Abstract
As part of the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia‐European Studies on Trace Gases and Atmospheric Chemistry (LBA‐EUSTACH), size‐fractionated aerosol samples were collected at a primary rain forest in the Brazilian Amazon during two field campaigns in April–May and September–October 1999. These two periods encompassed parts of the wet and dry seasons, respectively. Daytime‐nighttime‐segregated sampling was carried out at three different heights (above, within, and below canopy level) on a 54‐m meteorological tower at the forest site in order to better characterize the aerosol sources. The samples were analyzed for up to 19 trace elements by particle‐induced X‐ray emission analysis and for carbonaceous components by thermal‐optical analysis. Equivalent black carbon (BCe) and gravimetric analyses were also performed. The average mass concentrations for particles <2 μm diameter were 2.2 and 33.5 μg m−3 for the wet and the dry seasons, respectively. The elements related to biomass burning and soil dust generally exhibited highest concentrations above the canopy and during daytime, while forest‐derived aerosol was more concentrated underneath the canopy and during nighttime. These variations can be largely attributed to daytime convective mixing and the formation of a shallow nocturnal boundary layer, along with the possibility of enhanced nighttime release of biogenic aerosol particles. Mass scattering (αs) and mass absorption efficiency (αa) data indicate that scattering was dominated by fine aerosol, while fine and coarse aerosol both contributed significantly to absorption during both seasons. The data also suggest that components other than elemental carbon were responsible for a substantial fraction of the absorption.