Search

One of the most commonly employed instruments in the category of handheld condensation particle counters (CPCs) is the TSI 3007, which employs a simplified system for achieving a temperature difference between its saturator and its condenser in view of increasing portability, energy efficiency and autonomy. As a result, particle growth and consequently the detection efficiency of the instrument can be affected by the measurement conditions. In this work we measure the detection efficiency of the TSI 3007 CPC under temperatures and pressures that differ from standard conditions. Our results show that the performance of the CPC has a strong temperature dependence in the range of 5–30 °C, whereas it is not affected by ambient pressure when this varies between 0.7 and 1.0 atm. The temperature dependent detection efficiency of the instrument becomes significant at sizes below 20 nm. Recording the temperature at which this CPC is operated is therefore strongly advised, especially when required to determine the number concentration of sub-20 nm aerosol particles.

Particle deposits in an inertial impactor stage can be classified as primary or secondary. The primary deposits are directly downstream of each nozzle. Secondary deposits can be in the form of a halo around the primary deposit or of straight-lines between adjacent nozzles. Certainly, these additional secondary deposits lead to a deviation from the theoretical efficiency curve in which only a single primary deposit existed and was considered. The jet-to-jet interaction in multi-nozzle inertial impactors and the formation of secondary deposits has been analyzed and quantified as a function of the distance between nozzles and the Reynolds number. This work has been studied experimentally (Part I) and numerically (Part II), using CFD (Computational Fluid Dynamics) analysis to provide insight into the physical mechanisms for the formation of the secondary deposits and to support the experimental results. This paper provides detailed experimental impactor collection efficiency curves for impactor plates with 3 nozzles spaced in the range of 2.5W to 8W, where W is the nozzle diameter, and Reynolds numbers between 465 and 2210, which quantify the effects of the jet-to-jet interactions and the magnitude of the primary and secondary deposits. From the experimental results it may be concluded that the jet-to-jet interactions can produce four non-ideal effects: premature primary deposits with a half-moon shape, straight-line deposits between nozzles, nozzle plate deposits between nozzle exits (losses), and dispersion of the halo deposits. The secondary deposits and premature primary deposits reduce the slope of the efficiency curve, and/or displace the √Stk 50 towards smaller values and/or generate tails at the low end. All of which result in the collection of small particles that otherwise would be collected at another stage with a lower cut point. With respect to the design criteria, to avoid the formation of the additional deposits generated by jet-to-jet interactions (premature primary deposits, straight-line and nozzle plate deposits), the nozzle-to-nozzle spacings have to be larger than 4W and/or the impactor should work at low Re (in this work Re≤465), even at 2.5W spacing.

In this paper, particulate gas flows over a circular cylinder were studied numerically for wide ranges of flow Reynolds numbers (5

The adhesive collision between a micro-sized particle and wall is important for many natural phenomena and industrial processes. The critical adhesion velocity and restitution coefficient are mainly related to the loss of kinetic energy during the impact. Although several classical models have established the theoretical framework, the practical difficulty is the determination of coefficients. For example, the damping coefficient is often left as the fitting parameter. Sometimes, in order to obtain a good agreement with experimental results, complicated empirical correlations have to be introduced. One reason is that the damping coefficient is defined at the whole particle (usually corresponding to the overlap), not at the local region where damping actually occurs. In this work, we study the particle-wall collision process by finite element method which is able to resolve local stress and strain rate. The surface adhesion is incorporated in the simulation by manually adding and updating the force at the contact area. The results of FEM simulation agree well with reported experimental data for a wide range of particle/wall properties, particle size and impact velocity. The damping coefficient is then linearly correlated with the restitution coefficient for different properties, size and impact velocity, after being nondimensionalized by Hertz collision time and adhesion number. The simple relation has the potential to be extended to more complicated scenarios e.g. non-spherical particle collisions.

Virtual impactors can be used as aerosol concentrators as well as particle size classifiers. The flow field and particle trajectory inside a linear slit-type virtual impactor were investigated using a commercial computational fluid dynamics software package. Effects of flow and slit geometry, including the inlet flow (Qin), the ratio of minor to total flow (r), and the configuration of the taper-lip slit nozzle and the collection slit nozzle, were studied. A new modified Stokes number (Stkc) for the virtual impactor was proposed to predict the d50 at different r values. Stkc,500.5 was found to be retained at approximately 0.9 under different Qin or r values, and it can be considered the characteristic performance parameter for the slit virtual impactor. The numerical simulation results show that the particle loss starts increasing when Stkc0.5 is larger than 2.0 due to a particle-crossing phenomenon. The range of the maximum concentration factor (CFmax) is influenced by the particle-crossing phenomenon. An expression was also developed to predict the size range of particles with the maximum CF under different inlet flow rates. Regarding the geometrical configuration, the arc-lip nozzle could postpone particle-crossing and lessen the internal losses of coarse particles, whereas it increased particle losses near d50. However, the divergent collection slit nozzle could effectively decrease particle losses on the wall of the nozzle without this side effect. The designing guidelines of a linear-slit virtual impactor are briefly summarized based on these findings.

International audience; This study fits in the general framework of developing methods for the measurement of the number size distribution of airborne submicrometer particles suited for field measurements. An approach based on the combination of a Particle Size Selector (PSS) and a portable Condensation Particle Counter (CPC) is proposed in this work. The methodology relies on the successive measurement of the number concentration under 6 operational configurations (n=0, 1, 2, 6, 12 and 18 diffusion screens). A first part of this work consisted of determining particle penetration through different sets of diffusion grids, both experimentally and theoretically. On that basis, a specific data processing was developed, assuming that aerosol size distribution follows a monomodal lognormal law. In the second part of this study, the combined PSS-CPC approach was applied to a series of test aerosols produced in the laboratory, covering a range of modal diameters from 9 to 270 nm. Compared to reference SMPS data, the output number size distributions calculated from the PSS-CPC approach were found to be within± 11% in terms of modal diameter,which is considered satisfactory.; Cette étude s'inscrit dans le cadre général du développement de méthodes de mesure de la distribution granulométrique des particules submicrométriques dispersées dans l’air adaptées aux mesures sur le terrain. Une approche basée sur la combinaison d'un sélecteur (PSS) et d'un compteur de particules portable (CPC) est proposée dans ce travail. La méthodologie repose sur la mesure successive de la concentration en nombre sous 6 configurations opérationnelles (n = 0, 1, 2, 6, 12 et 18 grilles de diffusion). Une première partie de ce travail a consisté à déterminer la pénétration des particules à travers les ensembles de grilles de diffusion mentionnés ci-dessus, à la fois expérimentalement et théoriquement. Sur cette base, un traitement spécifique des données a été développé, en supposant que la distribution de taille d'aérosol suit une loi lognormale monomodale. Dans la deuxième partie de cette étude, l'approche par couplage PSS-CPC a été appliquée à une série d'aérosols d'essai produits en laboratoire, couvrant une gamme de diamètres modaux allant de 10 à 270 nm. Comparées aux données SMPS de référence, les distributions en nombre calculées à partir de l'approche PSS-CPC se sont révélées être à ± 11% en termes de diamètre modal, ce qui est considéré comme satisfaisant.

