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

Abstract

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 5=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 K 0.1-0.4 (0.16±0.06 arithmetic mean and standard deviation). The overall median value of K0. 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 (K&0.1 at D»50nm; K∼0.2 at D∼200nm), 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 (f org) was on average as high as ∼ 90% in the Aitken mode (D<100nm) and decreased with increas-ing particle diameter in the accumulation mode (∼ 80% at D ∼ 200nm). The K values exhibited a negative linear cor-relation with Forg (R2=0.81), and extrapolation yielded the following effective hygroscopicity parameters for organic and inorganic particle components: Korg∼ ;0.1 which can be regarded as the effective hygroscopicity of biogenic sec-ondary organic aerosol (SOA) and Kinorg0.6 which is char-acteristic for ammonium sulfate and related salts. Both the size dependence and the temporal variability of effective particle hygroscopicity could be parameterized as a func-tion of AMS-based organic and inorganic mass fractions (Kp=KOrg+ Kinorgxfinorg). The CCN number concen-trations predicted with KP were in fair agreement with the measurement results (∼ 20% average deviation). The me-dian CCN number concentrations at S=0.1-0.82% ranged from AfacN.o.10∼35cm-3 to NccN, 0.82 ∼ 160cm -3, the me-dian concentration of aerosol particles larger than 30 nm was NCN, 30∼200cm-3, and the corresponding integral CCN ef-ficiencies were in the range of NccN, 0.10/NcN, 30 ∼ 0.l to WCCN, 0.82/ Ncn30 ∼0.8. Although the number concentrations and hygroscopicity parameters were much lower in pristine rainforest air, the integral CCN efficiencies observed were similar to those in highly polluted megacity air. Moreover, model calculations of NCCN,S assuming an approximate global average value of K∼03 for continental aerosols led to systematic overpredic-tions, but the average deviations exceeded ∼50% only at low water vapor supersaturation (0.1%) and low particle number concentrations (<100cm-3). Model calculations assuming a constant aerosol size distribution led to higher average devi-ations at all investigated levels of supersaturation: ∼ 60% for the campaign average distribution and ∼1600% for a generic remote continental size distribution. These findings confirm earlier studies suggesting that aerosol particle number and size are the major predictors for the variability of the CCN concentration in continental boundary layer air, followed by particle composition and hygroscopicity as relatively minor modulators. Depending on the required and applicable level of detail, the information and parameterizations presented in this paper should enable efficient description of the CCN properties of pristine tropical rainforest aerosols of Amazonia in detailed process models as well as in large-scale atmospheric and cli-mate models.