During the past few years the importance of soot aerosols as a climate-forcing agent has become evident both through field and modeling studies. The role played by soot aerosols in climate is likely to be determined by their optical and hygroscopic properties, which principally depend on particle composition, shape and size. These properties in turn are determined mainly by the combustion conditions that generate the soot and on the subsequent transformations of the particles in the atmosphere. In experiments discussed here, soot particles are produced in a propane/oxygen flame and size-selected by a Differential Mobility Analyzer (DMA). The particles are coated in a flow reactor by organics, inorganics, or acids, or exposed to relevant concentrations of atmospheric oxidizers. The processed soot particles are exposed to a controlled amount of water vapor and sampled by an Aerodyne Aerosol Mass Spectrometer (AMS) and a Scanning Mobility Particle Sizer (SMPS). From the measurements of these two instruments, the particle composition, morphology (size, density, shape, and fractal dimension), and hygroscopic properties are determined. Results of these experiments will be presented. In the first set of studies, soot particles were exposed to atmospherically relevant gas-phase organic vapors. Changes in particle morphology were observed.