Hydration, adsorption, and reaction on small aerosol particles

• Applicant: Dr. Jozef Lengyel
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 549903652

Nearly half of the cloud seeds in the troposphere originate from atmospheric particle formation, a process in which gaseous molecules are transformed into small aerosol particles. Most of them are formed by the condensation of vapor and gaseous molecules into small sub-nanometer sized (subnano) clusters that exhibit strong size effects. While small clusters form and break apart continuously, only a few reach a critical size above which further growth becomes spontaneous. This project aims to unravel the first steps of atmospheric particle formation and to determine the pickup efficiency depending on particle size, a key parameter for cloud formation modeling. In contrast to large particles, whose pickup efficiency can be described by geometrical cross sections, subnano clusters often exhibit enhanced cross sections due to long-range interactions. To achieve these objectives, we propose extending our ion beam experimental setup with a molecular beam source. This extension will result in a unique and versatile instrument, capable of a wide range of experiments with charged and neutral particles. Beyond ion-induced nucleation, we can then also access the interaction of molecules with neutral clusters - the dominant nucleation event in the atmosphere. The project focuses on the systematic investigation of hydrated acid clusters, namely binary sulfuric acid-water and methanesulfonic acid-water systems. To generate small clusters, we plan to employ a free jet expansion, while nanometer-sized particles will be obtained by integrating an aerosol generator into our cluster source. The hydrated acid clusters are investigated in pickup experiments. Here, cluster collisions with molecules are characterized by their pickup cross sections in a combination of mass spectrometry analysis and cluster velocity measurements. First, we will investigate collisions with oxidized organics to monitor the pickup efficiency as a function of cluster size. Second, we will test the interaction with molecules of different molecular properties (functional groups, steric extension, etc.) and correlate these with the observed pickup efficiency. Third, we intend to identify the factors that modulate the acid hydration in the clusters upon introduction of a complementary base and subsequent proton transfer - the mass spectra of acid-base clusters observed with different mass spectrometry techniques often miss water content. Discriminating ionization versus thermochemical pathways will help to introduce soft ionization protocols for future pickup experiments.

• DFG Programme: Research Grants
• Major Instrumentation: Scanning Mobility Particle Sizer
• Instrumentation Group: 1950 Partikelzählgeräte und -klassiergeräte (optisch, elektronisch, außer 35