Liquid-liquid phase separation in mixed organic/inorganic aerosol particles.

Currently, the physical state of mixed organic/inorganic aerosol particles is not well characterized, largely because of the still unclear chemical composition of the organic fraction and of its properties with respect to mixing with the inorganic fraction. To obtain insight in the possible phases and phase transitions of such aerosol particles, we investigated the ternary poly(ethylene glycol)-400/ammonium sulfate/water system as a representative model system with partially immiscible constituents. For this purpose, we used optical microscopy and micro-Raman spectroscopy on micrometer-sized particles deposited on a hydrophobically coated substrate. The particles show liquid-liquid phase separations both upon decreasing (approximately 90-85%) and increasing (during ammonium sulfate deliquescence) relative humidities. In dependence upon the organic-to-inorganic ratio, OIR (i.e., poly(ethylene glycol)-400 to ammonium sulfate dry mass), phase separation is observed to occur by fundamentally different mechanisms, namely, nucleation-and-growth (OIR = 8:1 to 2:1), spinodal decomposition (OIR = 1.5:1 to 1:1.5) and growth of a second phase at the surface of the particle (OIR = 1:2 to 1:8). For each of these mechanisms, after completion of the phase separation, the resulting morphology of the particles is an aqueous ammonium sulfate inner phase surrounded by a mainly poly(ethylene glycol)-400 containing outer phase. We depict the various physical states of the ternary system in the relative humidity/composition phase diagram, constructed from bulk data and single particle measurements. Given the complex chemical composition of the organic fraction in tropospheric aerosols, it is expected that repulsive forces between the organic and inorganic aerosol constituents exist and that liquid-liquid phase separations commonly occur. The presence of liquid-liquid phase separations may change the partitioning of semivolatile species between the gas and the condensed phase, whereas the predominantly organic shell is likely to influence heterogeneous chemical reactions, such as N(2)O(5) hydrolysis.

Rutherford backscattering to study the near-surface region of volatile liquids and solids.

Here we describe the use of Rutherford backscattering spectrometry (RBS) to measure quantitative in situ elemental profiles with high depth resolution, online and nondestructively, in volatile substances (liquid and frozen acids, ice). Samples for analysis are held in a chamber with controlled temperature and partial pressures designed to match conditions for aerosols in Earth's atmosphere. This technique is demonstrated in studies of water solubility in sulfuric acid, hydrochloric acid (HCl) on ice surfaces, the formation of a HCl-hexahydrate surface layer on evaporating HCl-doped ice, and the diffusion of water through this layer.