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We present measurements of aerosol physico-chemical properties below 5 kmaltitude over the tropical rain forest and the marine boundary layer (MBL)obtained during the LBA-CLAIRE 1998 project. The MBL aerosol sizedistribution some 50-100km of the coast of French Guyana and Surinameshowed a bi-modal shape typical of aged and cloud processed aerosol. Theaverage particle number density in the MBL was 383cm-3. The daytimemixed layer height over the rain forest for undisturbed conditions wasestimated to be between 1200-1500m. During the morning hours the heightof the mixed layer increased by 144-180mh-1. The median daytimeaerosol number density in the mixed layer increased from 450cm-3 inthe morning to almost 800cm-3 in the late afternoon. The evolution ofthe aerosol size distribution in the daytime mixed layer over the rainforest showed two distinct patterns. Between dawn and midday, the Aitkenmode particle concentrations increased, whereas later during the day, asharp increase of the accumulation mode aerosol number densities wasobserved, resulting in a doubling of the morning accumulation modeconcentrations from 150cm-3 to 300cm-3. Potential sources of theAitken mode particles are discussed here including the rapid growth ofultrafine aerosol particles formed aloft and subsequently entrained into themixed layer, as well as the contribution of emissions from the tropicalvegetation to Aitken mode number densities. The observed increase of theaccumulation mode aerosol number densities is attributed to the combinedeffect of: the direct emissions of primary biogenic particles from the rainforest and aerosol in-cloud processing by shallow convective clouds. Basedon the similarities among the number densities, the size distributions andthe composition of the aerosol in the MBL and the nocturnal residual layerwe propose that the air originating in the MBL is transported above thenocturnal mixed layer up to 300-400km inland over the rain forest bynight without significant processing

Spatial and temporal distribution of atmospheric aerosols in the lowermost troposphere over the Amazonian tropical rainforest