Exploring the hygroscopicity and chemical composition evolution in organic-inorganic aerosols: A study on internally mixed malonic acid-metal (Na+, Ca2+, Mg2+) nitrates.

Chemical transformations in mixed aerosols alter the particulate physical properties. Nitrates and water soluble dicarboxylic acids, such as malonic acid (MA), are major components of ambient aerosol particles. Various metal ions such as, Na+, Ca2+, Mg2+ also become part of these complex aerosol systems during their atmospheric lifetime. Interactions among the co-existing ionic and molecular species govern the chemical changes in the aerosol particles. In this work, we provide a comparative account of the effect of metal ion identity (Na+, Ca2+, Mg2+) on such chemical changes arising from ion-molecular interactions in NaNO3-MA, Ca(NO3)2-MA and Mg(NO3)2-MA mixed inorganic-organic aerosols. In-situ micro-Raman spectroscopy has enabled us to gain molecular level insight on formation of organic salt and simultaneously estimate nitrate depletion in these mixed aerosols during different stages of their hygroscopic cycle. In addition to the nitrate depletion often reported during the drying phase, this study has brought to light an intriguing observation: depletion of nitrate in the humidification phase as well, a phenomenon that has hitherto remained undocumented. For the mixed systems studied here, the extent of nitrate depletion follows the order Mg-MA (58%) > Ca-MA (43%) > Na-MA (15%). The comparatively huge forward shift in the acid displacement reaction equilibrium for the systems, Ca-MA and Mg-MA is driven by complexation. Our results highlight the profound effect of ion-molecular interactions on the acid displacement reaction equilibria in aerosols.

Adsorption of Iodine Species (I3-, I-, and IO3-) at the Nuclear Paint Monolayer-Water Interface and Its Relevance to a Nuclear Accident Scenario.

Following a nuclear accident, radioactive iodine causes great concern to public health and safety. Organic iodide, because of its ability to escape reactor containment building and high environmental mobility, constitutes a predominant fraction of airborne radioiodine at places far away from the accident site. As the iodine released from a reactor core is inorganic iodine, it is vital to understand the mechanism of organic iodide formation inside reactor containment. In this context, we investigated the surface prevalence and adsorption of various inorganic iodines, I-, I3-, and IO3-, at a nuclear paint (used in nuclear installations) monolayer-water interface, mimicking the painted inner walls of an accident-affected containment building that are exposed to the iodine-containing condensed water layer. Vibrational sum frequency generation (VSFG) measurements in the OH and CH stretch regions reveal that the paint-water interface changes its charge characteristics with the pH of the water that affects the degree of interaction with the iodine species. At the acidic condition (bulk pH < 7), the paint becomes positively charged and strongly adsorbs the negatively charged iodine species dissolved in the aqueous phase, whereas at the alkaline condition (bulk pH > 9.5), the paint becomes net neutral and weakly interacts with the iodine species. These interactions change the conformation of the paint such that its hydrophobic alkyl groups orient increasingly away from the aqueous phase. The order of adsorption increases as IO3- < I- < I3- for the different iodine species studied.

Restructuring of Hydration Shell Water due to Solvent-Shared Ion Pairing (SSIP): A Case Study of Aqueous MgCl2 and LaCl3 Solutions.

Hydration of ions plays a crucial role in interionic interactions and associated processes in aqueous media, but selective probing of the hydration shell water is nontrivial. Here, we introduce Raman difference with simultaneous curve fitting (RD-SCF) analysis to extract the OH-stretch spectrum of hydration shell water, not only for the fully hydrated ions (Mg2+, La3+, and Cl-) but also for the ion pairs. RD-SCF analyses of diluted MgCl2 (0.18 M) and LaCl3 (0.12 M) solutions relative to aqueous NaCl of equivalent Cl- concentrations provide the OH-stretch spectra of water in the hydration shells of fully hydrated Mg2+ and La3+ cations relative to that of Na+. Integrated intensities of the hydration shell spectra of Mg2+ and La3+ ions increase linearly with the salt concentration (up to 2.0 M MgCl2 and 1.3 M LaCl3), which suggests no contact ion pair (CIP) formation in the MgCl2 and LaCl3 solutions. Nevertheless, the band shapes of the cation hydration shell spectra show a growing signature of Cl--associated water with the rising salt concentration, which is a manifestation of the formation of a solvent-shared ion pair (SSIP). The OH-stretch spectrum of the shared/intervening water in the SSIP, retrieved by second-round RD-SCF analysis (2RD-SCF), shows that the average H-bonding of the shared water is weaker than that of the hydration water of the fully hydrated cation (Mg2+ or La3+) but stronger than that of the anion (Cl-). The shared water displays an overall second-order dependence on the concentration of the interacting ions, unveiling 1:1 stoichiometry of the SSIP formed between Mg2+ and Cl- as well as La3+ and Cl-.

Polyatomic Iodine Species at the Air-Water Interface and Its Relevance to Atmospheric Iodine Chemistry: An HD-VSFG and Raman-MCR Study.

Iodine plays a key role in tropospheric ozone destruction, atmospheric new particle formation, as well as growth. Air-water interface happens to be an important reaction site pertaining to such phenomena. However, except iodide (I-), the behavior of other iodine species, for example, triiodide (I3-) and iodate (IO3-, the most abundant iodine species in seawater) at the aqueous interface and their effect on the interfacial water are largely unknown. Using interface-specific vibrational spectroscopy (heterodyne-detected vibrational sum frequency generation), we recorded the imaginary-χ(2) spectra (Imχ(2); χ(2) is the second-order electric susceptibility in OH stretch region) of the air-water interface in the presence of IO3-, I3-, and I- (≤0.3 M) in the aqueous subphase. The Imχ(2) spectra reveal that the chaotropic I3- is the most surface-active anion among the iodine species studied and decreases the vibrational coupling and hydrogen-bonding of interfacial water. Interestingly, the IO3-, even being a kosmotrope, is quite prevalent in the interfacial region and preferentially orients the interfacial water as "H-down" (i.e., water dipole moment is pointed toward the bulk water). Mapping of the OH stretch response of ion-affected water at interface (i.e., ΔImχ(2) = Imχ(2)air-water-iodine salt - Imχ(2)air-water) with that in the hydration shell of the respective ion (hydration shell water response is obtained by Raman multivariate curve resolution spectroscopy) reveals a correlative link between the ion's influence on the interfacial water and their hydration shell structure. The distinct water structure of stronger as well as weaker H-bonding in the hydration shell of the polyatomic IO3- anion promotes the anion to stay at the interfacial region. Thus, the surface prevalence of the iodine species and their effect on the interfacial water are perceived to be crucial for the transfer of iodine from seawater to the atmosphere across the marine boundary layer and the chemistry of iodine at aqueous aerosol surface.