Lawrence Berkeley National Laboratory

Aerosol samples were collected at a pasture site in the Amazon Basin as part of the project LBA-SMOCC-2002 (Large-Scale Biosphere-Atmosphere Experiment in Amazonia - Smoke Aerosols, Clouds, Rainfall and Climate: Aerosols from Biomass Burning Perturb Global and Regional Climate). Sampling was conducted during the late dry season, when the aerosol composition was dominated by biomass burning emissions, especially in the submicron frac- tion. A 13-stage Dekati low-pressure impactor (DLPI) was used to collect particles with nom- inal aerodynamic diameters (Dp) ranging from 0.03 to 0.10 m. Gravimetric analyses of the DLPI substrates and filters were performed to obtain aerosol mass concentrations. The con- centrations of total, apparent elemental, and organic carbon (TC, ECa, and OC) were deter- mined using thermal and thermal-optical analysis (TOA) methods. A light transmission meth- 1 od (LTM) was used to determine the concentration of equivalent black carbon (BCe) or the absorbing fraction at 880 nm for the size-resolved samples. During the dry period, due to the pervasive presence of fires in the region upwind of the sam- pling site, concentrations of fine aerosols (Dp < 2.5 m: average 59.8 g m-3) were higher than coarse aerosols (Dp > 2.5 m: 4.1 g m-3). Carbonaceous matter, estimated as the sum of the particulate organic matter (i.e., OC x 1.8) plus BCe, comprised more than 90percent to the total aerosol mass. Concentrations of ECa (estimated by thermal analysis with a correction for char- ring) and BCe (estimated by LTM) averaged 5.2 1.3 and 3.1 0.8 g m-3, respectively. The determination of EC was improved by extracting water-soluble organic material from the samples, which reduced the average light absorption ngström exponent of particles in the size range of 0.1 to 1.0 m from >2.0 to approximately 1.2. The size-resolved BCe measured by the LTM showed a clear maximum between 0.4 and 0.6 m in diameter. The concentra- tions of OC and BCe varied diurnally during the dry period, and this variation is related to diurnal changes in boundary layer thickness and in fire frequency.