Flame and furnace aerosol reactors are used extensively for the synthesis of nanoparticles at high temperatures (800–2000 K), however, there is a limited understanding of the early stages of particle growth by collision at these temperatures. An experimental set-up to quantify the collisional growth rate and collisional correction factor (relative to the kinetic theory of gases) for flame synthesized TiO2 nanoparticles in the free molecular regime is described. Spherical charge-neutral nanoparticles with precise control of size are selected from a flame aerosol reactor (FLAR), and then passed through a downstream furnace reactor tube maintained at a high temperature. Initial and final particle size distributions (PSD) are measured using a scanning mobility particle sizer (SMPS), and the PSD is represented by a lognormal distribution. The general dynamic equation (GDE) for simultaneous coagulation and diffusive deposition is solved numerically for a laminar pipe flow using the method of moments. By comparing the experimental data with the model prediction, a correction factor to collisional growth rate is calculated. It was found that for particles in the size range 5–8 nm, there was significant enhancement in the collisional growth rate for TiO2 nanoparticles at 400 °C. This enhancement in growth rate decreased as temperature increased (upto 800 °C). The collisional correction factor values are compared to that of three existing models in the literature which predict similar trends with temperature, however, these models under-predict the value of the collisional correction factor. Reasons for these deviations from existing models are elucidated.

We have developed a water harvesting condensation particle counter (WHCPC) using lab-on-printed circuit board (PCB) technology. Our method provides an inexpensive, compact, and highly accurate system for the ubiquitous monitoring of airborne ultrafine particles (UFPs). Lab-on-PCB-based WHCPC uses water as the working fluid to monitor harmless UFPs; it does not require water replenishment using a moderated water condensation method. Lab-on-PCB-based WHCPC consists of two main parts, a UFP growth section and a mini-OPC, through which UFPs are grown into micro-sized droplets and the grown droplets are single-counted. The UFP growth section consists of two PCBs and a 3D-printed channel. The heater, temperature sensor, and hydrophilic micropillar array wick, which are essential for the growth of UFPs, are integrated on the PCBs. Simulations were performed to verify the saturated-air generation and water collection in the UFP growth section. The dew points of the inlet and outlet air samples were measured to verify water collection using the moderated water condensation method. Through quantitative experiments using UFPs artificially generated in the laboratory, particles of 9.4 nm could be counted. It was possible to precisely measure the number concentration (2500–270000 N cm−3) over a wide range. In addition, the stability of the proposed system was confirmed through a long-term comparison with the reference CPC in an outdoor environment. The proposed system realized the sensorization of CPC because it could monitor harmless UFPs, with minimal maintenance; the proposed system is also inexpensive and compact.

We present a new approach for micro- and nanoparticle production by in-situ charge reduction of electrospray droplets, which prevents their Coulombic instabilities and allow the efficient transport (extraction) of the particles. A unipolar ion source based on corona discharge generates a controllable ion flux of opposite polarity to the electrospray. The ions are introduced axially into the spray, while the Taylor cone is screened from the ions by an extractor ring electrode. Efficient and steady droplet discharge and extraction through an orthogonal aerosol-extraction tube was attained when the inlet of the tube was near the spray emission and the ring electrode, resulting in dramatic changes in droplets’ trajectories. The best extraction conditions (highest filter collections) were associated with the best discharging (lowest residual electrical charge) and the most globular particles. The size distributions on the particles collected on the filters were monomodal and homogeneous, with small relative standard deviations (as small as 10.6%). The use of corona ions significantly expands the range of polymer concentrations over which globular particles with monomodal size distribution can be made by electrospray.

In this work, we studied the coagulation process of two PAH clusters with diameter ∼ 2 nm using reactive molecular dynamics (MD) simulations. To describe the coagulation process quantitatively, the distance between the center of mass (COM) of the two PAH clusters, as well as the inter-cluster potential energy and kinetic energy of the COM of the clusters were calculated. Head-on coagulation efficiencies ( η ) of two PAH clusters at typical flame temperatures where soot inception is most likely to occur, i.e., 1500 K—2000 K, were determined based on hundreds of MD simulated trajectories. Our simulation results showed that η decreases with increasing temperature, which is mainly due to the increased kinetic energy of atoms within the PAH clusters at higher temperature. In addition, introduction of surface σ -radical site fraction in the range of 0.01 to 0.1 can only moderately improve η by ∼ 10% by forming carbon–carbon bonds between the two coagulating clusters, which suggests η of incipient soot nanoparticles with surface σ -radicals in high temperature flame regions is very low even if with reactive coagulation taken into consideration.

There is no universally agreed upon definition for ultrafine particles (UFP). Commonly used definitions for UFP are either particle number below 100 nm or total particle number, but without an agreed upon lower cut point. For example, a lower cut point of 3 nm compared to 10 nm could result in a substantially higher count. Another definition for UFP is total particle mass but without a commonly agreed upon aerodynamic diameter upper cut point, e.g., below 100 nm, 200 nm, 300 nm, etc. Yet another definition is lung deposited surface area weighted by lung deposition fraction, found mainly in the particle mobility diameter range from 20 to 400 nm. It is clear from these definitions that there are inconsistencies in the way UFP is used and defined in the literature. Sometimes these metrics are well correlated, sometimes not. In this paper we suggest three exposure metrics: UFP-N, UFP-M, and UFP-S, that we believe will add clarity. These metrics represent total number, mass, and surface area below 500 nm, respectively. For surface area and mass, the 500 nm cut point can be either aerodynamic or mobility diameter depending upon measurement methodology. For all metrics, this cut point captures nearly all of the primary particle emissions from mobile sources. Furthermore, UFP-N would include a lower cut point of 3–6 nm and would not require an upper size cut point because there is very little particle number above 500 nm or even above 100 nm. Thus, our definition of UFP-N is consistent with the current definition of ultrafine number except for, importantly, the specification of a lower cut point. These exposure metrics can help facilitate consistency in the characterization of both short- and long-term UFP ambient exposures and associated health effects in epidemiological studies.

