Analysis of heterogeneous vapor uptake by cluster ions

For investigations on new particle formation processes (nucleation) in the atmosphere, the ability to examine nanoparticles and clusters in the size range below 3 nm is crucial; nonetheless, experimental methods to study the influence of the physiochemical properties of these particles on the nucleation process are still limited. The goal of this study is to develop a new experimental technique to quantitatively study heterogeneous vapor molecule uptake (successive sorption reactions) by cluster ions. This technique will facilitate improved understanding of heterogeneous nucleation processes and the formation of stable clusters that may act as seed particles for nucleation/condensation growth processes, which is of fundamental importance in atmospheric chemistry and physics. The proposed method will make use of a DMA-MS, which is a combination of a high resolution, high flow differential mobility analyzer (DMA) in combination with a time-of-flight mass spectrometer (MS). The DMA-MS system enables simultaneous mobility and mass measurement for both molecular ions and cluster ions and therefore gives access to structure and density. The DMA-MS will be modified to examine vapor uptake by clusters by introducing a well- defined amount of vapor into the DMA. Cluster or molecular ions introduced into the DMA system will take up vapor at a certain rate depending on their physiochemical properties and the vapor concentration in the DMA. This will result in a shift in both the ion mobility and mass measured. A comparison of mobility mass measurements at different vapor concentrations can be used to determine equilibrium sorption coefficients and its dependency on the structure or chemical properties of the investigated molecular cluster or cluster ion. Uniquely, DMA-MS measurement with vapor uptake additionally enables isolation of molecules/clusters that are similar in size and mass by a difference in vapor uptake rate. Another key advantage of using a DMA as a reactor for vapor uptake is that the residence time of each ion in the DMA is well known and therefore the reaction time is very well defined. This offers an opportunity for studying other processes, such as oxidation kinetics or other chemical reactions. In combination with the research proposed for the return phase at the University of Vienna, which includes nucleation experiments at high saturation ratios in an expansion type CPC (condensation particle counter) for the same chemical compounds used in the DMA-MS, this project is expected to give an insight into until now experimentally inaccessible parameters that influence the nucleation process, and thus the particle formation process in the atmosphere.

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