Probing the impact of the organic fraction on hygroscopicity, reactive growth and optical properties of mixed aerosol particles

The composition of atmospheric aerosols impacts their optical and hygroscopic properties, cloud condensation ability and chemical reactivity in heterogeneous reactions. The laboratory work in this thesis probes, in particular, how organic material is incorporated into aerosols and the connection between chemical composition, hygroscopicity and optical properties.; The impact of a coating of the fatty acid palmitic acid on the hygroscopicity of ammonium sulfate aerosol at 273K was probed using FTIR. It was found that aerosols containing ∼20 wt% palmitic acid have the same deliquescence relative humidity as pure ammonium sulfate (80 +/- 3% RH). Particles with ∼50 wt% palmitic acid however, began to take up water at relative humidities as low at 69%RH and continued to slowly take up water to 85%RH without fully deliquescing. These mixed particles had the same efflorescence RH as ammonium sulfate. These studies suggest that even if insoluble substances coat salt particles in the atmosphere, there may be relatively little effect on the resulting water uptake and loss.; The uptake of gas phase hexanal into ammonium sulfate and sulfuric acid aerosols was studied. While non-acidic ammonium sulfate aerosols showed no organic uptake, the acidic aerosols took up substantial amounts of organic material when exposed to hexanal vapor through acid-catalyzed reactions of the hexanal. Both aldol condensation and hemiacetal products were identified in the sulfuric acid aerosols, with the dominant reaction products dependent upon the initial acid concentration of the aerosol. Aerosols up to 88.4 wt% organic/11.1 wt% H2SO4/0.5 wt% water were produced via these two dimerization reaction pathways.; The response of the light extinction, lambda=532nm, by internally mixed ammonium sulfate/water-soluble organic aerosols to changes in relative humidity was studied using a tandem cavity ring-down aerosol extinction spectrometer. The ratio of the extinction at high relative humidities (80%RH) to low RH (dry) is reported as fRHext(80%RH,dry). It was found that fRH ext(80%RH,dry) values were insensitive to the composition of the water-soluble organic fraction for various mixtures of ammonium sulfate with sugars, dicarboxylic acids and complex mixtures of water-soluble organics. In addition, fRH ext(80%RH, dry) was found to vary linearly (within 15%) with the organic/inorganic content.