Fourier transform infrared spectroscopy of a single aerosol particle

Throughout this thesis, the phenomenon of radiation-induced particle size change is studied both on a theoretical as well as experimental level. The thrust of this study is aimed at using the size changes due to heat absorption to develop a technique for obtaining the particle chemical composition. The experiments here involve charged particles, generated with an impulse jet, and trapped by the electric field of an electrodynamic balance. The particles under study are all aqueous solutions of non-volatile salts, where upon heating a partial evaporation of water occurs. The evaporation and subsequent condensation processes are modeled in both the continuum and the transition regimes. The models developed are tested and the agreement between theory and experimental results is demonstrated. The models are also used to extract the values of the water, thermal, and mass accommodation coefficients from the data. The results for the thermal accommodation show that its value is near unity, however the corresponding results for the mass accommodation are not conclusive. A method is developed for obtaining the molecular composition of a single suspended microparticle by Fourier transform infrared spectroscopy. The particle is irradiated simultaneously by the infrared output from a Michelson interferometer and the visible light from a dye laser. The laser is tuned to an edge of an optical resonance of the particle while the IR beam is chopped. Through evaporation and condensation the chopped IR beam causes a size modulation of the droplet, which in turn induces a fluctuation in the laser light scattered from the particle. The scattered light is detected at 90° with a photomultiplier, and the amplitude of the light fluctuation is measured with a lock-in amplifier. The lock-in signal is then inverted by a discrete fast Fourier transform routine (FFT), to yield the particle absorption spectrum. Spectra of (NH4)2SO4 droplets at different solute concentrations are presented.

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