The Amazon is one of the few continental regions where atmospheric aerosol particles and their effects on climate are not dominated by anthropogenic sources. During the wet season, the ambient conditions approach those of the pristine pre-industrial era. We show that the fine submicrometer particles accounting for most cloud condensation nuclei are predominantly composed of secondary organic material formed by oxidation of gaseous biogenic precursors. Supermicrometer particles, which are relevant as ice nuclei, consist mostly of primary biological material directly released from rainforest biota. The Amazon Basin appears to be a biogeochemical reactor, in which the biosphere and atmospheric photochemistry produce nuclei for clouds and precipitation sustaining the hydrological cycle. The prevailing regime of aerosol-cloud interactions in this natural environment is distinctly different from polluted regions.

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[1] Dust samples were collected onboard the UK community BAe-146 research aircraft of the Facility for Airborne Atmospheric Measurements (FAAM) operated over Niger during the winter Special Observation Period of the African Monsoon Multidisciplinary Analysis project (AMMA SOP0/DABEX). Particle size, morphology, and composition were assessed using single-particle analysis by analytical scanning and transmission electron microscopy. The aerosol was found to be composed of externally mixed mineral dust and biomass burning particles. Mineral dust consists mainly of aluminosilicates in the form of illite and kaolinite and quartz, accounting for up to 80% of the aerosol number. Fe-rich particles (iron oxides) represented 4% of the particle number in the submicron fraction. Diatoms were found on all the samples, suggesting that emissions from the Bodélé depression were also contributing to the aerosol load. Satellite images confirm that the Bodélé source was active during the period of investigation. Biomass burning aerosols accounted for about 15% of the particle number of 0.1-0.6 mm diameter and were composed almost exclusively of particles containing potassium and sulfur. Soot particles were very rare. The aspect ratio AR is a measure of particle elongation. The upper limit of the AR value distribution is 5 and the median is 1.7, which suggests that mineral dust particles could be described as ellipsoids whose major axis never exceeds 1.9 Â D p (the spherical geometric diameter). This is consistent with other published values for mineral dust, including the recent Aerosol Robotic Network retrieval results of Dubovik et al. (2006).

[1] Real-time measurements of ammonia, nitric acid, hydrochloric acid, sulfur dioxide and the water-soluble inorganic aerosol species, ammonium, nitrate, chloride, and sulfate were performed at a pasture site in the Amazon Basin (Rondônia, Brazil). The measurements were made during the late dry season (biomass burning), the transition period, and the onset of the wet season (clean conditions) using a wet-annular denuder (WAD) in combination with a Steam-Jet Aerosol Collector (SJAC). Measurements were conducted from 12 September to 14 November 2002 within the framework of LBA-SMOCC (Large-Scale Biosphere Atmosphere Experiment in Amazonia -Smoke Aerosols, Clouds, Rainfall, and Climate: Aerosols From Biomass Burning Perturb Global and Regional Climate). Real-time data were combined with measurements of sodium, potassium, calcium, magnesium, and low-molecular weight (LMW) polar organic acids determined on 12-, 24-, and 48-hours integrated filter samples. The contribution of inorganic species to the fine particulate mass (D p 2.5 mm) was frequently below 20% by mass, indicating the preponderance of organic matter. The measured concentration products of NH 3 Â HNO 3 and NH 3 Â HCl persistently remained below the theoretical equilibrium dissociation constants of the NH 3 /HNO 3 /NH 4 NO 3 and NH 3 /HCl/NH 4 Cl systems during daytime (RH < 90%). The application of four thermodynamic equilibrium models (EQMs) indicates that the fine mode aerosol anions NO 3 À , Cl À , and SO 4 2À were balanced predominantly by mineral cations (particularly pyrogenic K + ) during daytime. At nighttime (RH > 90%) fine-mode NH 4 NO 3 and NH 4 Cl are predicted to be formed in the aqueous aerosol phase. Probably, Cl À was driven out of the aerosol phase largely by reaction of pyrogenic KCl with HNO 3 and H 2 SO 4 . As shown by an updated version of the equilibrium simplified aerosol model (EQSAM2), which incorporates mineral aerosol species and lumped LMW polar organic acids, daytime aerosol NH 4 + was mainly balanced by organic compounds. -H 2 O aerosol system and its gas phase precursors at a pasture site in the Amazon Basin: How relevant are mineral cations and soluble organic acids?,