Ultrafine particle behavior in electro-hydrodynamic (EHD) flow induced by corona discharge is studied experimentally and numerically. The EHD flow serves as a primary particle aspiration/sampling mechanism, the collector does not require any additional flow generation. Multiphysics numerical model couples the ion transport equation and the Navier-Stokes equations (NSE) to solve for the spatiotemporal distribution of electric field, charge density, and flow field, the results are compared with experimental velocity profiles at the exit. The computed velocity and flow rate data are in good agreement with the experimental data; the maximum velocity is located at the axis and ranges from 1 m/s to 4 m/s as a function of corona voltage. Experimentally evaluated particle transmission trends for ambient and NaCl nanoparticles particles in the 20 nm - 150 nm range are in good agreement with the theoretical models. However, for particles in the 10 nm - 20 nm size range, the transmission is lower due to the increased particle charging resulted from their exposure to the high-intensity electric field and high charge density in the EHD driven flow. These conditions yield a high probability of particles below 20 nm to acquire and hold a unit charge. The transmission is lower for smaller particle (10 nm) due to their high charge to mass ratio, and it increases as the single-charged particles grow in mass up to 20 nm, resulting in their lower electrical mobility. For particles larger than 20 nm, the electrical mobility increases again as they can acquire multiple charges. The results shed insight into interaction of nanoparticle and ions in high electrical field environment, that occur in primary EHD driven flows and in the secondary flows generated by corona discharge., 20 pages, 9 figures, will be submitted to Journal of Aerosol Science

Online measurements of nanoparticles are necessary when rapid information about the particle size and mass distribution is needed. Currently, the application of online measurement techniques with commonly used instruments such as SMPS, CPMA and ELPI+ is not possible at low-pressure conditions. In this work, a commercial vacuum ejector is used as a simple tool to transfer nanoparticles from a low-pressure region to atmospheric pressure. The vacuum ejector is investigated for different process pressures between 120 and 170 mbar to measure size-selected aerosols in the range from 10 to 100 nm. It was found that the sampling with the vacuum ejector does not change the particle size. The gas and particle dilution factors as well as the particle losses are determined, so that quantitative measurements of the aerosol size distribution can be obtained. Additionally, the applicability of the vacuum ejector is tested during particle synthesis in a low-pressure microwave plasma reactor with a combination of online instrumentation. The direct transfer of the aerosol to atmospheric pressure allows real-time measurements. The primary particle size, mass mobility exponent and effective density are calculated exemplary based on parallel online ELPI+, SMPS and CPMA measurements and are compared to offline TEM analysis.

Rapid measurement of submicron particle size distributions enables the characterization of aerosols with fast changing properties, and is often necessary for measurements onboard mobile platforms (e.g., research aircraft). Aerosol mobility size distribution is commonly measured by a scanning mobility particle sizer (SMPS), which relies on voltage scanning or stepping to classify particles of different sizes, and may take about 1 minute or longer to obtain a complete size spectrum of aerosol particles. The recently developed fast integrated mobility spectrometer (FIMS) with enhanced dynamic size range separates and detects particles from 10 to ~600 nm simultaneously, allowing submicron aerosol mobility size distributions to be captured at a time resolution of 1 s. In this study, we present a detailed data inversion routine for deriving aerosol size distribution from FIMS measurements of aerosols with a wide size range. The inversion routine takes into consideration the FIMS transfer function, particle penetration efficiency in the FIMS, and multiple charging of aerosols. The accuracy of the FIMS measurement is demonstrated by comparing parallel FIMS and SMPS measurements of stable aerosols with a wide range of size spectrum shapes, including ambient aerosols and aerosols classified by a differential mobility analyzer (DMA). The FIMS and SMPS-derived size distributions show excellent agreements for all aerosols tested. In addition, total number concentrations of ambient aerosols were integrated from 1 Hz FIMS size distributions, and compared with those directly measured by a condensation particle counter (CPC) operated in parallel. The integrated and measured total particle concentrations agree well within 6.5%.

The detection efficiency of Condensation Particle Counters (CPCs) reduces drastically as particle size becomes smaller than 2 nm. Increasing the supersaturation in order to enhance the detection efficiency, has limited applicability because the onset supersaturation value of droplet formation by homogeneous nucleation is very close to the heterogeneous onset supersaturation for sub-2 nm particles. In this work we introduce a new method for increasing the detection efficiency of CPCs for sub-2 nm particles, which relies mainly on controlling the spatial distribution of the supersaturation profile by simply modifying the operating temperatures of the CPC. We evaluated the new method by generating monodisperse particles in the size range of 0.98–4.50 nm and used them to characterize a TSI 3025 CPC. We achieved significant increase of the detection efficiency for sub-2 nm particles. Furthermore, we calculated the supersaturation field developed in the condenser tube with a finite element model and used it to determine the detection efficiency according to heterogeneous nucleation theory. These calculations reveal that the observed increase of the detection efficiency can be explained by the manipulation of the spatial distribution of the supersaturation field. The method introduced here can greatly improve the detection efficiency of CPCs and in the meantime further extend their use for particle sizing purposes in the sub-2-nm range.

The measurement of airborne particles with sizes below 3 nm is critical, as it helps the understanding of atmospheric nucleation and elucidates important particle synthesis mechanisms in the gas phase. Condensation particle counters (CPCs) have been widely used to measure the concentration of aerosols. However, it is challenging for the CPCs to measure particles below 3 nm due to the insufficient activation of these particles via vapor condensation. Methods have been proposed to increase the saturation ratio of the condensing vapor to promote the detection efficiency of sub-3 nm particles in the CPCs. Different working fluids also make a considerable impact on particle detection. Given the various types of parameters and the wide range of values these parameters can take, modeling studies are needed in searching for the optimal operating conditions of a CPC. In this work, we simulated the sub-3 nm particle activation and growth in a laminar flow CPC using COMSOL Multiphysics®, which has the advantages of simulating complex flow conditions and interfacing with post-processing software such as MATLAB. Our simulation incorporates the influence of temperature-dependent air and working fluid properties on particle activation and the impact of latent heat and non-continuum effects on droplet growth. Following the method introduced by Iida, Stolzenburg and McMurry (2009), particle activation is optimized for a given working fluid and condenser temperature by adjusting the saturator temperature to achieve a homogeneous nucleation rate of 1 s−1. The results, characterized by Dkel,0 (largest particle size that cannot be activated) and Dkel,50 (particle size activated with 50% efficiency), were compared against the analytical Graetz model used in Stolzenburg (1988). Our COMSOL simulations show that glycerine, diethylene glycol, ethylene glycol, 2-aminoethanol, and dimethyl phthalate are the best five working fluids achieving the smallest Dkel,50 among 45 commonly used solvents. The Dkel,50 values simulated by COMSOL under a condenser temperature of 10 °C for the five working fluids are 1.56, 1.88, 1.92, 1.98, and 2.10 nm, respectively, while the values simulated by the analytical Graetz solution differ slightly from 0.4% to 0.7%. The results demonstrate excellent agreement between these two simulation methods. For the five best working fluids activating the same 2.1 nm particles, the droplets can grow to sizes detectable by a second-stage CPC. The sensitivity of the COMSOL solution to the inlet condition and the form of convective diffusion equations is investigated. We also discussed the effect of CPC operating conditions, such as the condenser geometry and flow conditions, on particle activation for optimizing the performance of the CPC in detecting sub-3 nm particles.

An experimental study on the dynamics of an electrospray (ES) injected into a test chamber with a gaseous crossflow is presented. The study is relevant to the ion introduction into mass spectrometers that use ES as the ion source. Bending and trajectory of the ES plume is determined for a range of air crossflows and for different ES injection locations with respect to the crossflow centerline. It is shown that the ES injection conditions correlate with the intensity detections of a mass spectrometer. The effect of crossflow on the cone jet and ES performance are also discussed. A new hysteresis phenomenon for the electrospray in crossflow is detected.

