Research Topics

My research focuses on the physical and chemical properties of aerosol particles and their interaction with the atmosphere. Aerosol particles play a key role in air pollution, cloud formation, and global warming. Phase transitions of aerosol particles e.g. the formation of ice and its corresponding impact on the global radiation budget is one of the least understood processes in the atmosphere. Aerosol particles also provide a medium for gas-to-particle reactions, so-called heterogeneous reactions, which can significantly alter atmospheric chemistry. The Antarctic ozone hole is the most prominent example for efficient heterogeneous chemistry. Heterogeneous reactions can change the physical and chemical properties of particles, thus influencing air quality, source apportionment, cloud formation, and climate.

The growth, crystallization, nucleation, and freezing of aerosol particles are studied in the laboratory under atmospherically relevant conditions. The understanding of these particle phase transitions is crucial to accurately predict the effects of aerosol particles in the atmosphere. The particles studied here range from aqueous inorganic solution droplets and field-collected particles to marine bio-aerosol particles. In the laboratory phase transitions of aerosol particles are investigated using an aerosol nucleation cell coupled to an optical microscope. We use single particle micro-spectroscopic analyses such as computer controlled scanning electron microscopy with energy dispersive analysis of X-rays (CCSEM/EDX) and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) to infer chemical composition and morphological features of the particles. Continuous development of these experimental techniques is performed to improve our understanding of the microphysical and chemical processes that govern atmospheric aerosol particles.

Heterogeneous oxidation of particles by atmospheric trace gases such as O3, NO2, N2O5, NO3, and OH are investigated in the laboratory using a flow reactor coupled to a chemical ionization mass spectrometer (CIMS). This approach allows to infer heterogeneous kinetics and products. Additionally, a high-resolution proton-transfer time-of-flight mass spectrometer (HR-PTR-ToF-MS) is applied to determine volatile organic reaction products. Our focus is to study the chemical transformation of biomass burning aerosol particles and its effect on particle water uptake and ice nucleation. Chemical transformation during atmospheric transport of bio-molecular markers representative of biomass burning aerosol can significantly affect aerosol source apportionment. Another research focus lies on the implementation of detailed heterogeneous chemistry in atmospheric models. This allows to assess the importance of various heterogeneous reaction pathways for condensed and gas-phase chemical composition.