Abstract
Abstract. 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 fraction. A 13-stage Dekati low-pressure impactor (DLPI) was used to collect particles with nominal aerodynamic diameters (Dp) ranging from 0.03 to 0.10 μm. Gravimetric analyses of the DLPI substrates and filters were performed to obtain aerosol mass concentrations. The concentrations of total, apparent elemental, and organic carbon (TC, ECa, and OC) were determined using thermal and thermal-optical analysis (TOA) methods. A light transmission method (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 sampling site, concentrations of fine aerosols (Dp<2.5 μm: average 59.8 μg m−3) were higher than coarse aerosols (Dp> 2.5 μm: 4.1 μg m−3). Carbonaceous matter, estimated as the sum of the particulate organic matter (i.e., OC × 1.8) plus BCe, comprised more than 90% to the total aerosol mass. Concentrations of ECa (estimated by thermal analysis with a correction for charring) and BCe (estimated by LTM) averaged 5.2 ± 1.3 and 3.1 ± 0.8 μg m−3, respectively. The determination of EC was improved by extracting water-soluble organic material from the samples, which reduced the average light absorption Ångström exponent of particles in the size range of 0.1 to 1.0 μm 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 μm in diameter. The concentrations 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.
Highlights
Biomass burning in the tropics introduces huge concentrations of aerosol particles into the atmosphere (Artaxo et al, 2002)
As part of the LBA-SMOCC-2002 field experiment, we determined the carbonaceous fraction in aerosol samples dominated by biomass burning, using different analytical methods
Results from thermal, thermal-optical (TOA), and optical techniques applied to water-extracted and non-extracted samples showed that most of the aerosol mass was due to carbonaceous material, and that the fine fraction was greater than the coarse fraction
Summary
Biomass burning in the tropics introduces huge concentrations (up to 40 000 cm−3) of aerosol particles into the atmosphere (Artaxo et al, 2002). Findings from the European contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH) (Andreae et al, 2002; Graham et al, 2002; Mayol-Bracero et al, 2002a) demonstrated that aerosols were predominantly in the fine fraction (accumulation mode particles) and that most of the carbonaceous material was water-soluble organic carbon (WSOC) (45%–75% of the OC). We present the size-resolved concentrations of carbonaceous aerosol particles collected at a pasture site in Rondonia, Brazil, during the biomass-burning dominated part of the SMOCC-2002 campaign, giving special attention to the determination of ECa or BCe using several thermal and optical methods
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