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 the ambient temperature and relative humidity conditions, organic aerosols can change from a liquid, to a semi-solid, or to glassy phase state. Glassy organic aerosols are thought to promote heterogenous ice nucleation, 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.