The optical properties, health effects, atmospheric lifetime, and climate impact of ambient aerosols are influenced directly by their size distribution, chemical composition, and phase. The aerosol hygroscopicity, which is also a function of composition, governs the size and phase changes of these particles when subjected to varying ambient relative humidities (RH). This thesis presents results from a wide variety of studies involving laboratory and ambient measurements of aerosol size distributions and water uptake properties in the subsaturated regime. Time evolutions of particle size and hygroscopic growth were investigated for various secondary organic aerosol (SOA) systems generated in a smog chamber from ozonolysis of cycloalkenes and photooxidation of biogenic terpenes. SOA yields were measured at various initial parent hydrocarbon concentrations and correlated with the structure of the parent compound. The amount of water uptake of the aerosol at a reference RH was found to inversely correlate with the SOA yield. The hygroscopicity of many atmospherically relevant pure organic species was also studied using an unconventional particle generation scheme employing a nonaqueous solution. Experimental results were compared with predictions from an equilibrium thermodynamic model. In these works, organic aerosols are shown to exhibit complex hygroscopic growth, dependent on the particle chemistry, phase, and surrounding RH. Implications of the experimental techniques used on the observation of particle growth, deliquescence, and efflorescence are discussed. A number of other studies incorporating aircraft-based measurements of aerosol size distributions and hygroscopicity with other ambient measurements into various cloud microphysics models are also presented.
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