Phase transitions of aerosol particles in the sulfate-nitrate-ammonium-proton system

Atmospheric particles are important in Earth's radiation budget because they can scatter light directly back to space and serve as nuclei for cloud formation. Aerosol particles also play a critical role in the chemistry of the atmosphere by providing surfaces for heterogenous reactions and serving as sinks and sources of atmospheric gases. Sulfate-nitrate-ammonium (SNA) particles are particularly important because, on a global basis, they make the largest anthropogenic contribution to the aerosol mass budget. The magnitude of the effect of SNA particles on radiative forcing and chemical reactions depends on their physical state. Aqueous particles scatter light more efficiently than do crystalline particles and chemical reaction rates are usually faster on aqueous particles than dry particles. Therefore, accurate prediction of the effect of SNA particles on radiative forcing and reaction rates requires knowledge of their physical state and how physical state changes with relative humidity. While deliquescence relative humidities can be accurately predicted using thermodynamic models, there is currently no theory that can accurately predict crystallization relative humidities.; In this thesis, the crystallization properties at 293 K of aerosol particles composed of SO42-, NO3-, NH4+, and H+ are studied using aerosol flow tube infrared spectroscopy. An innovative experimental protocol is employed to restore water content to the aerosol particles and thus remove the ambiguity of their physical state after exposure to low relative humidity. The six crystals formed include (NH4)2SO4, (NH4) 3H(SO4)2, NH4HSO4, NH 4NO3, 2NH4NO3·(NH4) 2SO4, and 3NH4NO3·(NH4 )2SO4.; The infrared aerosol spectra for the six solids formed are determined based upon a linear decomposition analysis of the recorded spectra. Small nonzero residuals occur in the analysis because aerosol spectra depend on particle morphology, which changes slightly across the range of compositions studied. Although particles of NH4NO3(aq) and NH 4HSO4(aq) do not crystallize even at 1% relative humidity, additions of 0.05 mole fraction SO42-(aq) or NO 3-(aq) ions promote crystallization, respectively. 2NH 4NO3·(NH4)2SO4(s) and (NH4)3H(SO4)2(s) appear to serve as good heterogeneous nuclei for NH4NO3(s) and NH4HSO4(s), respectively. 2NH4NO3·(NH 4)2SO4(s) crystallizes over a greater range of aqueous compositions than 3NH4NO3·(NH4) 2SO4(s).; The liquid water content of the particles after exposure to complex relative humidity histories is compared to the predictions of the Aerosol Inorganics Model. Depending on composition and relative humidity history, it is possible for a particle to follow one of several different crystallization pathways. This results in an external mixture of solid particles from an initially homogeneous population of aqueous particles. Increasing the number of cycles between high and low relative humidity allows particles to reconstruct and increases the percentage of particles following the most thermodynamically stable pathway.