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Particles from smoldering biomass burning (BB) represent a major source of carbonaceous aerosol in the terrestrial atmosphere. In this study, mass specific absorption spectra of laboratory-generated smoldering wood particles (SWP) from 3 hardwood and 3 softwood species were measured in situ. Absorption data spanning from λ = 500 to 840 nm were collected using a photoacoustic spectrometer coupled to a supercontinuum laser with a tunable wavelength and bandwidth filter. SWP were size- (electrical mobility) and mass-selected prior to optical characterization allowing data to be reported as mass-specific absorption cross sections (MAC). The median measured MAC at λ = 660 nm for smoldering oak particles was 1.1 (0.57/1.8) × 10 -2 m 2 g -1 spanning from 83 femtograms (fg) to 517 fg (500 nm ≤ mobility diameter ≤950 nm), MAC values in parentheses are the 16 th and 84 th percentiles of the measured data (i.e., 1σ). The collection of all six wood species (Oak, Hickory, Mesquite, Western redcedar, Baldcypress, and Blue spruce) had median MAC values ranging from 1.4 × 10 -2 m 2 g -1 to 7.9 × 10 -2 m 2 g -1 at λ = 550 nm with absorption Ångström exponents (AAE) between 3.5 and 6.2. Oak, Western redcedar, and Blue spruce possessed statistically similar (p > 0.05) spectra while the spectra of Hickory, Mesquite, and Baldcypress were distinct (p < 0.01) as calculated from a point-by-point analysis using the Wilcox rank-sum test.

Optical absorption spectra of laboratory generated aerosols consisting of black carbon (BC) internally mixed with nonabsorbing materials (ammonium sulfate, AS, and sodium chloride, NaCl) and BC with a weakly absorbing brown carbon surrogate derived from humic acid (HA) were measured across the visible to near-IR (550 to 840 nm). Spectra were measured in situ using a photoacoustic spectrometer and step-scanning a supercontinuum laser source with a tunable wavelength and bandwidth filter. BC had a mass-specific absorption cross section (MAC) of 7.89 ± 0.25 m(2) g(-1) at λ = 550 nm and an absorption Ångström exponent (AAE) of 1.03 ± 0.09 (2σ). For internally mixed BC, the ratio of BC mass to the total mass of the mixture was chosen as 0.13 to mimic particles observed in the terrestrial atmosphere. The manner in which BC mixed with each material was determined from transmission electron microscopy (TEM). AS/BC and HA/BC particles were fully internally mixed, and the BC was both internally and externally mixed for NaCl/BC particles. The AS/BC, NaCl/BC, and HA/BC particles had AAEs of 1.43 ± 0.05, 1.34 ± 0.06, and 1.91 ± 0.05, respectively. The observed absorption enhancement of mixed BC relative to the pure BC was wavelength dependent for AS/BC and decreased from 1.5 at λ = 550 nm with increasing wavelength while the NaCl/BC enhancement was essentially wavelength independent. For HA/BC, the enhancement ranged from 2 to 3 and was strongly wavelength dependent. Removal of the HA absorption contribution to enhancement revealed that the enhancement was ≈1.5 and independent of wavelength.

We report the first mass-specific absorption and extinction cross sections for size- and mass-selected laboratory-generated soot aerosol. Measurement biases associated with aerosols possessing multiple charges were eliminated using mass selection to isolate singly charged particles for a specified electrical mobility diameter. Aerosol absorption and extinction coefficients were measured using photoacoustic and cavity ring-down spectroscopy techniques, respectively, for lacey and compacted soot morphologies. The measurements show that the mass-specific absorption cross sections are proportional to particle mass and independent of morphology, with values between 5.7 and 6 m(2) g(-1). Mass-specific extinction cross sections were morphology dependent and ranged between 12 and 16 m(2) g(-1) for the lacey and compact morphologies, respectively. The resulting single-scattering albedos ranged from 0.5 to 0.6. Results are also compared to theoretical calculations of light absorption and scattering from simulated particle agglomerates. The observed absorption is relatively well modeled, with minimum differences between the calculated and measured mass absorption cross sections ranging from ∼ 5% (lacey soot) to 14% (compact soot). The model, however, was unable to satisfactorily reproduce the measured extinction, underestimating the single-scattering albedo for both particle morphologies. These discrepancies between calculations and measurements underscore the need for validation and refinement of existing models of light scattering and absorption by soot agglomerates.

