Physical and chemical properties of aerosols in the wet and dry seasons in Rondônia, Amazonia

Abstract

As part of the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA), a large study of aerosol and trace gas properties was conducted in Amazonia during the dry and wet seasons and for pasture and primary forest sites. Aerosol mass and elemental composition were measured on filters for both fine (d p < 2.0 μm) and coarse (2.0 < d p < 10 μm) modes using polycarbonate filters mounted in stacked filter units (SFU). Measurements with real‐time aerosol and trace gas monitors were made, parallel to the collection of filter samples to obtain high time resolution (1–30 min). Black carbon equivalent (BCE) was determined with a multiwavelength Aethalometer as well as by reflectance in the fine‐mode SFU samples. Elemental carbon as well as total and organic carbon was measured using an R&P 5400 real‐time carbon monitor. PM 10 aerosol mass concentrations were determined with a Tapered Element Oscillating Monitor (TEOM) monitor every 5 min. CO, O 3, NO, and NO 2 were analyzed with Thermo Environment monitors with trace level capabilities for NOx. A TSI 3010 particle counter measured the total particle number concentration (d p > 10 nm) every minute. A 55‐m tower in a primary forest in Rondônia and a facility built on a pasture site were used to operate the instrumentation for 4 months in the wet season and 3 months in the dry season of 1999. Particle‐Induced X‐Ray Emission (PIXE) was used to measure the concentration of 22 trace elements for fine‐ and coarse‐mode aerosol. During the wet season, very clean atmospheric conditions were observed at both sites. Particle concentration averaged 900 cm−3, black carbon averaged 250 ng m−3, and mean fine‐mode mass concentration was 2.9 μg m−3. In sharp contrast, very high concentrations were observed in the dry season for all the parameters at both sampling sites. Aerosol mass concentrations up to 250 μg m−3 and particle number concentrations of more than 40,000 cm−3 were observed, while the CO peaked at 8 ppm and NO 2 reached 16 ppb. High correlation was observed between BCE, NO 2, CO, aerosol particle number, mass, aerosol optical thickness, and other properties linked to biomass‐burning emissions. Relatively large concentrations of phosphorus in the coarse‐mode particles were observed, especially at nighttime. The emission of biogenic particulate P could have an effect in the nutrient cycling of this essential and key nutrient, which is present almost exclusively in the form of aerosol particles. Phosphorus is exchanged in the aerosol phase and mostly for coarse‐mode particles and during nighttime.