during thewet-to-dry transition. On the other hand, elevated O₃ levels(40–60 ppb) were seen during the dry-to-wet transition to the east andsouth of Manaus, where biomass burning emissions of O₃ precursors werepresent. Chemistry simulations with the CCATT-BRAMS and WRF-Chem models arewithin the error bars of the observed O₃ profiles in the boundary layer(0–3 km a.s.l.) in polluted conditions. However, the models overestimateO₃ in the boundary layer in clean conditions, despite lacking thepredominant NO source from soil. In addition, O₃ simulated by the modelswas either within the error bars or lower than BARCA observations inmid-levels (3–5 km a.s.l.), and lower than total tropospheric O₃retrieved from the OMI/MLS instruments, which is primarily comprised ofmiddle troposphere O₃ and thus reflects long-range transport processes.Therefore, the models do a relatively poor job of representing the freetroposphere-boundary layer gradient in O₃ compared with aircraft andsatellite observations, which could be due to missing long-range andconvective transport of O₃ at mid-levels. Additional simulations withWRF-Chem showed that the model O₃ production is very sensitive to boththe O₃ deposition velocities and the NO_x emissions, whichwere both about one-half of observed values. These results indicate thenecessity of more realistic model representations of emissions, deposition,and convective processes for accurate monitoring and prediction of increasesin O₃ production in the Amazon Basin as the regional population grows.