Images reveal that atmospheric particles can undergo liquid-liquid phase separations.

A large fraction of submicron atmospheric aerosol particles contains both organic material and inorganic salts. As the relative humidity cycles in the atmosphere and the water content of the particles correspondingly changes, these mixed particles can undergo a range of phase transitions, possibly including liquid-liquid phase separation. If liquid-liquid phase separation occurs, the gas-particle partitioning of atmospheric semivolatile organic compounds, the scattering and absorption of solar radiation, and the reactive uptake of gas species on atmospheric particles may be affected, with important implications for climate predictions. The actual occurrence of liquid-liquid phase separation within individual atmospheric particles has been considered uncertain, in large part because of the absence of observations for real-world samples. Here, using optical and fluorescence microscopy, we present images that show the coexistence of two noncrystalline phases for real-world samples collected on multiple days in Atlanta, GA as well as for laboratory-generated samples under simulated atmospheric conditions. These results reveal that atmospheric particles can undergo liquid-liquid phase separations. To explore the implications of these findings, we carried out simulations of the Atlanta urban environment and found that liquid-liquid phase separation can result in increased concentrations of gas-phase NO(3) and N(2)O(5) due to decreased particle uptake of N(2)O(5).

Studies of one and two component aerosols using IR/VUV single particle mass spectrometry: Insights into the vaporization process and quantitative limitations.

This paper presents the studies of one and two component particles using a CO(2) laser for vaporization and VUV ionization in an ion trap mass spectrometer. The degree of fragmentation for a one component system was demonstrated to be a function of CO(2) laser energy. In a two component system, the degree of fragmentation was shown to be a function of the particle composition. This observation indicates that the analysis of mixed particles may be far more complicated than anticipated for a two step process with soft vaporization. In addition to showing that fragmentation is a function of CO(2) laser energy and particle composition, we also show that a key parameter that determines the extent of fragmentation is the energy absorbed by the particle during desorption. The ionization delay profile in a one component system is also shown to be strongly dependent on the vaporization energy. In a two component system, the delay profile is shown to strongly depend on the composition of the particle. The combined data suggest that the key parameter that governs the delay profile is the energy absorbed by the particle during desorption. This finding has implications for potential field measurements. Finally, for a two component system where the absorption crosssections are different, the change in the degree of fragmentation with particle composition resulted in a non-linear dependence of ion signal on composition. This makes any attempt at quantification difficult.

A study of oleic acid and 2,4-DHB acid aerosols using an IR-VUV-ITMS: insights into the strengths and weaknesses of the technique.

An investigation of oleic acid and 2,4-dihydroxybenzoic (DHB) acid aerosols was carried out using an aerosol mass spectrometer with pulsed lasers for vaporization and ionization and an ion trap for mass analysis. The extent of ion fragmentation was studied as a function of both vaporization energy and ionization wavelength. Low CO2 laser energies in the vaporization stage and near-threshold single photon ionization resulted in the least fragmented mass spectra. For DHB, only the molecular ion was observed, but for oleic acid fragmentation could not be eliminated. Tandem MS of the main fragment peak from oleic acid was carried out and provided a tool for compound identification. Photoionization efficiency curves were also collected for both DHB and oleic acid and the appearance energies of both parent and fragment ions were measured. Evidence for fragmentation occurring post-ionization is given by the similar appearance energies for both the parent and fragment ions. The results from this study were compared with those from similar experiments undertaken with time-of-flight (TOF) mass analyzers. The degree of fragmentation in the ion trap was considerably higher than that seen with TOF systems, particularly for oleic acid. This was attributed to the long storage interval in the ion trap which allows time for metastable ions to decay. Differences in the degree of fragmentation between the ion trap and TOF studies also provided further evidence for fragmentation occurring post-ionization. For 2,4-dihydroxybenzoic acid, the long delay prior to mass analysis also allowed time for reactions with background gases, in this case water, to occur.