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.

Atmospheric aerosol particles play a major role in our climate system. Acting as condensation nuclei aerosol particles are crucial for the cloud formation processes. The attachment of vapor molecules to the surface of particles, caused by the random motion of the vapor molecules (diffusion), is called nucleation. Understanding the nucleation process is of major importance for predictions in our climate systems as this process not only drives cloud formation, but also enables new particle formation from gaseous precursors in the atmosphere. A lot of research effort has been put into experimentally investigating this process in the atmosphere but also in lab experiments in the past. These studies have led to the development of a number of theories, which are implemented into climate models. The key steps of nucleation process take place in the sub-2-nanometer size range. Charged particles (ions), which are also abundant in the atmosphere, have a beneficial influence on the nucleation process. However, this size range has not been experimentally accessible until the past decade because of instrument limitations for executing such measurements. The limited data that has been available so far has shown discrepancies from existing theoretical predictions. Specifically, the models fail to explain the contribution of the particle charge of the ion to the nucleation process. This project aimed toward providing more experimental data in the sub-2-nanometer range by carrying out experiments in very well defined and controllable lab experiments using ions of known chemical composition and exposing them to controlled amounts of vapor. Subsequently the particle growth due to vapor attachment was recorded. The measurements were done by covering a wide range of vapor concentrations. This gave us the opportunity to observe the nucleation process during all steps, first the attachment of only a first layer of vapor molecules to the particle surface, but with increasing concentrations also activating them to grow into macroscopic sized droplets. The results were compared to existing model predictions. The experimental work resulted into a qualitative and quantitative explanation of observed effects of ion polarity on the nucleation process. Additionally it was found that already at very low vapor concentrations the first layer of vapor molecules forms around the ions and make them grow. This effect has been predicted by theory, but with the high-resolution measurement technique applied we could measure the exact number of vapor molecules attached to the ion and thus, have a direct comparison with the model. Also it was found that small ions, consisting of only one single atom can significantly lower the required vapor concentration needed for the phase change from vapor to liquid phase. Meaning that big liquid droplets are more likely to form, than in the absence of these ions. All of the experimental results could also be explained by adapting existing theories and modeling approaches. We hope that the results of this project subsequently contribute to a better understanding of nucleation processes also in the atmosphere.