Atmospheric aerosols play a very important role in atmospheric processes and have a major influence on the global climate. In this paper, we report results of a molecular dynamics study on the unique properties of organic-coated water droplets. In particular, we find that, for particles preferring an inverted micelle structure, the lower chain-chain interaction, with increasing radial distance from the water-organic interface, results in a negative internal radial pressure profile for the organic layer. As a result, a coated particle behaves as though the surface tension is "negative" and implies that such a particle will inherently have an inverse Kelvin vapor pressure effect, resulting in increased water condensation.

The kinetics of the reaction between soot nanoparticles and molecular oxygen were studied by tandem differential mobility analysis (TDMA). The particles were extracted from the tip of an ethene diffusion flame. Reactions were studied at atmospheric pressure in mixtures of nitrogen and oxygen. The studies involved particles of an initial mobility diameter of 30 nm over broad ranges of temperature (500-1100 degrees C) and oxygen volume fraction (0-1). Measurements as a function of oxygen partial pressure establish that the oxidation kinetics are not first-order in oxygen volume fraction (F(O2)). Rather, the oxidation rate increases rapidly and linearly with F(O2) between 0 and 0.05 and then more slowly but still linearly between 0.05 and 1. Temperature dependent measurements are consistent with a reaction pathway involving two kinetically distinguishable oxidation sites which interconvert thermally and through oxidation. Results and conclusions are compared to those of earlier studies on the oxidation of soot.

A new polymorph of iodic acid, δ-HIO3, synthesized via aerosol spray pyrolysis was characterized with powder X-ray diffraction and its crystal structure was solved. We find that a previously misidentified phase of I4O9 hydrate is in fact a new polymorph of HIO3, which crystalizes in the orthorhombic space group P212121. [ABSTRACT FROM PUBLISHER]

High iodine containing oxides are of interest as biocidal components in energetic applications requiring fast exothermic reactions with metallic fuels. Aerosol techniques offer a convenient route and potentially direct route for preparation of small particles with high purity, and are a method proven to be amenable and economical to scale-up. Here, we demonstrate the synthesis of various iodine oxide/iodic acid microparticles by a direct one-step aerosol method from iodic acid. By varying temperature and humidity, we produced near phase pure δ-HIO3, HI3O8, and I2O5 as determined by X-ray diffraction. δ-HIO3, a previously unknown phase, was confirmed in this work. In addition, scanning electron microscopy was used to examine the morphology and size of those prepared iodine oxide/iodic acid particles and the results show that all particles have an irregularly spherical shape. Thermogravimetric/differential scanning calorimetry measurement results show that HIO3 dehydrates endothermically to HI3O8, and then to I2O. I2O5 decomposes to I2 and O2. [ABSTRACT FROM PUBLISHER]

The effect of an aligning electric field on the mobility of both flame generated smoke and smolder generated smoke was studied. A pulsed differential mobility analyzer was used to study the alignment without changing the DMA flows. No detectable change in the mobility was observed for the smolder smoke, while a small but detectable effect of up to 5% decrease in the mobility diameter with increasing field was observed for the largest aggregates with a mobility diameter of 200 nm. We modeled the friction coefficient tensor of soot fractal aggregates as an equivalent prolate spheroid to obtain the field induced mobility as a function of aspect ratio. The alignment probability distribution function was determined by computing the polarizability tensor for simulated fractal aggregates. One interesting result was the smallness of the prolate sphere aspect ratio of 1.2 to 1.3 compared to the much larger aspect ratio from TEM analysis and from the polarizability ratio. An explanation for the low value based on the contribution to the friction coefficient from the individual spheres for a fractal is given. Another interesting observation is the broadening of the mobility size distribution with decreasing field. This is shown to be related to the polydispersity of the aspect ratio. The fact that all three aggregate sizes appear to fit the same spheroid aspect ratio is interesting, and offers a first-order approach to describing transport properties of aggregates. An estimate of the rotation relaxation time of the fractal aggregate was made to verify that the rotation time was much shorter than the duration of the zero electric field period during each cycle. Copyright © 2016 American Association for Aerosol Research [ABSTRACT FROM PUBLISHER]

