dc.description.abstract | Particles of sizes between 8 and 100 nm in diameter are ubiquitous in the lower troposphere. The way in which these particles including the inorganic salt take up water is critical in determining their role in forming haze and clouds as well as in serving as sites for heterogeneous chemical reactions [1]. This hygroscopic growth plays a significant role in atmospheric phenomena such as cloud-droplet nucleation, earth’s radiation budget, optical properties and human health assessment.
As relative humidity increases, water would be adsorbed onto the surface of inorganic salt particles, such as pure NaCl, (NH4)2SO4 aerosols, or thier externally and internally mixed aerosols. These inorganic particles would initiate a transition from the crystalline to aqueous phase (deliquescence) at the deliquescence relative humidity (DRH). However, the current two physical models proposed in the literatures predicted opposite trend of the size effect on DRH (because of different σc/σaq ratio) [2]. Russell and Ming attributed the inconsistent predictions to the different assumptions of dry particle surface and of surface coated with a few monolayers of water molecules prior to deliquescence [1]. Therefore, for studying the hygroscopic behavior of inorganic salt particles, it is important to investigate the interactions between water and particles at or prior to DRH as well as the effects of particle size, morphology and chemical composition etc..
Many studies have claimed that the inorganic particles would start to be more compacted and spherical due to water adsorbed onto the particle surface, i.e. forming water film, at or prior to DRH. However, only few have experimentally observed the phenomena during deliquescence transition using Nano-DMA measurements [3]. Accordingly, both the hygroscopic tandem differential mobility analyzer (HTDMA, Electrostatic Classifier, TSI 3080, with DMA, TSI 3081/3085) system and the aerosol particle mass analyzer (APM, Kanomax 3601) were applied to investigate the change of particle size and mass in real time. Combining the HTDMA and APM measurements, the particle density changes caused by forming water film during deliquescence transition was observed. The measurements reveals approximately 7 to 8 monolayers of water momlecules on the surface of 100 nm crystalline sodium chloride at relative humidity 73%. This phenomenon may changes the values of σc/σaq as well as surface energy, which provides the explanation why the DRH of the nanoparticles increases as particle size decreases. In addition, we developed a semi-empirical model for predicting the CDRH of internally mixing particles at different NaCl/(NH4)2SO4 ratios, which was also validated through the experimental results. The relationship between the size effects on DRH and σc/σaq was also discussed in the thesis.
| en_US |