A Fast Integrated Mobility Spectrometer (FIMS) with a wide dynamic size range has been developed for rapid aerosol size distribution measurements. The design and model evaluation of the FIMS are presented in the preceding paper (Paper I, Wang et al., 2017 ), and this paper focuses on the experimental characterization of the FIMS. Monodisperse aerosol with diameter ranging from 8 to 600 nm was generated using a Differential Mobility Analyzer (DMA), and was measured by the FIMS in parallel with a Condensation Particle Counter (CPC). The mean particle diameter measured by the FIMS is in good agreement with the DMA centroid diameter. Comparison of the particle concentrations measured by the FIMS and CPC indicates the FIMS detection efficiency is essentially 100% for particles with diameters of 8 nm or larger. For particles smaller than 20 nm or larger than 200 nm, FIMS transfer function and mobility resolution can be well represented by the calculated ones based on simulated particle trajectories in the FIMS. For particles between 20 and 200 nm, the FIMS transfer function is boarder than the calculated, likely due to non-ideality of the electric field, including edge effects near the end of the electrode, which are not represented by the 2-D electric field used to simulate particle trajectories.

High-resolution DMAs requiring hundreds of liters per minute of sheath gas flow Q to classify 1 nm particles have not been previously examined and optimized under the modest Q values (tens of liters per minute) needed to classify particles well above 10 nm. Here we study the resolving power R (based on the relative width of the transfer function) of the Halfmini DMA, at sample flow rates q beyond 1 l/min. The charge-reduced electrosprayed ovalbumin protein used as test aerosol has an accurately determinable Gaussian mobility distribution, which limits the measurable R to at most 25 to 30. Non-ideal DMA response can however be precisely probed by comparing measured peak widths with the convolution of the protein's mobility distribution with the triangular Knutson-Whitby distribution associated to the finite qi/Q ratio. For the unmodified Halfmini DMA, R departs considerably from ideal at qi/Q values as modest as 2%, revealing a problem with the aerosol inlet flow. This imperfection is not removed by either increasing or decreasing the slit width, nor by reorienting axially the sample flow jet as it merges with the sheath gas. By introducing a ring with many perforations upstream of the inlet slit, which favors a more symmetric distribution of the sample flow over the slit perimeter, R values close to ideal are approached with qi/Q ratios as large as 6 to 12%. This improvement enables (in principle) classification of particles larger than 30 nm., Comment: 17 pages, 9 figures, Published in J. Aerosol Science, 2017

How dust grains aggregate into planetesimals is still an open question. It is experimentally observed that binary collisions between micron-sized dust grains result in adhesion for collisions up to ∼ 1 ms−1. However, aggregates at scales ∼ 1 mm and above have been shown to exhibit fragmentation or bouncing at relevant collision velocities, resulting in a barrier preventing the formation of larger aggregates. One key factor is the weak adhesion observed between dust particles in aggregates leading to ruptures even in low-velocity collisions. To better understand the structure and strength of the adhered interface resulting from collisions, we performed molecular-dynamics simulations of head-on collisions between amorphous FeO nanoparticles. The results demonstrate several important phenomena which indicate how bonding between aggregates might become stronger than is often observed in experiments. For nanograins, it is shown that strong attractive interactions result in a minimum relative collision speed v c determined just before impact. The values of v c for particle radii 7 nm and below are computed to be in the range 40 – 100 ms−1. This high collision speed is shown to result in strong bond reordering at the interface. Moreover, increasing the incident collision speed v r e l is shown to increase the work of adhesion W a d h , which is correlated to substantial bond rearrangement. Specifically, for values of v r e l in the range between 10 – 90 ms−1, the interface between adhered grains is structured very closely to perfectly coordinated FeO amorphous solid. The reported results suggest stronger bonding results from collisions between particles with unpassivated surfaces, especially when particles are small, have amorphous surface structures, or when collisions occur at higher relative speeds. These factors should generate aggregates with stronger bonds that are less easily broken in subsequent collisions, and hence might be responsible for aggregates less likely to fragment at larger length scales.

A butanol-based fast condensation particle counter (Fast CPC, Model 3650, Kanomax Inc., Japan) was characterized in this study with three different types of monodisperse particles, including positively and negatively charged nickel-chromium (Ni/Cr) particles, positively charged tetraheptyl ammonium bromide (THABr) and tetrabutyl ammonium bromide (TBABr) particles, and positively charged polyethylene glycol (PEG) particles. The PEG particles were characterized with varied numbers of charges, whereby the charge number effect on particle activation was investigated. The performance of the Fast CPC was compared with a TSI® 3775 CPC and a TSI® 3776 CPC, respectively, under both factory temperature settings and boosted temperature settings, the latter of which corresponded to the brink of butanol homogeneous nucleation. Our results show that size-resolved Ni/Cr particles were detected by the Fast CPC with comparable detection efficiencies regardless of the charge polarity. When the charge number on a PEG particle increased from 1 to 3, an increasing tendency of the detection efficiency was observed, hinting that multiple charges on a particle will facilitate its activation. Particle activation was affected by the particle chemical composition as well, which is suggested by the higher activation efficiencies of PEG particles than those of Ni/Cr particles. The effects of multiple charges and chemical composition on particle activation were both more significant under factory temperature settings, but became less pronounced under boosted temperature settings. In addition to a quicker response time, the Fast CPC was capable of detecting sub-3 nm particles under boosted temperature settings, with the detection efficiency higher than that of the 3775 CPC but lower than that of the 3776 CPC, and can be used as a good candidate for the measurement of rapidly varying concentrations of small particles.

Micrometer-sized graphite particles are widely found in nature and industry. Particularly, the graphite dust is an important problem for the pebble-bed high temperature gas-cooled reactor (HTGR), regarding the coupling with the fission products (FPs). Due to the highly irregular disk-like shape, a great gap still exists for understanding the transport-related behaviors of real graphite particles, including the settling velocity and drag coefficient. To address this issue, we present an experimental investigation on the free settling of micrometer-sized natural, IG-110, NG-CT-10 and A3-3 graphite particles. The steady settling process is filmed using a high-speed camera together with a microscope. Both the terminal settling velocity and the drag coefficient are measured. In the Stokes region, although the shape is highly irregular and diverse, the statistical results show that the drag coefficient still follows the form of Stokes law and scales inversely proportional to the particle Reynolds number. Adopting the Stokes form and modifying the coefficient, a new correlation is proposed for the drag coefficient. The fitted value is 30.7 ± 10.2 for natural graphite, 26.2 ± 7.5 for IG-110, 28.1 ± 8.4 for NG-CT-10 and 32.7 ± 8.3 for A3-3 particles, which are larger than 24 of the classical spherical Stokes solution. In a wide range of particle Reynolds number from 10−4 to 3, the Bagheri and Bonadonna's correlation well represents the drag coefficient of graphite particles. The results of this work can be easily incorporated into Eulerian-Lagrangian simulations and aerosol models in system analysis codes, and helps to improve the understanding and modelling of irregular disk-like graphite particles.

