Jiaxi Fang

Comparison of Measured Particle Lung-Deposited Surface Area Concentrations by an Aerotrak 9000 Using Size Distribution Measurements for a Range of Combustion Aerosols

Surface area in addition to mass concentration is increasingly being emphasized as an important metric representing potential adverse health effects from exposure to inhaled particles. Lung-deposited surface area (SA) concentrations for a variety of aerosols: coal, biomass, cigarette, incense, candle, and TiO2 were measured using an AeroTrak 9000 (TSI Incorporated) and compared with those calculated from number size distributions from a scanning mobility particle sizer (SMPS). Three methodologies to compute the SA concentrations using the International Commission on Radiological Protections (ICRP) Lung Deposition model and an SMPS were compared. The first method calculated the SA from SMPS size distributions, while the second method used lognormal size distribution functions. A third method generated a closed-form equation using the method of moments. All calculated SMPS SA data against which the measured SA data were compared were generated using the first method only; however, the SA concentrations calculated from each of the three methods demonstrated strong correlations with each other. Overall, results between measured and calculated lung-deposited SA indicated strong positive linear associations (R 2 0.78 - >0.99), moderately dependent on the type of aerosol. In all cases, the measured SA concentrations slightly underestimated those calculated from the SMPS data, with the exception of coal combustion particles. Although some dependency on aerosol material exists, the instrument measuring lung-deposited SA demonstrated consistent reliability across a range of concentrations for a range of materials. For optimal results however, applying a correction factor (CF) before taking the instrument to the field is recommended. Copyright 2013 American Association for Aerosol Research

Measurement of sub-2 nm clusters of pristine and composite metal oxides during nanomaterial synthesis in flame aerosol reactors.

Measuring stable clusters to understand particle inception will aid the synthesis of well-controlled nanoparticles via gas-phase aerosol routes. Using a Half Mini differential mobility analyzer, the presence of monomers, dimers, trimers, and tetramers was detected for the first time in a flame aerosol reactor during the synthesis of pristine TiO2 and TiO2/SiO2 nanocomposites. Atomic force microscopy confirmed the presence and the size of sub-2 nm clusters. The detection of these clusters elucidated the initial stages of particle formation during combustion synthesis and supported previous hypotheses that collisional growth from stable monomers of metal oxides is the first step of particle growth.

Synthesis of Titanium Dioxide Aerosol Gels in a Buoyancy-Opposed Flame Reactor

Aerosol gels are a novel class of materials with potential to serve in various energy and environmental applications. In this work, we demonstrate the synthesis of titanium dioxide (TiO2) aerosol gels using a methane-oxygen coflow diffusion flame reactor operated in down-fired configuration (fuel flow in the direction opposite to buoyancy forces). Titanium tetraisopropoxide was fed as a precursor to the flame under different operating conditions. Control of the monomer size and crystalline phase of TiO2 gel particles was achieved by adjusting the flame operating conditions, specifically the flame temperature, which was shown to significantly influence the phase transformation and rate of particle growth and sintering. The resulting materials were characterized for their physical and optical properties. Results showed that the TiO2 aerosol gels had effective densities in the range 0.021–0.025 g/cm3, which is 2 orders of magnitude less than the theoretical mass density of TiO2. The monomer size distribution, crystalline phase, and UV-Vis absorbance spectra of the gels showed distinct characteristics as a function of flame temperature. Copyright

An Apparatus for Generating Aged Cigarette Smoke for Controlled Human Exposure Studies

Research has shown that cigarette smoke changes chemically and physically after it is released into indoor air, that these changes can increase secondhand smoke (SHS) toxicity, and that acute exposures to even low levels of SHS increase the risk of cardiopulmonary disease. We designed a system to reproduce realistic SHS exposures in the laboratory for use in controlled human exposure studies. We generated cigarette smoke with a smoking machine, diluted it and conducted it through a 6 m3 stainless steel flow reactor at rates equivalent to the upper ranges of normal residential air exchange rates, to create aged cigarette smoke as a model for secondhand cigarette smoke. We observed that approximately 50% of the particle mass deposited within the system and that particle deposition percentage was higher when absorbent materials were placed within the system. The particle size ranges and deposition percentages, coefficients and velocities observed for this smoke aerosol are in good agreement with published values for SHS observed in residences and vehicles. This apparatus also permits the study of the physical and chemical interactions between SHS and indoor surfaces. The apparatus delivers stable aerosol concentrations to a human subject, which will permit accurate analysis of dose-response relationships in studies of the cardiovascular and respiratory effects of SHS exposure. Copyright 2012 American Association for Aerosol Research

Cluster formation mechanisms of titanium dioxide during combustion synthesis: Observation with an APi-TOF

ABSTRACT Few studies reported the formation of Ti-containing clusters in the initial stages of TiO2 flame synthesis. The conversion from synthesis precursor to TiO2 monomers was commonly assumed to take place through global reaction such as thermal decomposition and/or hydrolysis at high temperatures. More recent studies have been able to identify stable intermediates of Ti-containing monomers, most commonly Ti(OH)4, as the final step before the formation of TiO2. However, no larger Ti-containing cluster formation mechanisms or interactions between these monomers have been tracked. To investigate cluster formation pathways of TiO2 during flame synthesis, Charged clusters were measured in an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer. TiO2 nanoparticles were synthesized by adding titanium tetraisopropoxide (TTIP) precursor to a premixed CH4/O2/N2 flat flame aerosol reactor. Pure TiO2 clusters were not detected by the APi-TOF. Results from measured mass spectra and mass defect plots show that for positively charged clusters, the abstraction of CH2 groups occurs simultaneously with the clustering of larger intermediate organometallic species. For negatively charged clusters, NOx formation pathways in the flame may play a role during the initial stages of TiO2 formation, since a lot of Ti-containing clusters were attached with nitrate-related species. These research findings provide insights on quantum dot synthesis and molecular doping where rapid dilution of the flame synthesized nanoparticles is needed to better control the particle size and chemical composition. The possible influences of and potential artifacts brought by the dilution system on observing the incipient particle formation in flames were also discussed.

The initial stages of multicomponent particle formation during the gas phase combustion synthesis of mixed SiO2/TiO2

ABSTRACT The ability to properly scale the synthesis of advanced materials through combustion synthesis routes is limited by our lack of knowledge regarding the initial stages of particle formation. In flame aerosol reactors, the high temperatures, fast reaction rates, and flame chemistry can all play a critical role in determining the properties of the resulting nanomaterials. In particular, multicomponent systems pose a unique challenge as most studies rely on empirical approaches toward designing advanced composite materials. The lack of predictive capabilities can be attributed to a lack of data on particle inception and growth below 2 nm. Measurements for the initial stages of particle formation during the combustion synthesis of SiO2 and composite SiO2/TiO2 using an atmospheric pressure inlet time-of-flight mass spectrometer are presented. Both positively and negatively charged clusters can be measured and results show the presence of silicic acid species which grow through dehydration, hydrogen abstraction, and interactions with hydroxyl radicals. In the case of composite SiO2/TiO2 particle formation, new molecular species containing Ti atoms emerge. Tandem differential mobility analysis-mass spectrometry (DMA-MS) provided further insight into the size-resolved chemistry of particle formation to reveal that at each cluster size, further hydroxyl-driven reactions take place. From this we can conclude that previous assumptions on collisional growth from simple monomer species of SiO2 and TiO2 do not sufficiently describe the collisional growth mechanisms for particle growth below 2 nm. Copyright