Gas-Phase Reactions of Water Cluster Anions with HNO3 and NO

The proposed research project aims at a detailed understanding of elementary processes intimately tied to the atmospheric chemistry of HNO3 and NO in the gas-phase and at the air-water interface. We will study reactions with transient species in water cluster anions, specifically the hydrated electron, hydrated atomic oxygen radical anion, and hydrated ozonide. For all proposed reactions, we will study the thermochemistry by means of nanocalorimetry, chemical kinetics to determine the reaction rates, and the dynamics, i.e. explore the reaction mechanisms. First an extension of the Cluster apparatus available at the Leopold-Franzens-Universität Innsbruck is planned. A new pick-up cell will be designed and implemented in a vacuum chamber, and new schemes on ion-depletion by laser excitation will be developed. The proposed combination of FT- ICR mass spectrometry and the newly developed laser depletion technique accompanied by ab initio calculations and molecular dynamics simulations will enable us to gain a deep understanding of cluster ion chemistry. One of the specific questions to be solved is the importance of solvent induced effects and the convergence to bulk behavior at larger cluster sizes. This will be investigated by looking at a wide range of cluster sizes. Experiments will be performed with size selection, which will allow observing size- effects, size threshold-phenomena, etc. For the first time, we will be able to study the chemistry of oxygen-based radical anions selectively for the hydrated atomic radical anion, as well as the hydrated ozonide ion. We will implement various strategies to generate cluster distributions that contain only the species of interest. This will resolve several open questions in the scientific literature, which originate from the use of mixed cluster distributions. These innovative aspects will help to reveal the possible ion chemistry pathways and the associated reaction mechanisms, dynamics and kinetics of the studied processes, the solvent effects, and other details. On the application side, the results obtained within the proposed project will improve our understanding of elementary steps in the atmospheric chemistry of anions, which can ultimately be exploited in atmospheric chemistry models.

Even very simple chemical reactions consist of dozens of elementary steps, which can occur consecutively, in parallel or in competition. Scientists are then expected to describe the whole mechanism in order to understand chemistry at the molecular level. In contrast to macroscopic bulk chemistry, gas-phase studies of isolated molecules are quite convenient for such an exploration due to several oversimplifications, e.g., avoiding the interaction with solvent molecules. However, most of the reactions in nature occur in the presence of water. We can then ask whether we can learn something about reactions in nature once we neglect the environment of the studied system. The corresponding answer can be demonstrated on an example from atmosphere: why is there an ozone hole above Antarctica when ozone- depleting molecules are present throughout all latitudes? The reason is the specific chemistry on surfaces of small ice particles that does not occur in the gas phase.Within the present project, we revealed fundamental aspects of how hydration affects chemical reactions by adding water molecules one by one. We have investigated gas-phase reactions of electrons and ions with atmospherically relevant molecules, such as freons, NOx, HNO3 or H2SO4, in the presence of water environment. We designed our experimental setup at the University of Innsbruck to produce well-defined ice particles mimicking small atmospheric aerosols in a laboratory environment. We experimentally measured whether the reactions take place, and how fast they are. In combination with theoretical calculations, we investigated these reaction mechanisms to an unprecedented level of detail.Efficient ion formation was found, e.g., in electron-induced processes in HNO3/H2O and H2SO4/H2O particles. A very surprising hydration effect was found for a reaction between an electron and the HNO3 molecule. By varying the environment, i.e. selective hydration of one reactant, the reaction yields different products. We described the reaction pathways for reactions of HNO3/H2O and NOx/H2O particles with several hydrated ions, such as O? O2-, O3, OH and CO2- that are relevant in the atmosphere. We introduced a new experimental technique to understand the uptake of molecules by HNO3/H2O ice particles. All these results provided detailed insight into processes relevant to atmospheric chemistry and should be considered in modelling of atmospheric aerosol particles.