Abstract View
Characterization of Particle Charge from Aerosol Generation Process: Investigations of Material Reactivity
ERIN M. DURKE, Monica McEntee, Meilu He, Suresh Dhaniyala, US Army CCDC CBC
Abstract Number: 51
Working Group: Aerosol Chemistry
Abstract
Aerosols are one of the most important and significant surfaces in the atmosphere. They can influence weather, absorption and reflection of light, and reactivity of atmospheric constituents. A notable feature of aerosol particles is the presence of surface charge, a characteristic imparted via the aerosolization process. The existence of charge can complicate the interrogation of aerosol particles, therefore many researchers remove or neutralize aerosol particles before characterization. However, the charge is present in real world samples, and likely has an effect on the physical and chemical properties of an aerosolized material. In our studies, we aerosolized different materials in an attempt to characterize the charge imparted via the aerosolization process and determine what impact it has on the aerosolized materials’ properties.
The metal oxides, TiO2 and SiO2, were aerosolized expulsively and then characterized, using several different techniques. Particle charge distribution measurements were conducted via the employment of a custom scanning mobility particle sizer. Determination of the degree of surface charging led to the use of non-traditional techniques to explore the impact of additional surface charge on the overall reactivity of the metal oxides, specifically TiO2.
TiO2 was aerosolized, again expulsively, onto a gold-coated tungsten mesh, which was then evaluated with transmission infrared spectroscopy in an ultra-high vacuum environment. The TiO2 aerosols were exposed to O2, H2 and CO, respectively. Exposure to O2 resulted in a decrease in the overall baseline of the aerosol spectrum, suggesting O2 removed some of the surface charge imparted during aerosolization. Upon exposure to H2, there was no observable rise in the baseline of the IR spectrum, as is typically seen for TiO2, due to the population of electrons into the shallow trapped states and subsequent promotion of the electrons into the conduction band. This result suggests that the additional charge imparted via aerosolization fills the trapped states, therefore no rise is seen upon exposure to H2. Dosing the TiO2 aerosols with CO showed no adsorption of CO on the surface, even at lower temperatures (~100 K), indicating the additional charge on the aerosol surface prevents the CO molecules from adsorbing to the TiO2 surface.
The TiO2 was also exposed to methanol vapor. The reaction was performed on both aerosolized TiO2 and unmodified TiO2 powder. The results for the reaction with the aerosolized sample differ drastically from those recorded for the methanol exposed powder. The results observed for the exposure of aerosolized TiO2 suggest that the additional charge imparted via aerosolization impacts the reaction with methanol, causing a divergence from the known, well-documented pathway.
Finally, a high-flow dual-channel Differential Mobility Analyzer (HDDMA) was employed to determine the particle charge distribution for the polydisperse TiO2 samples. The HDDMA data indicates as many as 230 charges per particle for a 700 nm particle of TiO2. Also of note is the polarity of the aerosol particles. The concentration of positively charged particles versus negatively charged particles appears to be dependent upon the material being aerosolized, as opposed to the generation process simply resulting in an even bipolar distribution.