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. We examine processes driving the vertical distribution of biomass burningpollution following an integrated analysis of over 200 pollutant andmeteorological profiles measured in situ during the South AMerican BiomassBurning Analysis (SAMBBA) field experiment. This study will aid future workexamining the impact of biomass burning on weather, climate and air quality. During the dry season there were significant contrasts in the compositionand vertical distribution of haze between western and eastern regions oftropical South America. Owing to an active or residual convective mixinglayer, the aerosol abundance was similar from the surface to ∼1.5 km in the west and ∼3 km in the east. Black carbon massloadings were double as much in the east (1.7  µg m−3 ) than the west (0.85  µg m−3 ), but aerosol scattering coefficients at 550  nm weresimilar ( ∼120 Mm−1 ), as too were CO near-surfaceconcentrations (310–340  ppb ). We attribute these contrasts to the moreflaming combustion of Cerrado fires in the east and more smoulderingcombustion of deforestation and pasture fires in the west. Horizontal windshear was important in inhibiting mixed layer growth and plume rise, inaddition to advecting pollutants from the Cerrado regions into the remotetropical forest of central Amazonia. Thin layers above the mixing layerindicate the roles of both plume injection and shallow moist convection indelivering pollution to the lower free troposphere. However, detrainment oflarge smoke plumes into the upper free troposphere was very infrequentlyobserved. Our results reiterate that thermodynamics control the pollutantvertical distribution and thus point to the need for correct modelrepresentation so that the spatial distribution and vertical structure of biomassburning smoke is captured. We observed an increase of aerosol abundance relative to CO with altitudeboth in the background haze and plume enhancement ratios. It is unlikelyassociated with thermodynamic partitioning, aerosol deposition or localnon-fire sources. We speculate it may be linked to long-range transport fromWest Africa or fire combustion efficiency coupled to plume injection height.Further enquiry is required to explain the phenomenon and explore impacts onregional climate and air quality.

The vertical distribution of biomass burning pollution over tropical South America from aircraft in situ measurements during SAMBBA