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Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) arekey elements of the hydrological cycle and climate. We have measured andcharacterized CCN at water vapor supersaturations in the range of S=0.10–0.82%in pristine tropical rainforest air during the AMAZE-08 campaign in central Amazonia. The effective hygroscopicity parameters describing the influence of chemicalcomposition on the CCN activity of aerosol particles varied in the range ofκ≈0.1–0.4 (0.16±0.06 arithmetic mean and standard deviation).The overall median value of κ≈0.15 was by a factor of two lowerthan the values typically observed for continental aerosols in other regionsof the world. Aitken mode particles were less hygroscopic than accumulationmode particles (κ≈0.1 at D≈50 nm; κ≈0.2 atD≈200 nm), which is in agreement with earlier hygroscopicity tandemdifferential mobility analyzer (H-TDMA) studies. The CCN measurement results are consistent with aerosol mass spectrometry(AMS) data, showing that the organic mass fraction (forg) wason average as high as ~90% in the Aitken mode (D≤100 nm) anddecreased with increasing particle diameter in the accumulation mode(~80% at D≈200 nm). The κ values exhibited a negative linearcorrelation with forg (R2=0.81), and extrapolation yielded thefollowing effective hygroscopicity parameters for organic and inorganicparticle components: κorg≈0.1 which can be regarded as theeffective hygroscopicity of biogenic secondary organic aerosol (SOA) andκinorg≈0.6 which is characteristic for ammonium sulfate andrelated salts. Both the size dependence and the temporal variability ofeffective particle hygroscopicity could be parameterized as a function ofAMS-based organic and inorganic mass fractions (κp=κorg×forg+κinorg×finorg).The CCN number concentrationspredicted with κp were in fair agreement with the measurement results(~20% average deviation). The median CCN number concentrations atS=0.1–0.82% ranged from NCCN,0.10≈35 cm−3 toNCCN,0.82≈160 cm−3, the median concentration of aerosolparticles larger than 30 nm was NCN,30≈200 cm−3, and thecorresponding integral CCN efficiencies were in the range ofNCCN,0.10/NCN,30≈0.1 to NCCN,0.82/NCN,30≈0.8. Although the number concentrations and hygroscopicity parameters were muchlower in pristine rainforest air, the integral CCN efficiencies observedwere similar to those in highly polluted megacity air. Moreover, modelcalculations of NCCN,S assuming an approximate global average value ofκ≈0.3 for continental aerosols led to systematic overpredictions,but the average deviations exceeded ~50% only at low water vaporsupersaturation (0.1%) and low particle number concentrations (≤100 cm−3).Model calculations assuming a constant aerosol size distributionled to higher average deviations at all investigated levels ofsupersaturation: ~60% for the campaign average distribution and~1600% for a generic remote continental size distribution. Thesefindings confirm earlier studies suggesting that aerosol particle number andsize are the major predictors for the variability of the CCN concentrationin continental boundary layer air, followed by particle composition andhygroscopicity as relatively minor modulators. Depending on the required and applicable level of detail, the informationand parameterizations presented in this paper should enable efficientdescription of the CCN properties of pristine tropical rainforest aerosolsof Amazonia in detailed process models as well as in large-scale atmosphericand climate models

atmospheric chemistry and physics

Cloud condensation nuclei in pristine tropical rainforest air of Amazonia: size-resolved measurements and modeling of atmospheric aerosol composition and CCN activity