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The chemical aging of organic aerosols by OH-initiated heterogeneous oxidation was investigated using both model organic and ambient aerosol particles. Organic aerosol particles were exposed to OH radicals in an aerosol flow tube and the modification of their chemical composition and particle properties was studied. Overall, this work has shown that OH-initiated heterogeneous oxidation enhanced the degree of oxidation and the Cloud Condensation Nucleus (CCN) activity of organic aerosol particles for equivalent OH exposure timescales of a few days to a week. Aerosol Mass Spectrometer (AMS) measurements showed that the heterogeneous uptake kinetics of OH radicals onto model primary organic aerosols was efficient. The heterogeneous reaction of organic aerosols with OH led to the production of high molecular weight particle-phase species with the addition of multiple oxygenated functional groups. These results were consistent with the observed increase in particle density with OH exposure. With the exception of solid organic aerosols, the particle volume and mass of organic particles were reduced by less than 20% from OH oxidation at high OH exposures due to volatilization of particle-phase reaction products. The degree of oxidation of the organic fraction of urban ambient aerosols was significantly enhanced for an equivalent atmospheric OH exposure time of 4 days for a daily average atmospheric OH concentration of 2×106 cm-3. Ambient aerosol particles sampled from a sparsely populated, forested region were initially more oxygenated than the urban aerosol particles and did not become more oxidized from reaction with OH radicals. The modification of the hygroscopicity of model primary and secondary organic aerosols from chemical aging was investigated by measuring the CCN activity of organic aerosols exposed to OH radicals. Primary organic aerosols, initially CCN inactive, became as CCN active as secondary organic aerosols due to heterogeneous reaction, where surface tension reduction played a major role. The CCN activity for model secondary organic aerosols was also enhanced due to OH oxidation, but changes were less dramatic than for the model primary organic aerosols.

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The interaction between sunlight and our atmosphere is one of the most fundamental processes affecting weather and climate. The majority of the Sun�s radiation is produced in the ultraviolet, visible and near-infrared regions of the electromagnetic spectrum. These spectral regions correspond to the energies of vibrational overtone and electronic transitions. The composition of our atmosphere is complex, and many trace species have a large influence on its chemistry and dynamics. Hydrogen bound hydrated complexes are trace species that could play an important role in the Earth�s atmosphere. However, before this role can be quantified, spectral identification and characterisation of these complexes is essential. We have developed vibrational local mode Hamiltonians to simulate the absorption spectra of hydrated complexes. To test the approximations made in the vibrational model, we have peformed calculations on the diatomics OH, HF and CO, which can be considered to act as pure local modes. When highly correlated ab initio methods and large basis sets are used to calculate the potential energy and dipole moment curve, the simulated vibrational transitions of the diatomics are in excellent agreement with experiment. We have derived approximate vibrational Hamiltonians which describe the OH-stretching and HOH-bending modes of vibration in the complexes H₂O�H₂O, H₂O�HO₂ and H₂O�HO. The calculated spectrum of H₂O�H₂O has been used to assess its atmospheric importance, and to succesfully guide experimental efforts to detect H₂O�H₂O in the laboratory and the Earth�s atmosphere. The calculated transition energies and intensities of H₂O�H₂O and H₂O�HO are in good agreement with experimental matrix isolation and gas-phase studies. To investigate the effect of low frequency modes on OH-stretching overtone spectra we have simulated the spectrum of HOONO. We have derived a Hamiltonian that couples the NOOH-torsional mode to the high frequency OH-stretching and OOH-bending modes. The simulated spectrum is in good agreement with the experimentally observed spectrum. We find that the OH-stretching spectra are perturbed strongly only if the barrier to torsion is low. We have also investigated changes in the electronic spectrum of hydrated complexes and the corresponding parent monomers. Upon complex formation, the lowest-lying electronic transition in the hydroxyl radical is strongly redshifted outside the region of monomeric absorption.

Thesis (M.S.)--Oregon Graduate Center, 1985.