The Effect of Cloud Processing on the Phase State, Morphology, and Ice Nucleation Behavior of Internally Mixed Ammonium Sulfate–Secondary Organic Material Particles

Internally-mixed aerosol particles containing organic materials, inorganic salts, and water are an abundant particle type in the atmosphere. Depending on the temperature and the relative humidity, such organic-inorganic-water particles can undergo different phase transitions and exist in multiple morphologies. Important processes include deliquescence and efflorescence of the inorganic constituent, liquid-liquid phase separation, as well as the glass transition of the organic matter, given that aqueous solutions of a large number of organic substances do not crystallize but tend to form highly-viscous or glassy phases at low temperature/relative humidity. The particles’ phase state crucially influences a variety of atmospheric processes, e.g., the absorption and scattering of solar radiation, the reactive uptake of trace gases, and the particles’ ability to act as cloud condensation nuclei (CCN) and/or ice nucleating particles (INP). An inorganic-organic particle type that has been frequently used in previous laboratory studies is the internal mixture of ammonium sulfate and secondary organic material (SOM) from the ozonolysis of alpha-pinene. Its atmospheric relevance with respect to ice nucleation was e.g. evidenced by mass spectrometric analyses of cirrus residuals, where mixtures of sulfate and organics were found to be one of the most abundant particle types.

Whereas crystalline ammonium sulfate particles were found to be efficient INPs at cirrus temperatures, promoting heterogeneous ice formation already at critical ice saturation ratios below about 1.3, pure alpha-pinene derived SOM particles were mostly shown to nucleate ice only at or above the homogeneous freezing threshold at ice saturation ratios above 1.5. In a companion paper of this symposium (“Effect of secondary organic coating on the ice nucleation ability of solid ammonium sulfate aerosol” by Bertozzi et al.), we show that thin coating layers of SOM from the ozonolysis of alpha-pinene are sufficient to completely deactivate the heterogeneous ice nucleation mode of crystalline ammonium sulfate particles. These experiments were conducted in the AIDA aerosol and cloud chamber at the Karlsruhe Institute of Technology.

In this contribution, we investigate the effect of cloud-processing on the phase state, morphology, and ice nucleation behavior of the SOM-coated ammonium sulfate aerosol particles. Specifically, we probe whether the initial core-shell morphology of the particles (i.e., a crystalline core of ammonium sulfate surrounded by a shell of organic matter) can be modified to a partially engulfed structure when the particles are exposed to complex atmospheric trajectories involving deliquescence, droplet activation, freezing, and re-crystallization of ammonium sulfate as simulated in the AIDA chamber. In a partially engulfed structure, the SOM would not fully encapsulate the crystalline inorganic surface, suggesting that the heterogeneous freezing mode of ammonium sulfate would not be completely suppressed as in the case of the core-shell particle morphology. Infrared extinction as well as light scattering and depolarization measurements are used to characterize the phase state and morphology of the aerosol particles, whereas the cloud-processing and the analysis of the particles’ ice nucleation ability are performed via expansion cooling in the AIDA chamber.