The fine particles serving as cloud condensation nuclei in pristine Amazonian rainforest air consist mostly of secondary organic aerosol. Their origin is enigmatic, however, because new particle formation in the atmosphere is not observed. Here, we show that the growth of organic aerosol particles can be initiated by potassium-salt-rich particles emitted by biota in the rainforest. These particles act as seeds for the condensation of low- or semi-volatile organic compounds from the atmospheric gas phase or multiphase oxidation of isoprene and terpenes. Our findings suggest that the primary emission of biogenic salt particles directly influences the number concentration of cloud condensation nuclei and affects the microphysics of cloud formation and precipitation over the rainforest.

The influence of second-generation products on the particle mass yield of beta-caryophyllene ozonolysis was systematically tested and quantified. The approach was to vary the relative concentrations of first- and second-generation products by adjusting the concentration of ozone while observing changes in particle mass yield. For all wall-loss corrected organic particle mass concentrations M-org of this study (0.5 < M-org < 230 mu g m(-3)), the data show that the particle-phase organic material was composed for the most part of second-generation products. For 0.5 < M-org < 10 mu g m(-3), a range which overlaps with atmospheric concentrations, the particle mass yield was 10 to 20% and was not sensitive to ozone exposure, implying that the constituent molecules were rapidly produced at all investigated ozone exposures. In contrast, for M-org > 10 mu g m(-3) the particle mass yield increased to as high as 70% for the ultimate yield corresponding to the greatest ozone exposures. These differing dependencies on ozone exposure under different regimes of M-org are explained by a combination of the ozonolysis lifetimes of the first-generation products and the volatility distribution of the resulting second-generation products. First-generation products that have short lifetimes produce low-volatility second-generation products whereas first-generation products that have long lifetimes produce high-volatility second-generation products. The ultimate particle mass yield was defined by mass-based stoichiometric yields alpha(i) of alpha(0) = 0.17 +/- 0.05, alpha(1) = 0.11 +/- 0.17, and alpha(2) = 1.03 +/- 0.30 for corresponding saturation concentrations of 1, 10, and 100 mu g m(-3). Terms alpha(0) and alpha(1) had low sensitivity to the investigated range of ozone exposure whereas term alpha(2) increased from 0.32 +/- 0.13 to 1.03 +/- 0.30 as the ozone exposure was increased. These findings potentially allow for simplified yet accurate parameterizations in air quality and climate models that seek to represent the ozonolysis particle mass yields of certain classes of biogenic compounds.