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Aerosol particle number size distributions and hygroscopic properties weremeasured at a pasture site in the southwestern Amazon region (Rondonia). Themeasurements were performed 11 September-14 November 2002 as part ofLBA-SMOCC (Large scale Biosphere atmosphere experiment in Amazonia - SMOkeaerosols, Clouds, rainfall and Climate), and cover the later part of the dryseason (with heavy biomass burning), a transition period, and the onset ofthe wet period.Particle number size distributions were measured with a DMPS (DifferentialMobility Particle Sizer, 3-850nm) and an APS (Aerodynamic Particle Sizer),extending the distributions up to 3.3 µm in diameter. An H-TDMA (HygroscopicTandem Differential Mobility Analyzer) measured the hygroscopic diametergrowth factors (Gf) at 90% relative humidity (RH), for particles with drydiameters (dp) between 20-440 nm, and at several occasions RH scans(30-90% RH) were performed for 165nm particles. These data provide themost extensive characterization of Amazonian biomass burning aerosol, withrespect to particle number size distributions and hygroscopic properties,presented until now. The evolution of the convective boundary layer over thecourse of the day causes a distinct diel variation in the aerosol physicalproperties, which was used to get information about the properties of theaerosol at higher altitudes.The number size distributions averaged over the three defined time periodsshowed three modes; a nucleation mode with geometrical median diameters(GMD) of ~12 nm, an Aitken mode (GMD=61-92 nm) and an accumulationmode (GMD=128-190 nm). The two larger modes were shifted towards largerGMD with increasing influence from biomass burning.The hygroscopic growth at 90% RH revealed a somewhat external mixturewith two groups of particles; here denoted nearly hydrophobic (Gf~1.09for 100 nm particles) and moderately hygroscopic (Gf~1.26). While thehygroscopic growth factors were surprisingly similar over the periods, thenumber fraction of particles belonging to each hygroscopic group variedmore, with the dry period aerosol being more dominated by nearly hydrophobicparticles. As a result the total particle water uptake rose going into thecleaner period. The fraction of moderately hygroscopic particles wasconsistently larger for particles in the accumulation mode compared to theAitken mode for all periods. Scanning the H-TDMA over RH (30-90% RH)showed no deliquescence behavior. A parameterization of both Gf(RH) andGf(dp), is given

Size distribution and hygroscopic properties of aerosol particles from dry-season biomass burning in Amazonia