In this report, a flame spray pyrolysis setup has been examined with various in situ extraction methods of particle samples along the flame axis. First, two precursor formulations leading to the formation of iron oxide nanoparticles were used in a standardized SpraySyn burner system, and the final particle outcome was characterized by a broad range of established powder characterization techniques (TEM/HRTEM, SAXS, XRD, BET). The characterization of the powder products evidenced that mostly homogeneous gas-to-particle conversion takes place when applying an acidic precursor solution, whereas the absence of the acid leads to a dominant droplet-to-particle pathway. Our study indicates that a droplet-to-particle-pathway could be present even when processing the acidic formulation. However, even if a secondary pathway might take place in this case as well, it is not dominant and nearly negligible. Subsequently, the in situ particle structure evolution was investigated for the dominant gas-to-particle pathway, and particles were extracted along the flame axis for online SMPS and offline TEM/HRTEM analysis. Due to the highly reactive conditions within the flame (high temperatures, turbulent flow field, high particle number concentrations), the extraction of representative samples from spray flames is challenging. In order to handle the reactive conditions, two extraction techniques were tailored in this report. To extract an aerosol sample within the flame for SMPS measurement, a Hole in a Tube probe was adjusted. Thus, the mobility particle diameter as well as the corresponding distribution widths were obtained at different heights above the burner along the flame axis. For TEM/HRTEM image analysis, particle samples were collected thermophoretically by means of a tailored shutter system. Since all sampling grids were protected until reaching the flame axis and due to the low sampling time, momentary captures of local particle structures could be extracted precisely. The particle morphologies have clearly shown an evolution from spherical and paired particles in the flame center to fractal and compact agglomerates at later synthesis stages.

Current diesel particulate filters (DPFs) can effectively capture the exhaust particles, but they add to engine backpressure and accumulate particles during their operation, which results in the need to regenerate the DPFs by burning off the collected particles periodically. This regeneration results in aerosol emissions, especially in the 10–30 nanometer size range and contributes to ultrafine particle pollution. In this research, we designed and developed a prototype of a novel diesel exhaust control device: the Electrostatic Screen Battery for Emissions Control (ESBEC). The device features high particle collection efficiency without adding to the exhaust backpressure and without the need for thermal regeneration of the collected particles. The ESBEC consists of a series of metal mesh screens coated with a superhydrophobic substance and an integrated carbon fiber ionizer to charge the incoming particles. Multiple pairs of screens (e.g., 5 pairs) are arranged in a battery, in which one screen of each pair is supplied with high voltage, and the other is grounded, producing electrostatic field produced across the screens. The application of a superhydrophobic coating onto the screens allows easy removal of the collected particles using liquid without the need for thermal regeneration. The current prototypes of the device were tested with fluorescent polystyrene latex (PSL) particles of 0.2 and 1.2 μm in size and at 25 and 105 L/min sampling flow rates. The average collection efficiency was ~87% for 0.2 μm and ~95% for 1.2 μm PSL particles. In addition, the ESBEC was tested with actual diesel exhaust particles; here its performance was verified by visually inspecting deposition of particles on an after-filter with the device ON and OFF. In the next stages of this work, the ESBEC will be challenged with diesel exhaust at different mass concentrations and for different collection time periods.

Information on the ambient aerosol number size distribution is essential to address various scientific questions related to aerosol particles in the atmosphere. However, due to the wide size and concentrations ranges of ambient aerosol particles, no single instrument alone is capable of measuring them all. Instruments based on different measurement principles are engaged in the measurement of atmospheric aerosols. Intercomparison of such instruments is necessary to cross-validate the reliability of obtained data. In this study, a verification of a Novosibirsk Aerosol Diffusion Spectrometer (ADS) in the size classification of aerosol particles was carried out in laboratory and via a field intercomparison with a Differential Mobility Particle Sizer (DMPS) and a Neutral cluster and Air Ion Spectrometer (NAIS). The laboratory experiments affirmed the good accuracy of the ADS on sizing and concentration measurement. The ADS was satisfactorily comparable with the DMPS and the NAIS for the measurement of ambient aerosols within the size range 3–200 nm. The differences between condensation sinks derived from the ADS and DMPS measurements were smaller on days with new particle formation (NPF) than on non-NPF days. Similar formation rates and growth rates were acquired based on the DMPS, NAIS and ADS measurements, proving a reasonable ability of the ADS in measuring the concentrations and size distribution of nucleation-mode particles.

A scanning mobility particle sizer was utilized for the real-time measurement of particles in the size range of 14.6 nm to 661.2 nm from August 14 to November 18, 2014, to characterize the evolution of particle number size distribution and new particle formation (NPF) events in Lanzhou, China. The mean number concentrations of nucleation mode, Aitken mode, accumulation mode, and total particles were 2833, 12,898, 6210, and 21,940 cm −3 , respectively. The mean number size distribution was obviously unimodal, with a peak at around 60 nm. The diurnal variation of nucleation mode particles was mainly influenced by NPF events. Two peaks at traffic rush hours were observed for Aitken mode, accumulation mode, and total particles; the peaks were related to traffic and cooking soot emissions. The particle number and mass concentrations were all higher on workdays than on weekends mainly because of the enhancement in human activities and traffic density. Daytime also had higher values of number concentrations than nighttime. The mass concentrations of total submicron particles, sulfate, nitrate, and ammonium had much higher values at daytime than at nighttime, however, organics had slightly higher mass concentrations at nighttime mainly because of heating activities at night and intensified evening peaks. During the measurement period, the frequency of NPF events was calculated to be 34%. The average apparent formation and growth rates of nucleation mode particles were 1.71 cm −3 s −1 and 6.10 nm h −1 , respectively. A typical NPF event with two-stage particle growth was selected and analyzed to provide insight into the evolution of particle number size distributions. Back trajectory clustering analysis indicated that air masses that originated from the west had high values of both particle number and mass concentrations associated closely with local source emissions. Air masses from the north and northwest represented the relatively clean air mass arriving in Lanzhou.

Aerodynamic Particle Size Distribution (APSD) is an important factor governing if aerosols can be inhaled to different parts of a human's respiratory system. To understand the APSD of the aerosol produced from several brands of E-cigarettes, measurements using the Next Generation Pharmaceutical Impactor (NGI) were performed. The NGI impactor was operated at two different temperature conditions, ambient room temperature condition (20–25 °C) and in a cold condition (4–8 °C). The relative humidity (RH) in the latter case, cooled impactor, was close to 100%, similar to the human's respiratory system. Obtained Mass Median Aerodynamic Diameters (MMAD) for the investigated electronic cigarettes were in the range 0.5–0.9 µm. The different NGI temperature conditions had a significant effect on the result and this is probably due to dynamic processes during the aerosol transport in the NGI pass ways. For each NGI measurements performed the nicotine present in both the droplets and in the gaseous phase were measured. This is different from the previously published data using physical aerosol sizing techniques (such as Scanning Mobility Particle Sizer, SMPS), where obtained droplet sizes is a result of all chemical components in the aerosol, not only nicotine. In principle, it could be only excipients in the droplets and all nicotine in gaseous phase. Thus, impactor and a gas trap with nicotine specific quantification is a good complement for more thorough understanding of droplet size distributions of E-cigarette. The APSD in vitro data was used in theoretical modeling using the Multiple-Path Particle Dosimetry Model (MPPD v 2.11), for estimation of the lung deposition. Using the model it was estimated that 75–90% of the nicotine droplets will be exhaled and 10–25% deposited in the respiratory system. However, when an aerosol is inhaled to a human's respiratory system it will be diluted and the evaporation of nicotine will increase. This will lead to overestimation of the fraction of exhaled nicotine using the MPPD model.