There is currently a need for low oxide content nanoaluminum as a component in high-energy-density fuels. A gas-phase passivation coating of perfluoropentanoic acid on in situ generated bare nanoaluminum accomplished in an aerosol stream and resulting in an air-stable product is demonstrated. Transmission electron microscopy inspection demonstrates a 1-2 nm coating layer, and thermogravimetric analysis reveals an 80% active fuel content, an increase of 17% from untreated product. X-ray photoelectron spectroscopy confirms both the presence of the fluorocarboxylic acid on the aluminum surface and the thinner coating layer compared with the untreated case. A bridge bonding coordination between the carboxylate group and aluminum is indicated via Fourier transform infrared spectroscopy. The coated product demonstrates reduced ignition temperature in thermite combinations for temperature-jump fine wire combustion tests and X-ray photoelectron spectroscopy verifies formation of AlF3 in burned product. [ABSTRACT FROM AUTHOR]

The mobility of a nonspherical particle is a function of both particle shape and orientation. In turn, the higher magnitude of electric field causes nonspherical particles to align more along the field direction, increasing their mobility or decreasing their mobility diameter. In previous works, Li et al. developed a general theory for the orientation-averaged mobility and the dynamic shape factor applicable to any axially symmetric particles in an electric field, and applied it to the specific cases of nanowires and doublets of spheres. In this work, the theory for a nanowire is compared with experimental results of gold nanorods with known shape determined by TEM images. We compare the experimental measured mobility sizes with the theoretical predicted mobility in the continuum, free molecular, and the transition regime. The mobility size shift trends in the electric fields based on our model, expressed both in the free molecular regime and in the transition regime, are in good agreement with the experimental results. For rods of dimension: widthdr= 17 nm and lengthLr= 270 nm, where one length scale is smaller than the mean free path and one larger, the results clearly show that the flow regime of a slender rod is mostly controlled by the diameter of the rod (i.e., the smallest dimension). In this case, the free molecule transport properties best represented our nanorod. Combining both theory and experiment we show how, by evaluating the mobility as a function of applied electric field, we can extract both rod length and diameter. Copyright 2013 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The article presents an illustration of nanoenergetic gas generators based on periodate salts. It notes the development of versatile aerosol spray drying approach to make periodate salt nanoparticles using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Moreover, it provides information on the relative reactivity of nanoenergetic formulations of periodate.

Abstract: The substrate-free hydrolysis of Zn nanocrystals was investigated as the second step in a ZnO/Zn solar water-splitting thermochemical cycle. In this work, we combined two different ion-mobility schemes in series to study the hydrolysis kinetics of size-selected zinc nanocrystals (NCs). The first mobility characterization size selects particles with a differential mobility analyzer (DMA). The second mobility characterization employs an aerosol particle mass analyzer (APM) and measures changes in mass resulting from a controlled hydrolysis of the Zn NCs. A low temperature reaction mechanism is proposed to explain the mass change behavior of Zn NCs hydrolysis at 100–250°C. An Arrhenius law was used to extract the reaction kinetic parameters. The hydrolysis activation energy and the order of the reaction with respect to water mole fraction were found to be 24±2kJ/mol and 0.9±0.1, respectively. Complete conversion of 70nm Zn NCs was achieved at 175°C with a residence time of about 10s and water vapor mole fraction of 19%. [Copyright &y& Elsevier]

