Surface organization of aqueous MgCl ₂ and application to atmospheric marine aerosol chemistry

Abstract

Inorganic salts in marine aerosols play an active role in atmospheric chemistry, particularly in coastal urban regions. The study of the interactions of these ions with water molecules at the aqueous surface helps to elucidate the role of inorganic cations and anions in atmospheric processes. We present surface vibrational sum frequency generation (SFG) spectroscopic and molecular dynamics (MD) studies of aqueous MgCl 2 surfaces as models of marine aerosol. Spectroscopy results reveal that the disturbance of the hydrogen bonding environment of the air/aqueous interface is dependent on the MgCl 2 concentration. At low concentrations (<1 M) minor changes are observed. At concentrations above 1 M the hydrogen bonding environment is highly perturbed. The 2.1 M intermediate concentration solution shows the largest SFG response relative to the other solutions including concentrations as high as 4.7 M. The enhancement of SFG signal observed for the 2.1 M solution is attributed to a larger SFG-active interfacial region and more strongly oriented water molecules relative to other concentrations. MD simulations reveal concentration dependent compression of stratified layers of ions and water orientation differences at higher concentrations. SFG and MD studies of the dangling OH of the surface water reveal that the topmost water layer is affected structurally at high concentrations (>3.1 M). Finally, the MgCl 2 concentration effect on a fatty acid coated aqueous surface was investigated and SFG spectra reveal that deprotonation of the carboxylic acid of atmospherically relevant palmitic acid (PA) is accompanied by binding of the Mg 2þ to the PA headgroup. magnesium chloride | fatty acid | air/aqueous interface | sum frequency spectroscopy | molecular dynamics I norganic salts present in marine boundary layer (MBL) aerosol originate from turbulent wave action at the surface of the ocean (1). These aerosols, typically of the micron size range and smaller, travel over continental regions by being entrained in the air mass in which they were created, and have been detected more than 900 km inland (2). Aerosols play a key role in the modification of global climate through their effect on cloud condensation nuclei prevalence, radiative balance, and level of precipitation (3). Aerosol composition and size have also been correlated to thunderstorm severity (4). Alkali metals (Na, K, Li, and Rb), alkaline earth metals (Mg, Ca), and ammonium (NH þ 4 ) make up the majority of cationic species, and halides (F, Cl, Br, and I) and oxidized sulfur and nitrogen ions make up the majority of inorganic anionic species found in MBL aerosol (5, 6). While calcium and magnesium are the most prevalent divalent cations in seawater and MBL aerosols, chloride anion is the dominant halide species (5). Although recent cloud drop measurements in the MBL show that calcium concentrations are about four times higher than magnesium concentrations, (6) the small size and high charge density of Mg 2þ gives rise to a strongly hydrated complex in aqueous solut...

Effect of cation enrichment on dipalmitoylphosphatidylcholine (DPPC) monolayers at the air-water interface

“…These studies, however, used significantly lower concentrations (<50 mM) of MgCl 2 or PCs with different chain lengths and degree of saturation, which makes direct comparison to results here difficult. In any case, the similarity in DPPC phase behavior on MgCl 2 compared to NaCl and KCl solutions suggests that Mg 2+ (most likely Mg 2+ hexahydrate) [51] interacts with DPPC in a manner similar to monovalent ions, i.e., with one cation binding per lipid headgroup (1:1 binding). This is likely due to the preference of Mg 2+ to strongly retain its solvation shell [51].…”

“…In any case, the similarity in DPPC phase behavior on MgCl 2 compared to NaCl and KCl solutions suggests that Mg 2+ (most likely Mg 2+ hexahydrate) [51] interacts with DPPC in a manner similar to monovalent ions, i.e., with one cation binding per lipid headgroup (1:1 binding). This is likely due to the preference of Mg 2+ to strongly retain its solvation shell [51]. Density functional theory calculations have determined that the net charge of a solvated Mg 2+ ion is +1.29, which could account for the similar behavior to monovalent ions [52].…”

Unique ocean-derived particles serve as a proxy for changes in ocean chemistry

“…The bubble bursting process is known to lyse cells [ Cherry and Hulle , 1992] potentially releasing organics and/or chlorophyll a into the water column that could be the source of enhanced organic material and Mg 2+ in the particle phase. Furthermore, Mg 2+ has been shown to deprotonate and associate with the polar head groups of surface active fatty acids creating a particle interface rich in Mg 2+ and organics [ Casillas‐Ituarte et al , 2010]. The Mg‐type particles detected by ATOFMS could, thus, primarily reflect an organic‐rich particle coating when water insoluble organic material is present in sea spray aerosols.…”

Direct aerosol chemical composition measurements to evaluate the physicochemical differences between controlled sea spray aerosol generation schemes

“…The tendency for inorganic cations to coordinate, or strongly associate, with organic and biological material in the ocean and in SSA particles has long been recognized (e.g., Duce and Hoffman, 1976). Divalent cations, such as Mg 2+ or Ca 2+ , have the ability to stabilize organic supramolecular structures (Verdugo, 2012) and coordinate surface-active organic ligands at interfaces (Casillas-Ituarte et al, 2010). Magnesium has been shown to be a good tracer for SSA produced from bacteria-rich seawater and has been observed in aerosol over the ocean in association with biological activity (Gaston et al, 2011).…”