The Connection between Microphysical Morphology and Atmospheric Particle Phase Transitions
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CitationSmith, Mackenzie Lynn. 2012. The Connection between Microphysical Morphology and Atmospheric Particle Phase Transitions. Doctoral dissertation, Harvard University.
AbstractThe phase of atmospheric particles can influence heterogeneous chemistry, cloud formation, and radiative forcing. Sulfate particles ranging from sulfuric acid to ammonium sulfate are the largest contributor arising from anthropogenic activities to the global fine aerosol burden. Multi-component inorganic particles of neutralized or partially neutralized crystalline components and acidic solutions can form via uptake of gaseous ammonia. Under some atmospheric conditions, secondary organic material can combine with sulfate species to form internally mixed organic-sulfate particles. The way in which interactions between the constituents in ambient multi-component particles affect the phase of the overall particle are not well understood, especially for complex mixtures containing secondary organic material. This thesis presents measurements of relative humidity-dependent phase transitions of atmospherically relevant multicomponent particles, measured using tandem differential mobility analyzer (TDMA) setups. Particle morphologies implied by these measurements and their impact on other atmospheric processes are discussed. The connection between particle morphology and hygroscopic phase transitions is first discussed in the context of internally mixed ammonium bisulfate and letovicite particles. Unexpected differences in the deliquescence behavior of individual particles of identical diameter and overall composition were observed. From the deliquescence data, we hypothesized composition- and diameter-dependent morphologies based on heterogeneous crystallization of the particles that explained the variable water uptake and deviations from thermodynamic predictions of deliquescence. In multi-component particles containing both organic and inorganic material, phase separation can occur in addition to the deliquescence and efflorescence transitions. The phase transitions and hygroscopic growth of particles composed of ammonium sulfate mixed with secondary organic material are presented and interpreted in a morphological context. Secondary organic material was generated in the Harvard Environmental Chamber via the photo-oxidation of isoprene, the ozonolysis of \(\alpha\)-pinene, and the photo-oxidation of \(\alpha\)-pinene. The occurrence of liquid-liquid phase separation between aqueous ammonium sulfate and \(\alpha\)-pinene-derived organic material, not directly observable from hygroscopic growth measurements, was inferred from the minimal influence of the organic material on the ammonium sulfate phase transitions (less than 4% RH deviation from pure ammonium sulfate values). In contrast, the influence of isoprene photo-oxidation products on the phase transitions of ammonium sulfate was substantial: Both the deliquescence relative humidity and the efflorescence relative humidity of the mixed particles were decreased by over 30% RH from pure ammonium sulfate values for high organic volume fraction. These results implied that dissolved ammonium sulfate, organic molecules, and water were mixed in a uniphasic solution. The dependence of secondary organic material phase on relative humidity and growth factor was investigated via concurrent measurements of particle bounce and hygroscopic growth. Particles were composed of ammonium sulfate mixed with the products of isoprene photo-oxidation, \(\alpha\)-pinene ozonolysis, or \(\alpha\)-pinene photo-oxidation. Particle bounce gradually decreased as RH was increased. The bounce behavior of all three types of particles were closely correlated with growth factor. These results implied that the uptake of particle-phase water caused gradual softening of the particles.
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