Electrophoretic losses at the outlet of Differential Mobility Analyzers result from the need to transport the aerosol from the high voltage VBias typically applied to the inner electrode, to the ground potential at which a particle detector is usually connected. Here we examine the sample transmission efficiency η for mobility-selected ions brought to ground through a commercial Static Dissipative Polyurethane tube. η(VBias) is defined as the ratio of the ion current transmitted through the tube with bias voltage VBias over that transmitted when VBias=0. We find η values close to unity under common DMA operational conditions. η is measured also under adverse fields strong enough for all charged particles to be lost, with a sample transmission efficiency comparable to that expected for a uniform velocity profile within the tube. Outlet tube Reynolds numbers are examined in the range 446

Engineering nanoparticles for applications such as drug delivery, needs understanding of dependence of particle size on process and precursor parameters. The size of nanoparticles prepared through the aerosol route using solution droplet drying technique is traditionally governed by a 1/3rd scaling law between dry particle diameter and precursor solute concentration. Here, we experimentally investigate this relationship, for precursor solutions of lipids in organic solvents. Nanoparticles were prepared using air-jet atomization of precursor solutions of stearic acid in organic solvents (acetone, chloroform, methanol and ethanol), in a pulse heat aerosol reactor, at a process temperature of 50 degrees C. The relationship between median mobility diameter of prepared nanoparticles (40-155 nm) and solute concentration was governed by a scaling law exponent (SLE) in the range 0.104-0.182. The observed exponents showed significant deviations from the classical 1/3rd law. The experimental investigation indicated an unexpected increase in particle number concentration with increasing solute concentration for all solvents studied. Simulations with a critical super saturation based solute diffusion model and theoretical calculations of size dependent external factors such as coagulation and wall losses during transit, could not explain the deviation in SLE, nor the increased number concentrations. These considerations point to possible effects of solute concentration on the mechanism of droplet formation during the atomization process. A preliminary model based on an area conservation law has been proposed to explain the twin effects of deviation in SLE and increased droplet number concentration with increasing solute concentration. The present study provides empirical data on the relationship between dry particle diameter and solute concentration in the selected lipid-organic solvent systems. Also, it opens avenues for further understanding nanoparticle formation by air-jet atomization. (C) 2016 Elsevier Ltd. All rights reserved.

The resolidified fuel debris from the damaged Fukushima Daiichi reactors must be cut into small pieces prior to removing it from reactor buildings. Submicron radioactive aerosol particles are likely to be generated and dispersed into the primary containment vessel (PCV) atmosphere during this cutting process. However, traditional sprays cannot effectively scavenge aerosol particles with diameters in the range of 0.1–1 μm from gas environments. In this paper, a new aerosol agglomeration method is accordingly proposed employing water mist as a pretreatment to enlarge the aerosol particles prior to removing them using water sprays. First, the water mist is injected into a vessel in which aerosol particles are primarily dispersed. Following a short waiting time, subsequent spray injection is then able to more effectively remove the enlarged aerosol particles. The application of water mist was found to improve the aerosol spray scavenging efficiency by coalescing with aerosol particles under both diffusiophoresis and different coagulation mechanisms to form large-sized agglomerated mist–aerosol particles that can subsequently be removed by the water spray. The efficiency of removal is increased due to the enhanced collection mechanisms of inertial impaction and improved particle surface hydrophilicity. The optimized spray system evaluated in this study will be used to effectively control radioactive aerosol suspension inside the Fukushima Daiichi PCVs and protect workers and the public from any unknown consequences of internal radiation exposure during reactor decommissioning. The proposed method can also be applied throughout the nuclear industry to reduce the risk posed by small-sized aerosol particles during maintenance.

Particle shape strongly influences the diffusion charging of aerosol particles exposed to bipolar/unipolar ions and accurate modeling is needed to predict the charge distribution of non-spherical particles. A prior particle-ion collision kernel β i model including Coulombic and image potential interactions for spherical particles is generalized for arbitrary shapes following a scaling approach that uses a continuum and free molecular particle length scale and Langevin dynamics simulations of non-spherical particle-ion collisions for attractive Coulomb-image potential interactions. This extended β i model for collisions between unlike charged particle-ion (bipolar charging) and like charged particle-ion (unipolar charging) is validated by comparing against published experimental data of bipolar charge distributions for diverse shapes. Comparison to the bipolar charging data for spherical particles shows good agreement in air, argon, and nitrogen, while also demonstrating high accuracy in predicting charge states up to ± 6 . Comparisons to the data for fractal aggregates reveal that the LD-based β i model predicts within overall ± 30 % without any systematic bias. The mean charge on linear chain aggregates and charge fractions on cylindrical particles is found to be in good agreement with the measurements ( ~ ± 20 % overall). The comparison with experimental results supports the use of LD-based diffusion charging models to predict the bipolar and unipolar charge distribution of arbitrary shaped aerosol particles for a wide range of particle size, and gas temperature, pressure. The presented β i model is valid for perfectly conducting particles and in the absence of external electric fields; these simplifications need to be addressed in future work on particle charging.

We investigated a spray-drying aerosol generator that can generate micro-particles at high concentration using an ultrasonic nozzle. Unlike a conventional spray-drying aerosol generator that uses vertical spray with co-current sheath flow, we designed a new aerosol generator by using horizontal spray with co-current sheath flow combined with clean counter flow to increase particle generation efficiency. A drying chamber was designed using the ANSYS Fluent solver to complete the evaporation of droplets and to minimize the loss of post-evaporation particles inside the chamber. Both numerical prediction and experimental data showed that droplets would completely evaporate at a temperature of 353 K. Both KCl aqueous solution and silica powder suspension were sprayed using an ultrasonic nozzle to generate micro-sized particles with a size range of 1–10 μm. When horizontal spray with co-current sheath flow was operated with clean counter flow the loss of particles on the chamber wall was minimized and thus the total particle number concentration was 2.4 times higher than when the clean counter flow was inactive. The particle generator in the present study can generate larger particles than a Collison atomizer which generates particles up to 3 μm.

Concentration and size distribution of airborne particles (size range 10-1100 nm), using a SMPS+C Scanning Mobility Particle Sizer, and concentrations of positive and negative cluster ions (size range 0.36-1.6 nm), using several CDI-06 Gerdien-type integral air ion detectors, have been monitored in three rooms in rural dwellings of Serbia and Slovenia, during periods when nobody was present in the room and while smoking and heating were taking place. The highest particle generation rate was 2.4 x 10(11) min(-1) while cigarette smoking was taking place and was 1.5 x 10(11) min(-1) during heating with a cast iron stove with wood burning at 150 degrees C, and 1.1 x 10(10) min(-1), during heating with an Alpine -type oven at 40-50 degrees C. The related particle loss rate constants were 0.0603 min(-1), 0.0442 min(-1), and 0.0067 min(-1). The estimated mean values of the effective ion attachment rate beta(eff) vary between (2.2-5.4)10(-6) cm(3) s(-1). A correlation between ions concentration and particles concentration and their sizes has been sought, and findings are discussed and shown. (C) 2016 Elsevier Ltd. All rights reserved.

