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

Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements of the hydro- logical cycle and climate. We have measured and charac- terized 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 chemical composition on the CCN activ- ity of aerosol particles varied in the range of 0.1-0.4 (0.16±0.06 arithmetic mean and standard deviation). The overall median value of 0.15 was by a factor of two lower than the values typically observed for continental aerosols in other regions of the world. Aitken mode particles were less hygroscopic than accumulation mode particles ( 0.1 at D 50 nm; 0.2 at D 200 nm), which is in agreement with earlier hygroscopicity tandem differential mobility ana- lyzer (H-TDMA) studies. The CCN measurement results are consistent with aerosol mass spectrometry (AMS) data, showing that the organic mass fraction (forg) was on average as high as 90% in the Aitken mode (D 100 nm) and decreased with increas- ing particle diameter in the accumulation mode ( 80% at D 200 nm). The values exhibited a negative linear cor-

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