A combination of a differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM) is used to measure the mass of NIST Standard Reference Materials (SRM®) PSL spheres with 60 and 100 nm nominal diameter, and NIST traceable 300 nm PSL spheres. The calibration PSL spheres were previously characterized by modal diameter and spread in particle size. We used the DMA to separate the particles with modal diameter in a narrow mobility diameter range. The mass of the separated particles is measured using the APM. The measured mass is converted to diameter using a specific density of 1.05. We found that there was good agreement between our measurements and calibration modal diameter. The measured average modal diameters are 59.23 and 101.2 nm for nominal diameters of 60 and 100 nm (calibration modal diameter: 60.39 and 100.7 nm) PSL spheres, respectively. The repeatability uncertainty of these measurements is reported. For 300 nm, the measured diameter was 305.5 nm, which is an agreement with calibration diameter within 1.8%. The effect of spread in particle size on the APM transfer function is investigated. Two sources of the spread in “mono-dispersed” particle size distributions are discussed: (a) spread due to the triangular DMA transfer function, and (b) spread in the calibration particle size. The APM response function is calculated numerically with parabolic flow through the APM and diffusion broadening. As expected from theory, the calculated APM response function and measured data followed a similar trend with respect to APM voltage. However, the theoretical APM transfer function is narrower than the measured APM response. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate gas-phase (aerosol) generation of diameter-controlled carbon nanotubes (CNTs) by employing size-controlled monodisperse nickel nanoparticles produced by the combination of pulsed laser ablation and electrostatic classification. The electrostatic classifier sorted agglomerated mono-area nickel particles, and then a subsequent heating process at ∼1200 °C created sintered single primary particles with very narrow size distribution. These isolated single primary particles were then sent to an aerosol reactor where free-flight CNTs were grown with acetylene and hydrogen mix at temperature of ∼750 °C. The resulting CNTs formed in this continuous gas-phase process were found to have a uniform diameter, which is commensurate with the diameter of the size-controlled catalytic nickel particles. [Copyright &y& Elsevier]

The corrected rotational diffusion coefficient for a rod in the free molecular regime is given in this work and a simplified derivation using the differential drag forces for a rotating rod in the free molecular regime is presented to explain the difference between our result and the one from Kim's approach. Finally, we compare the rotational diffusion coefficient between a rod and an ellipsoid with the same aspect ratio and the same volume. Copyright 2014 American Association for Aerosol Research [ABSTRACT FROM PUBLISHER]

We demonstrate a method to restructure graphene oxidesheets intomonodisperse solid 17 nm nanospheres by tuning the solution ionicstrength. This method enables the preparation of both two-dimensionalself-assemblies comprising three-dimensional GO nanospheres and three-dimensionalsuper assemblies of GO clusters via dispersal into an aerosol. TheGO super assemblies are more thermally stable than single crumpledsheets. Finally, we demonstrate that GO nanospheres and their assembliescan be thermally processed to form reduced GO with high aromatic characterwhile still maintaining their spherical conformation. [ABSTRACT FROM AUTHOR]

Abstract: We show a low temperature gas-phase synthesis route to produce faceted aluminum crystals in the aerosol phase. Use of triisobutylaluminum whose decomposition temperature is below the melting point of elemental aluminum enabled us to grow nanocrystals from its vapor. TEM shows both polyhedral crystalline and spherical particle morphologies, but with the addition of an annealing furnace one can significantly enhance the production of just the polyhedral particles. The results on surface passivation with oxygen suggest that these nanocrystals are less pyrophoric than the corresponding spherical aluminum nanoparticles, and combustion tests show an increase in energy release compared to commercial nanoaluminum. [ABSTRACT FROM AUTHOR]

