Seeded Crystal Growth of Internally Mixed Organic-Inorganic Aerosols: Impact of Organic Phase State.

Atmospheric aerosols are complex with both inorganic and organic components. The soluble inorganics can transition between aqueous and crystalline phases through efflorescence and deliquescence. This study focuses on the efflorescence of (NH4)2SO4/organic particles by seeded crystal growth through contact with a crystal of (NH4)2SO4. Seeded crystal growth is known to effectively shut down supersaturation of aqueous aerosols. Here, we investigate whether organics can inhibit seeded crystal growth. We demonstrate that poly(ethylene glycol) 400 (PEG-400), which phase-separates from the aqueous (NH4)2SO4 and forms a core-shell structure, did not inhibit seeded crystal growth of (NH4)2SO4 at all relative humidity (RH) values below deliquescence RH. The PEG-400 layer was not viscous enough to prevent the diffusion of species through the coating. In contrast, we find that although raffinose, which stays homogeneously mixed with (NH4)2SO4, did not inhibit seeded crystal growth at RH > 45%, it did inhibit heterogeneous efflorescence at lower humidities. Viscosity measurements using an electrodynamic balance show a significant increase in viscosity as humidity was lowered, suggesting that inhibited diffusion of water and ions prevented efflorescence. The observed efflorescence at the higher RH also demonstrates that collisions can induce efflorescence of mixed aerosols that would otherwise not homogeneously effloresce.

Immersion and Contact Efflorescence Induced by Mineral Dust Particles.

The phase state of inorganic salt aerosols impacts their properties, including the ability to undergo hygroscopic growth, catalyze heterogeneous reactions, and act as cloud condensation nuclei. Here, we report the first observation of contact efflorescence by mineral dust aerosol. The efflorescence of aqueous ammonium sulfate ((NH4)2SO4) and sodium chloride (NaCl) droplets by contact with three types of mineral dust particles (illite, montmorillonite, and NX illite), were examined using an optical levitation chamber. Immersion mode efflorescence was also studied for comparison. We find that in the presence of mineral dust particles, crystallization occurred at a higher relative humidity (RH) when compared to the homogeneous phase transition. Additionally, crystallization by contact mode efflorescence occurred at a higher RH than the corresponding immersion mode. Crystallization efficiencies in the contact mode exhibited an ion-specific trend consistent with the Hoffmeister series. Estimates for lifetimes of a salt droplet to collide with dust particles suggests that collisions between the two aerosol types are likely to occur before the salt aerosol is removed by other atmospheric processes. Such collisions could then lead to the crystallization of salt droplets that would otherwise have remained liquid, changing the overall impact that salt aerosols have on atmospheric chemistry and climate.

Contact efflorescence as a pathway for crystallization of atmospherically relevant particles.

Inadequate knowledge of the phase state of atmospheric particles represents a source of uncertainty in global climate and air quality models. Hygroscopic aqueous inorganic particles are often assumed to remain liquid throughout their atmospheric lifetime or only (re)crystallize at low relative humidity (RH) due to the kinetic limitations of efflorescence (salt crystal nucleation and growth from an aqueous solution). Here we present experimental observations of a previously unexplored heterogeneous nucleation pathway that we have termed "contact efflorescence," which describes efflorescence initiated by an externally located solid particle coming into contact with the surface of a metastable aqueous microdroplet. This study demonstrates that upon a single collision, contact efflorescence is a pathway for crystallization of atmospherically relevant aqueous particles at high ambient RH (≤80%). Soluble inorganic crystalline particles were used as contact nuclei to induce efflorescence of aqueous ammonium sulfate [(NH4)2SO4], sodium chloride (NaCl), and ammonium nitrate (NH4NO3), with efflorescence being observed in several cases close to their deliquescence RH values (80%, 75%, and 62%, respectively). To our knowledge, these observations represent the highest reported efflorescence RH values for microdroplets of these salts. These results are particularly important for considering the phase state of NH4NO3, where the contact efflorescence RH (∼20-60%) is in stark contrast to the observation that NH4NO3 microdroplets do not homogeneously effloresce, even when exposed to extremely arid conditions (<1% RH). Considering the occurrence of particle collisions in the atmosphere (i.e., coagulation), these observations of contact efflorescence challenge many assumptions made about the phase state of inorganic aerosol.