Contact efflorescence as a pathway for crystallization of atmospherically relevant particles

Abstract

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 [(NH 4 ) 2 SO 4 ], sodium chloride (NaCl), and ammonium nitrate (NH 4 NO 3 ), 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 NH 4 NO 3 , where the contact efflorescence RH (∼20-60%) is in stark contrast to the observation that NH 4 NO 3 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.efflorescence | coagulation | aerosol | climate | air quality N ucleation of the solid phase from a liquid solution (crystallization) is an important process in pharmaceuticals, manufacturing, and atmospheric science (1). In the atmosphere, the phase state and water content of particulate matter influences both heterogeneous chemistry and the aerosol direct and indirect effect on climate (2-6). Despite its importance, there is no comprehensive understanding of the phase state of atmospheric particulate, and the aerosol radiative forcing remains one of the largest uncertainties in climate predictions (6).A significant fraction of aqueous atmospheric aerosol contains soluble inorganics such as chlorides, sulfates, and nitrates that can undergo efflorescence, i.e., the process of salt crystallization and water evaporation. Efflorescence often occurs at a significantly lower relative humidity (RH) than the reverse process of deliquescence (2, 7-12). A potent example of this hydration hysteresis is demonstrated with ammonium nitrate (NH 4 NO 3 ), a hygroscopic component of atmospheric aerosol (9-11). NH 4 NO 3(s) crystals will deliquesce to form an aqueous droplet at ∼62% RH (T = 295 K) (13). However...

Crystal nucleation initiated by transient ion-surface interactions at aerosol interfaces

“…The lowest ${\mathrm{\sigma }}_{0.5}^{\text{RH}}$ (highest $\mathrm{normalR}{\mathrm{H}}_{0.5}^{\text{CE}}$) was observed for Na + , yet colloidal studies have suggested that this ion does not adsorb to negatively charged hydrophobic PSL surfaces ( 21 ), preferring to remain well hydrated away from the hydrophobic surface. This suggests that destabilizing hydration-mediated ion-surface interactions may be facilitating crystal nucleation upon contact, which would be in contrast to the prevailing theories of heterogeneous nucleation, which assume a stabilizing solute-substrate interaction via, for example, active sites ( 24 ), epitaxy ( 7 ), or templating ( 12 ). …”

Tandem configuration of differential mobility and centrifugal particle mass analysers for investigating aerosol hygroscopic properties

“…In this case, due to the isochemical collision, no activation barrier is required for nucleation, and efflorescence will occur at ERH close DRH. Contact efflorescence experiments by Davis et al (2015) have shown that upon collision of ((NH 4 ) 2 SO 4 ) solid with ((NH 4 ) 2 SO 4 ) droplet and NaCl solid with NaCl droplet , the efflorescence of the metastable droplets occurred at 79 ± 2 % RH for (NH 4 ) 2 SO 4 and at 74 ± 2 % RH for NaCl. In contrast, heterochemical collision did not significantly influence ERH.…”

Hygroscopic behavior of atmospheric aerosols containing nitrate salts and water-soluble organic acids

“…Previous observations indicated that pure NH 4 NO 3 droplets were difficult to homogeneously crystallize and thus remained in the liquidlike state even under extremely dry conditions (< 1 % RH) while it could efflorescence with the presence of the solid core in the droplets (Lightstone et al, 2000). Soluble inorganic crystalline particles acting as the contact nuclei were found to induce crystallization of aqueous ammonium nitrate (Davis et al, 2015). It was found that the phase state of NH 4 NO 3 tended to be substantially affected by coexisting species such as ammonium sulfate and succinic acid (Lightstone et al, 2000;Liu et al, 2016).…”