[Display omitted] Unlike liquid phase colloidal assembly, significantly changing the structure of fractal aggregates in the aerosol phase, is considered impractical. In this study, we discuss the possibility of applying external magnetic and electric fields, to tune the structure and fractal dimension (D f) of aggregates grown in the aerosol phase. We show that external fields can be used to induce dipole moments in primary nanoparticles. We found that an ensemble of particles with induced dipole moments will interact through directional attractive and repulsive forces, leading to the formation of linear, chain-like aggregates with D f ~ 1. The aggregate structure transition is dependent on the primary particle sizes, temperature and applied field strength which was evaluated by performing a hybrid ensemble/cluster-cluster aggregation Monte Carlo simulation. We demonstrate that the threshold magnetic field strength required to linearly assemble 10–500 nm particle sizes are practically achievable whereas the electric field required to assemble sub-100 nm particles are beyond the breakdown strength of most gases. To theoretically account for the enhanced coagulation rates due to attractive interactions, we have also derived a correction factor to both free molecular and transition regime coagulation kernel, based on magnetic dipolar interactions. A comparison has been made between the coagulation time-scales estimated by theory and simulation, with the estimated magnetization time-scales of the primary particles along with oscillation time period of the magnetic field, to demonstrate that sub-50 nm superparamagnetic primary particles can be magnetized and assembled at any temperature, while below the Curie temperature ferromagnetic particles of all sizes can be magnetized and assembled, given the applied field is higher than the threshold. [ABSTRACT FROM AUTHOR]

Mass absorption coefficient spectra were measured between λ  = 500 nm and 840 nm for nine forms of highly-absorbing carbonaceous aerosol: five samples generated from gas-, liquid- and solid-fueled flames; spark-discharge fullerene soot; graphene and reduced graphene oxide (rGO) crumpled nanosheets; and fullerene (C 60 ) assemblies. Aerosol absorption spectra were measured for size- and mass-selected particles and found to be dependent on fuel type and formative conditions. Flame-generated particles had morphologies consistent with freshly emitted black carbon (BC) with mass absorption coefficients (MAC) ranging between 3.8 m 2  g −1 and 8.6 m 2  g −1 at λ  = 550 nm. Absorption Ångström exponents ( AAE ) – i.e. MAC spectral dependence – ranged between 1.0 and 1.3 for flame-generated particles and up to 7.5 for C 60 . The dependence of MAC and AAE on mobility diameter and particle morphology was also investigated. Lastly, the current data were compared to all previously published MAC measurements of highly-absorbing carbonaceous aerosol. [ABSTRACT FROM AUTHOR]

Ultrasmall metal nanoparticles are inherently unstable because of their high specific surface area. This work investigates how growth and aggregation of these nanostructures can be circumvented by incorporating them into a polymer matrix in an on-the-fly growth process. We demonstrate the formation of sub-5 nm particles of Ni, Co, and Cu nanoparticles in a polymer matrix using an aerosol single-drop reactor approach. The rapid thermal pulse given to the aerosol particles enables the formation of nuclei and growth, with subsequent rapid quenching to freeze in the structure. The role of the temperature as well as the precursor concentration of the resulting size and morphology is discussed. A characteristic time analysis and an analysis of the particle size distributions lead to the conclusion that growth is governed by nucleation and surface growth, with little coagulation or Ostwald ripening. Finally, we note that this aerosol route is amenable to scale-up for large-scale production of nanoclusters that can either be used as is within the polymer or released by solvent extraction, depending on the application. [ABSTRACT FROM AUTHOR]

