The aerosol phase behavior is a fundamental property that remains insufficiently understood for many particle types. The phase behavior denotes the number and type of particle phases in individual aerosols. The number and type (or state) of phases impact particle reactivity and their ability to form clouds. In addition, aerosol phase behavior affects the gas-particle partitioning, i.e., interaction of species in the gas and in the particle phase, which in turn govern particle growth and total particulate mass concentrations. All of these processes are closely connected to atmospheric chemistry, air quality and climate.
The type of phases in atmospheric particles can often be characterized by their dynamic viscosity as a liquid, semisolid or a solid phase state. When transported through the atmosphere, aerosols particles can change their phase state by undergoing phase transitions, in response to changes in ambient relative humidity or temperature. Examples of phase transitions relevant for atmospheric aerosols are efflorescence and deliquescence, that take place during particle water loss and water uptake, respectively. Efflorescence denotes the transition from a liquid particle to a crystalline solid particle. It is controlled by kinetics since there is a free energy barrier to formation of solid nuclei. Conversely, deliquescence is controlled by thermodynamics and denotes the phase transition from a crystalline solid particle to a liquid particle. In addition to deliquescence and efflorescence, particles containing organic material can also undergo phase separation and mixing, which changes the number of phases in these particles: Separation refers to the phase transition from a particle with one to a particle with two coexisting phases. Conversely, mixing denotes the transition from a two-phase to a one-phase particle.
In our group we use microscopy techniques, to study phase behavior in supermicron particles. Through specially developed flow cells, that can be coupled to a microscope, we explore dependencies of the aerosol phase behavior on e.g. RH and determine humidity conditions for separation (SRH), mixing (MRH), deliquescence (DRH) and efflorescence (ERH). The results of these studies are used to build parameterizations that can be used in models to predict aerosol phase behavior.