The cloud settling effect for aerosol particles is examined based on approximation of long-range particle–particle hydrodynamic interactions. It is indicated that the normalized cloud settling velocity (CSV) exhibits a linear relationship with the product of particle number and particle size ratio. A particle-level analysis, combined with the Stokes/Oseen dynamics simulation, is applied to study the effects of fluid inertia and cloud shape on the CSV. Prediction functions for both spherical and columnar clouds, considering the effect of fluid inertia, are obtained through both simulation regression and analytical derivation.

The accuracy of instruments measuring the alveolar lung deposited surface area (LDSA) concentration based on unipolar diffusion charging has been assessed. Monodisperse particles with sizes between 10 nm and 700 nm were used. The results indicate that the LDSA concentration can be measured with at least ±30% accuracy for particle sizes between 20 nm and 400 nm. The LDSA concentrations of particles 400 nm increasingly underestimated by the instruments. The LDSA concentration of 685 nm particles was on average underreported by a factor of approximately 2. LDSA concentrations measured with agglomerated particles were consistently higher than those measured with spherical particles of the same electrical mobility diameter. The accuracy was, however, still mostly within ±30% for a size range from 20 nm to 400 nm. Due to the strong and increasing deviation for large particles, the use of an appropriate preseprator is recommended when measuring LDSA concentrations.

Stability of airborne nanoparticle agglomerates is important for occupational exposure and risk assessment in determining particle size distribution of nanomaterials. In this study, we developed an integrated method to test the stability of aerosols created using different types of nanomaterials. An aerosolization method, that resembles an industrial fluidized bed process, was used to aerosolize dry nanopowders. We produced aerosols with stable particle number concentrations and size distributions, which was important for the characterization of the aerosols׳ properties. Next, in order to test their potential for deagglomeration, a critical orifice was used to apply a range of shear forces to them. The mean particle size of tested aerosols became smaller, whereas the total number of particles generated grew. The fraction of particles in the lower size range increased, and the fraction in the upper size range decreased. The reproducibility and repeatability of the results were good. Transmission electron microscopy imaging showed that most of the nanoparticles were still agglomerated after passing through the orifice. However, primary particle geometry was very different. These results are encouraging for the use of our system for routine tests of the deagglomeration potential of nanomaterials. Furthermore, the particle concentrations and small quantities of raw materials used suggested that our system might also be able to serve as an alternative method to test dustiness in existing processes.

Physical and chemical characterization of gas borne particles in the nanometer to sub micrometer size range includes knowledge about particle size distribution and chemical composition, possibly with a high time resolution. However, most of the available techniques are offline methods and/or not able to provide such physical and chemical information simultaneously. Scanning Mobility Particle Sizer (SMPS) is a well established and widely used technique for determining size distribution and number concentration of such particles, within scan durations of a few minutes. Inductively Coupled Plasma Mass Spectrometry (ICPMS) is a highly sensitive multi element technique, allowing the determination of elemental composition with excellent defection limits and a wide dynamic concentration range. In this work, SNIPS and ICPMS were coupled to one hyphenated system which allows achieving on-line and simultaneously size and chemical information with SMPS-Lypical time resolutions. A Rotating Disk Diluter was used as sample introduction interface, and the SMPS flow concept was re designed to fulfill the requirements of both different analytical methods. The developed setup was successfully tested with a model aerosol containing airborne silver nanoparticles. Proper SMPS operation under argon atmosphere instead of air was validated. The presented setup was applied to scan the particle size range from 7 to 156 um within 120s, Similar sensitivities and efficiencies compared to that of state-of-the-art SMPS and ICPMS instruments were achieved. (C) 2015 Elsevier Ltd. All rights reserved.

Electrical Mobility is a transport property that describes a particle behavior in the gas phase. When dealing with the free molecular regime, ascertaining the shape of a nanoparticle or an ion directly from measurements of mobility becomes quite difficult as the particle no longer can be assumed to have spherical shape. Here we propose an efficient parallelized tool, IMoS, that makes use of all-atom models to calculate the mobility of nanoparticles in a variety of gases. The program allows for different types of calculations that range from the efficient Projection Approximation (PA) algorithm to the 4-6-12 Lennard-Jones potential Trajectory Method. It also includes a diffuse inelastic simulation that achieves Millikan's predicted 1.36 value over PA. When compared to experimental results, the error of the most efficient calculations is shown to be approximately 2–4% on average.

A low cost, easy-to-manufacture particle classification device for coarse mode, supermicrometer particles and droplets remains important for a variety of industries and environmental measurements, including agricultural sprays, pharmaceutical powders, spray drying processes, precipitation, and dust storms. While optical techniques can typically be used to analyze particle sizes in this range (20 μm–100 μm), there are few instruments available for size-segregated particle collection. Here, we demonstrate that a horizontal, parallel-plate sheathed elutriator can be designed and 3D printed, enabling modest-to-high resolution size segregated particle and droplet collection in the supermicrometer size range. In such elutriators, gravitational settling drives sampled particles vertically down the device (between two plates), while a clean sheath flow drives motion orthogonally; the net result is aerodynamic diameter-specific particle and droplet trajectories. In designing a sheathed elutriator, we develop both an analytical deposition location distribution function, similar to the transfer function of sheath mobility analyzers, as well as a 3D computational fluid dynamics model of a prototype instrument. The prototype is built solely from 3D printed parts (and PVC sheath lines) without any high voltage sources or high power pumps required. The performance of the prototype is tested by examining the deposition location distribution of monodisperse uranine-doped oleic acid droplets in the 26 μm–81 μm size range. Uranine signal intensity was determined using both traditional fluorometric analysis and via yellow-color intensity analysis from a digital camera, with both measurements in good agreement with one another. Overall excellent agreement is observed between the analytical model, computational fluid dynamics simulations, and experimental measurements, demonstrating that inexpensive elutriators provide a means to collect coarse-mode particles in a size selective manner, and further suggesting that they can be designed for aerodynamically monodisperse particle selection.

