Speaker：Asst. Prof. Fabian Mahrt（Aarhus University）
Time：10:00-11:00, 13 September (Friday)
Venue：Lecture Hall 221, Building No. 16, Tianjin University
Abstract：
Organic aerosol particles are ubiquitous in the atmosphere. Depending on their composition, as well as the ambient temperature and relative humidity conditions, the phase behavior, i.e., the number and types of phases in these particles can vary, with important implications for air quality and climate. For example, compared to single-phase particles, phase separated particles with multiple organic phases can more readily act as cloud condensation nuclei and form liquid clouds. Next to the number also the type of phases critically impacts their role for atmospheric processes: Depending on atmospheric conditions, organic aerosols can change from a liquid, to a semi-solid, or to glassy phase state. While liquid organic aerosols can act as cloud condensation nuclei in warm clouds, glassy organic aerosols are thought to promote heterogenous ice nucleation, thus impacting the formation and microphysical properties of ice clouds. While the importance of aerosol phase behavior is well recognized, it remains insufficiently understood for particles that contain organic material, largely due to the complexity and variety of organic aerosols in the atmosphere.
The research presented here, focusses on the phase behavior of major types of atmospheric organic particles, namely, primary organic aerosol (POA), secondary organic aerosol (SOA) and mixtures thereof. Employing fluorescence microscopy, we directly observed the humidity-dependent phase behavior of particles that contained POA and SOA. Using the oxygen-to-carbon (O/C) ratio to describe the composition of the particles, we demonstrate that the difference in the average O/C ratio between the POA and SOA component of a mixture is a good predictor of the phase behavior, with two-phase particles always forming for ΔO/C ≥ 0.265. Using this novel ΔO/C-framework, that can be implemented into models, we find the majority (88%) of POA+SOA mixtures to form two-phase particles and assess potential implications for cloud condensation nuclei activity. We also explored the phase state of pure SOA particles and investigated how it is affected by atmospheric aging. Our results show that photochemical aging of SOA promotes the formation of a glassy phase state and could represent an unrecognized source of nuclei for ice clouds.
Overall, our results shed new insights into the phase behavior of organic aerosols and highlight possible implications for air pollution and climate.
About the speaker：
Asst. Prof. Fabian Mahrt is an experimental atmospheric scientist. He obtained his PhD in 2019 from ETH Zurich (Switzerland), where he studied aerosol-cloud interactions with a focus on the ice nucleation abilities of soot particles. During his Postdoc at the University of British Columbia (Canada) and at the Paul Scherrer Institute (Switzerland), he studied aerosol physical chemistry of organic aerosol particles. Fabian is currently a Tenure Track Assistant Professor at the Chemistry Department of Aarhus University (Denmark). His group, affiliated with the Center for Chemistry of Clouds (C3), works on topics related to aerosol properties and cloud microphysics.
Research group website: https://chem.au.dk/en/research/research-areas-and-groups/physicalchemistry/aerosol-and-cloud-microphysics
Honours (selected):
- Prix Schläfli in Geosciences 2021, Swiss Academy of Sciences
- Invited participant of ACCESS XVI (Atmospheric Chemistry Colloquium for Emerging Senior Scientists), 2021
