Margaret A. Tolbert

Aqueous inorganic ions are a major component of atmospheric aerosols, typically comprising 25 to 75 percent of the dry aerosol mass. At sufficiently low relative humidity (RH), aqueous inorganic aerosols can undergo crystallization with the loss of particle-phase water (efflorescence), which alters the aerosol’s effect on global climate and many heterogeneous reactions that influence air quality. Because of the importance of the aerosol phase, the efflorescence behavior of aqueous inorganic aerosols has been studied for decades. Despite advances, there is still little agreement on how crystals nucleate and grow from an aqueous solution. In particular, the effect of collisions on particle phase remains largely unexplored.

We previously showed that collisions between aqueous droplets and soluble crystalline contact nuclei (CN) resulted in a nonequilibrium CN-droplet interface that persisted long enough for crystal nucleation to occur epitaxially (that is, oriented by CN crystal lattice structure), although CN dissolution was predicted from bulk thermodynamics. The question arises whether nonequilibrium effects may exist for collisions with other CN particle types, such as insoluble amorphous organics. Here, we studied the effect of collisions between aqueous inorganic microdroplets and insoluble (hydrophobic) amorphous organic aerosols. Experiments were performed using optically levitated microdroplets of sodium chloride (NaCl), ammonium chloride (NH₄Cl), and sodium bromide (NaBr), which are important components of sea spray aerosol. In our optical levitation system, light-absorbing particles, such as soot, strongly absorb the trapping laser wavelength. Thus, to avoid these complications and to probe the potential relevance of ion-specific interactions on salt crystal nucleation, functionalized surfactant-free polystyrene latex spheres (PSLs), which are hydrophobic amorphous organic aerosols, with a range of surface functionality/net charge were used as CN proxies for atmospherically relevant insoluble organic aerosols. We found that collisions with PSLs induced crystallization of aqueous inorganic microdroplets at high RH, the effect of which was correlated with destabilizing water-mediated ion-specific surface interactions. These same organic aerosols did not induce crystallization once internally mixed in the droplet, pointing toward a previously unconsidered transient ion-specific crystal nucleation pathway that can promote aerosol crystallization via particle collisions.

An example of a collision induced contact efflorescence event is shown in the figure, where an aqueous droplet of NaCl is collided with a 400 nm PSL CN at 57% RH. In part A, the brightfield images of the NaCl particle are shown (scale bar 10 mm).  Frame 1 shows the aqueous droplet before contact, Frame 2 the droplet 20 ms after contact, and frames 3 and 4 the droplet 90 and 180 ms, respectively, after contact. Efflorescence begins immediately after contact, and as efflorescence proceeds, the loss of water causes the particle to move upward in the trap. Part B shows a different fully effloresced NaCl crystal levitated in the trap. Parts C and D show binary contrast images that allow the point of contact to be more visible (see arrow). This set of images clearly shows that it is the contact event with the amorphous organic CN that induced NaCl efflorescence at a relative humidity well above the homogeneous value of 47% RH. Ongoing studies will begin to probe contact ice nucleation, a process thought to be important in mixed phase clouds.