以HTDMA與HT DMA-APM系統探討無機鹽奈米微粒的吸溼行為

Investigating the Hygroscopic Behavior of Inorganic Nanoparticles Using HTDMA and HT DMA-APM System

陳可青(Ke-Ching Chen)

指導教授 ： 蕭大智

國立中央大學/工學院/環境工程研究所/碩士(2014年)

摘要

依據大氣研究調查發現，粒徑界於8-100 nm的氣膠微粒廣泛存在於大氣對流層中，且多屬於凝結核之範圍，其中包含無機鹽類氣膠微粒。而這些無機鹽類微粒大多具有吸溼性，其與環境水汽間的異質成核反應對於大氣中的雲凝結核濃度、地球的輻射收支、氣膠的光學性質、甚至是人體健康皆會造成顯著的影響。
隨著相對溼度上升，無機鹽氣膠微粒(NaCl、(NH4)2SO4與混合微粒)的表面會有水膜包覆，當微粒達到潮解相對溼度(DRH)時，則會造成固液態之間的相轉變。而Russell於2002的文獻中以理論模擬之方法，提出奈米微粒DRH因受粒徑效應(表面自由能)影響會有兩個完全相反的結果，其最主要之差異來自於潮解前微粒表面是否有水膜的包覆。而本研究將針對不同粒徑、構形及化學組成的無機鹽氣膠微粒，觀察其在潮解前與水汽的交互作用關係，以探討水膜對於微粒吸溼行為的改變與影響。
截至目前為止，許多文獻也提出微粒在DRH前會受水膜之包覆而使結構更扎實且趨近於圓球狀。然而卻僅有少數研究以實驗量測之方法，透過Nano-DMA觀察到在潮解前微粒粒徑大小波動之情形。有鑑於此，本研究分別以HTDMA與APM系統即時量測微粒在不同相對溼度條件下，微粒因吸溼增長所造成的粒徑與質量變化。結合HTDMA與APM系統(合稱HT DMA-APM)所得粒徑與質量兩者數據，即可推算出微粒在潮解前因水膜包覆而造成的密度變化。本研究依據HT DMA-APM量測結果發現100 nm氯化鈉微粒在潮解前(RH=73%)約受7-8個水分子厚度的水膜包覆。水膜包覆之現象即可能改變微粒固、液相表面張力的比值，進而對奈米微粒表面自由能造成影響，以此解釋為何微粒奈米DRH會隨粒徑下降而上升。除此之外，本研究中也以半理論半經驗的多項式函數預測不同NaCl/(NH4)2SO4比例條件的內混微粒DRH，所得結果與實驗結果吻合。而有關於σc/σaq比值改變而影響微粒DRH的粒徑效應關係亦於本論文中進行探討。

關鍵字

奈米微粒 ； 吸溼 ； 無機鹽

並列摘要

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.

並列關鍵字

nanoparticles ； hygroscopic ； inorganic

參考文獻

1. Russell, L.M. and Y. Ming, Deliquescence of small particles. The Journal of chemical physics, 2002. 116(1): p. 311-321.

2. Biskos, G., et al., Nanosize effect on the deliquescence and the efflorescence of sodium chloride particles. Aerosol Science and Technology, 2006. 40(2): p. 97-106.

3. Romakkaniemi, S., et al., Adsorption of water on 8-15 nm NaCl and (NH4) 2SO4 aerosols measured using an ultrafine tandem differential mobility analyzer. The Journal of Physical Chemistry A, 2001. 105(35): p. 8183-8188.