The theoretical drift velocity of a randomly oriented nonspherical aerosol particles in an external field has been previously computed both in the limit of slow rotation and in the limit of fast rotation but not in the intermediate interval. The low rotation limit has been widely used to calculate the drift velocity for a range of nonspherical particles. The fast rotation limit, which is equivalent to the projected area method, has been used for molecular ions and agglomerates. A 1-D model equation containing the particle acceleration and an orientation dependent friction coefficient is proposed to predict the drift velocity between the two limits. This model has the essential physical phenomena without the complications of the 3-D treatment of the combined translation and rotation behavior. As an example, the drift velocity is computed for model parameters based on the friction tensor and rotational diffusion coefficient for circular cross section nanorods in the free molecular limit. For a momentum accommodation coefficient of 0.9 and a particle density of 1000 kg/m 3 , the largest percent deviation from the low rotation velocity limit is 14% and the deviation is at most 1% for nanorods of any length for diameters of 20 nm diameter or larger. Much larger changes in the velocity ratio are shown to occur if the momentum accommodation coefficient is reduced. Also, examples are given where the dimensionless rotation rate increases by about a factor of 7 from either a change in density or a change in the mean free path of the background gas. The results of recent experiments and model calculation of the collision cross section and mobilities of large molecular ions are discussed in regard to our model predictions on the effect of rotation on the drift velocity. [ABSTRACT FROM AUTHOR]

An electrospray–differentialmobility analyzer (ES-DMA)was operated with an aerosol flow-mode, temperature-programmed approachto enhance its ability to characterize the particle size distributions(PSDs) of nanoscale particles (NPs) in the presence of adsorbed andfree ligands. Titanium dioxide NPs (TiO2-NPs) stabilizedby citric acid (CA) or bovine serum albumin (BSA) were utilized asrepresentative systems. Transmission electron microscopy (TEM) andinductively coupled plasma mass spectrometry were used to providevisual information and elemental-based PSDs, respectively. Resultsshow that the interference resulting from electrospray-dried nonvolatilesalt residual nanoscale particles (S-NPs) could be effectively reducedusing the thermal treatment process: PSDs were accurately measuredat temperatures above 200 °C for CA-stabilized TiO2-NPs and above 400 °C for BSA-stabilized TiO2-NPs.Moreover, TEM confirmed the volumetric shrinkage of S-NPs due to thermaltreatment and also showed that the primary structure of TiO2-NPs was relatively stable over the temperature range studied (i.e.,below 700 °C). Conversely, the shape factor for TiO2-NPs decreased after treatment above 500 °C, possibly due toa change in the secondary (aggregate) structure. S-NPs from BSA-stabilizedTiO2-NPs exhibited higher global activation energies towardinduced volumetric shrinkage than those of CA-stabilized TiO2-NPs, suggesting that activation energy is dependent on ligand size.This prototype study demonstrates the efficacy of using ES-DMA coupledwith thermal treatment for characterizing the physical state of NPs,even in a complex medium (e.g., containing plasma proteins) and inthe presence of particle agglomerates induced by interaction withbinding ligands. [ABSTRACT FROM AUTHOR]

Abstract: Catalytic decomposition of liquid fuels (n-octane, iso-octane, 1-octene, toluene and methylcyclohexane) is achieved in a continuous tubular aerosol reactor as a model for the solar initiated production of hydrogen, and easily separable CO free carbonaceous aerosol product. The effects of fuel molecular structure and catalyst concentration on the overall hydrogen yield were studied. Iron aerosol particles used as the catalysts, were produced on-the-fly by thermal decomposition of iron pentacarbonyl. The addition of iron catalyst significantly decreases the onset temperature of hydrogen generation as well as improves the reaction kinetics by lowering the reaction activation energy. The activation energy without and with iron addition was 260 and 100 kJ/mol, respectively representing a decrease of over 60%. We find that with the addition of iron, toluene exhibits the highest hydrogen yield enhancement at 900 °C, with a 6 times yield increase over thermal decomposition. The highest H2 yield obtained was 81% of the theoretical possible, for n-octane at 1050 °C. The general trend in hydrogen yield enhancement is that the higher the non-catalytic thermal decomposition yield, the weaker the catalytic enhancement. The gaseous decomposition products were characterized using a mass spectrometer. An XRD analysis was conducted on the wall deposit to determine the product composition and samples for electron-microscopic analysis were collected exiting the furnace by electrostatically precipitating the aerosol onto a TEM grid. [Copyright &y& Elsevier]