Description
The investigation of the fundamental processes that enable and predict phase change is of great interest. Both homogeneous and heterogeneous nucleation/condensation processes are applied for e.g. nanoparticle synthesis, but also are responsible for new particle formation (NPF) in the atmosphere and thus, affect our climate and human health. With advances in aerosol instrumentation towards observation of smaller and smaller particles an increasing attention was drawn to the investigation of the process of heterogeneous nucleation for seed particles with diameters down to the Kelvin diameter and even below . With the introduction of the ultrafine condensation particle counter (CPC) by Stolzenburg and McMurry in 1991 (Atmospheric Environment, 34(12-14):1959-1999) particle detection down to about 3 nm becam e possible. For a long time the Kelvin diameter was seen as a fundamental lower detection limit for the CPCs. However, this assumption could not hold some experimental investigations. Seto et al. (1997) (The Journal of Chemical Physics.107 (5):1576-1585)f ound that ions of about 1.5 nm could be activated in a turbulent mixing CNC (condensation nuclei counter). Gamero and de la Mora (2000) (Journal of Aerosol Science.31(7):757-772) used dibutyl phthalate vapor and showed that there was no lower limit to the particle size that could be activated. The definite confirmation, that detection below the Kelvin limit is possible, was provided by observations of Winkler et al. (2008) (Science, 319:1374-1377) in an expansion type CPC. A universal theory predicting heterogeneous nucleation, especially for particles smaller than the Kelvin diameter, is still missing. The development of a theory is limited also due to the lack of meaningful experimental measurements. Accurate well-defined measurements of activation probabilities require highly monodisperse nuclei and uniform supersaturations (Fernandez de la Mora (2011). Aerosol Science and Technology. 45 (4):543-554). This still is difficult to achieve using general state-of-the-art aerosol instruments. In this study we were able to overcome these experimental challenges by using highly specialized aerosol instruments that can meet both conditions:on the one hand a high resolution and transmission DMA for the size selection of highly monodisperse seed ions of defined chemical composition and of both polarities, and on the other hand an expansion type CPC for exposing the seed ions to a well-defined uniform supersaturation. In addition to the experimental work, we compared the results to existing theories by applying the classical nucleation theory (CNT) for the case of insoluble and perfectly wetting seed particles. We examined the nucleation probability of monoatomic ions upon introduction of n-butanol into the size analyzing nuclei counter (SANC) (Wagner et al.(2003).Phys. Rev. E67:021605-1). Thereby we could Directly measure the onset saturation ratio and the additional number of n-butanol molecules to build a critical cluster by applying the Gumbel distribution. Furthermore, by applying the vapor uptake model (Raw at et al. (2015). Analyst. 140(20):6945-6954) we were able to calculate the mobility equivalent diameter of the ions at partial pressures of n- butanol below saturation (A. Maiβer & C. J. Hogan 2017) ChemPhysChem.18(21):3039-3046). With these results and the assumption that the seed ions are spherical and perfectly wettable, we were able to calculate the critical cluster size for different monoatomic ions of both polarities. In summary, we were able to quantitatively characterize the heterogeneous nucleation process for the pre-nucleation and critical nanoclusters using monoatomic singly charged ions of both polarities as seeds. In view of the small cluster sizes the prediction by the Kelvin-Thomson model can still be considered as fairly good.| Period | 6 Sept 2018 |
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| Event title | International Aerosol Conference 2018 |
| Event type | Conference |
| Location | St. Louis, United StatesShow on map |