Hygroscopic behavior and liquid‐layer composition of aerosol particles generated from natural and artificial seawater

Abstract

Sea‐salt aerosol (SSA) particles affect the Earth's radiative balance and moderate heterogeneous chemistry in the marine boundary layer. Using conventional and environmental transmission electron microscopes (ETEM), we investigated the hygroscopic growth and liquid‐layer compositions of particles generated from three types of aqueous salt solutions: sodium chloride, laboratory‐synthesized seawater (S‐SSA particles), and natural seawater (N‐SSA particles). Three levels of morphological change were observed with the ETEM as the laboratory‐generated particles were exposed to increasing relative humidity (RH). The first level, onset of observable morphological changes, occurred on average at 70, 48, and 35% RH for the NaCl, S‐SSA, and N‐SSA particles, respectively. The second level, rounding, occurred at 74, 66, and 57% RH for NaCl, S‐SSA, and N‐SSA particles, respectively. The third level, complete deliquescence, occurred at 75% RH for all particles. Collected ambient SSA particles were also examined. With the exception of deliquescence, they did not exhibit the same hygroscopic characteristics as the NaCl particles. The ambient particles, however, behaved most similarly to the synthesized and natural SSA particles, although the onset of morphological change was slightly higher for the S‐SSA particles. We used energy‐dispersive X‐ray spectrometry to study the composition of the liquid layer formed on the S‐SSA and N‐SSA particles. The layer was enriched in Mg, S, and O relative to the solid particle core. An important implication of these results is that MgSO4‐enriched solutions on the surface of SSA particles may be the solvents of many heterogeneous reactions.

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols

“…However, sea salt is a complex mixture ( Table 2) that contains hygroscopic salts such as MgCl 2 Á6H 2 O. 118,119 As a result, even at low relative humidity, the surface of sea salt particles has a liquid layer that is concentrated in the least soluble salts such as those associated with magnesium. 119,120 Surface reactions of such liquid layers can be quite different both in terms of kinetics and the mechanisms.…”

Mixed Chloride Aerosols and their Atmospheric Implications: A Review

“…The mixing of chloride salts with other inorganic salts, Wexler and Seinfeld (1991) compared the DRHs of mixtures of nitrates, chlorides, and sulfates and those of the individual salts, and found apparent decreases in DRH after mixing. Wise et al (2009) observed a significant reduction in deliquescence humidity when sodium chloride was mixed with a small amount of various inorganic substances, with the DRH dropping to 66% for laboratory-synthesized sea salts and to 57% for natural sea salts. For sulfate-coated and Mgrich chloride-coated sea salt particles collected from the ambient atmosphere, Semeniuk et al (2007a) also observed that the initial water uptake occurred at lower relative humidity (RH) than for common marine aerosol particles, and the coated sea salts underwent a complex multi-step deliquescence.…”

Horizontal distributions of aerosol constituents and their mixing states in Antarctica during the JASE traverse

“…The diurnal variation of the relative humidity can engender (1) phase transformation by deliquescence of available sea-salts on surface snow, (2) condensation and re-freezing of water vapour (e.g., formation of surface hoar) during local nighttime, and (3) enhancement of quasi-liquid layer and supercold liquid on the surface snow. Laboratory experiments conducted for earlier studies (e.g., Ge et al, 1998;Wise et al, 2009, Woods et al, 2010 revealed that chemical constituents with lower deliquescence relative humidity (DRH) can be localized in the outer layer (surface) around a solid core through phase transformation by deliquescence. Although relative humidity was minimal in the afternoon at Kohnen Station (Van As et al, 2005), the minimum relative humidity (∼ 89 %) was often higher than the deliquescence relative humidity (DRH) of plausible sea-salts.…”

“…Redistribution of chemical constituents can occur through (1) seawater freezing (Marion and Farren, 1999;Hara et al, 2012), and (2) phase transformation by deliquescence and efflorescence (e.g., Ge et al, 1998;Wise et al, 2009;Woods et al, 2010). Sea-salt fractionation in seawater freezing depends on the temperature (Marion and Farren, 1999;Hara et al, 2012).…”