Oxide nanoparticle growth from vapor phase precursors occurs in high temperature aerosol reactors first via the formation of nanoclusters, which are nanometer-scale condensed-phase species composed of 101-102 atoms. The binding rate for nanoclusters, defined as the rate at which two nanoclusters collide with and stick to one another, is hence of interest in predicting nanoparticle size distribution functions in an aerosol. We have utilized molecular dynamics (MD) trajectory calculations to determine the homogeneous (equal-sized) and heterogeneous (disparate-sized) binding rate coefficients of SiO2 (silica) nanoclusters composed of 18, 144, and 333 atoms. MD trajectory calculations incorporated all-atom models of nanoclusters using a combined Born–Huggins–Mayer-Lennard-Jones potential model, which accounts for both short range interactions and electrostatic interactions. MD calculations were utilized to determine the binding probability as a function of both initial relative velocity and impact parameter; integration of the binding probability across impact parameter and relative velocity space yields the binding rate coefficient. MD trajectory calculations reveal that the most common type of encounters between nanoclusters leading to binding are grazing collisions, i.e. instances where collision would not occur without induced dipole potential influences. The resulting binding rate coefficients are found to be extremely weakly dependent on system temperature, which is in contrast to the use of rate coefficient models which are the product of a hard-sphere collision rate coefficient and a constant enhancement factor (leading to a rate coefficient proportional to the square root of temperature). Enhancement factors defined from MD trajectory calculations fall in the range of 3–9 as system temperature decreases from 1500 K to 300 K. Such large values suggest that potential interactions need to be considered when modeling oxide nanocluster growth in the gas phase.

The generation of a secondary aerosol after impact, consisting of smaller droplets at a given velocity and mass flow, is relevant for various applications. Thus far, the investigations and modelling approaches on spray impact are based on extrapolation of the single-droplet impingement or empirical correlations. The validity of the models presented is limited to the given experimental setup and conditions such as initial droplet size, velocity and the impact surface characteristics. The aim of this work was to empirically evaluate the spray impact of a two-fluid nozzle on a sphere. A small-scale nozzle was used, which produced a primary aerosol with a mass median diameter of about 12 μ m (liquid-to-gas mass flow ratio = 1, gas pressure: ΔpG = 5 bar). After impact on a sphere, a multimodal distribution was observed and a higher mass flowrate of droplets in the small micrometer range ( 2 and 3 μ m ) was produced for a liquid mass flow rate in the range of 1.2–6 kg/h and an atomizing gas mass flow rate of 1–4 kg/h. For easier observation, a geometrically similar, larger nozzle was used, which produced an aerosol with a mass median diameter of about 80 μ m (liquid-to-gas mass flow ratio = 4, gas pressure: ΔpG = 1 bar). The measured droplet size after impact is smaller for a lower liquid-to-gas mass flow ratio and increased atomizing gas inlet pressure. Droplet formation mechanisms such as splashing, crown formation and spreading on the sphere surface were observed. A characteristic film with large variations in thickness was generated.

The properties of gas-borne aggregates are important in nano-technology and for potential health effects. Gold aggregates from three generators (one commercial and one custom built spark discharge generator and one high-temperature furnace) have been characterized. The aggregate surface areas were determined using five approaches - based on aggregation theory and/or measured aggregate properties. The characterization included mass-mobility relationships, effective densities (assessed by an Aerosol Particles Mass analyzer), primary particle analysis (based on Transmission Electron Microscopy), as well as total mass and number concentration outputs. The relationships between mass and mobility are well described by power-law functions with exponents of 2.18-2.35. For all generators, the primary particles of the aggregates were fused together by a bridge with a diameter typically similar to 60-70% of the primary particle diameter (5-10 nm). The total mass outputs were 6.1-48.1 mg/m3 and the predicted surface area outputs in the range 0.9 x 10(-3)-17 x 10(-3) cm(2)/cm(3). The aggregate effective densities differed considerably between generators. The difference could partly be explained by the differences in primary particle diameter, but not fully. This in turn may be explained either by a varying primary particle size with aggregate size, or by that there are slight differences in the morphology of the aggregates from the generators. (C) 2015 The Authors. Published by Elsevier Ltd. (Less)

The critical velocity of rebound was determined for spherical silver aerosol particles in the size range of 20–1000 nm. A novel instrument, a variable nozzle area impactor, was especially designed for measuring the particle–surface interaction as a function of the particle impact velocity. The experimental results were combined with a numerical model in order to obtain the impact velocities. The experiments were carried out using a plain aluminum collection substrate in the impactor. Our results show that the critical velocity of rebound decreases from 14 to 0.022 m/s as the particle size increases from 20 to 1000 nm. Furthermore, the critical velocity was found to be proportional to the power of −1.6 of the particle size, instead of the theoretical inverse proportionality. This result is in line with the previous studies for micron-sized particles. In the nanoparticle size range, the obtained values are approximately 3–10 times greater than the recent literature values. This discrepancy can most likely be explained by the different surface materials. All in all, our results give valuable information about the particle–surface interactions in the sub-micron size range.

The study was conducted to analyze the morphological aspects of particle emitted from improved and traditional cookstoves when used with different fuel combinations. The particles were sampled using hood method under actual cooking process performed in controlled laboratory conditions. The sample preparation was done using wet technique which was also compared to dry technique for transferring particles on to the transmission electron microscopy (TEM) grid. The density and parameter fractal dimension 2D-DDf and PDf was calculated using box count algorithm developed using Python language. The 3D-DDf was calculated using ensemble approach and other shape descriptors were also measured using image processing software. The distribution of primary particle, particle agglomerates and tar balls were also studied. The 3D-DDf for particles emitted from traditional cookstove with wood as fuel was 2.42 compared to a lower 3D-DDf of 1.82 in PF cookstove. The 3D-DDf along with fractal prefactor (kg) of 0.7 and 1.4 indicated circular and dense particles emitted from TR cookstove with wood as fuel. On the contrary, with Mix-2 and Mix-3, the particles emitted from improved cookstoves had 3D-DDf>2 indicating presence of super soot aggregates. The distribution of primary particle and tar balls indicated count median diameter (CMD) in the range of 15–25 nm and 64–104 nm respectively for all cookstove–fuel combinations. The distribution of equivalent area diameter of aggregates showed maximum particles of 300–400 nm range as evident in other physical characterization studies conducted using instruments that work light scattering principle and electrical mobility of particles.

Radiative forcing by aerosol particles containing black carbon (BC) may be positive or negative depending on specific atmospheric conditions. Black carbon itself absorbs solar radiation and thereby heats the surrounding environment. On the other hand, as a result of atmospheric aging, BC-containing particles may become hydrophilic due to oxidation, condensation, and/or coagulation of water-soluble material. The aged particles can act as cloud condensation nuclei (CCN) and contribute to cloud formation that may result either in cooling or heating. In this work, through a series of laboratory experiments, we investigate the transformation of soot particles from hydrophobic to hydrophilic and estimate the atmospheric residence time required for this transformation. Ethylene flame-generated soot particles were size-selected and exposed to OH radicals in a Potential Aerosol Mass flow reactor. Aging was simulated via OH exposures equivalent to atmospheric lifetimes over a range from hours to multiple days. The chemical composition of the organic coatings as a function of OH exposure was monitored with an Aerodyne Aerosol Mass Spectrometer. The CCN activity of the aged soot particles was measured as a function of OH exposure and chemical composition. Experimental measurements indicate that heterogeneous OH oxidation of initially CCN-inactive nascent soot produces CCN-active particles. Critical supersaturations at integrated OH exposures equivalent to 0.4, 2, and 10 days are 2.1%, 0.82%, and 0.40%. Corresponding values for the effective hygroscopicity parameter, κ eff , ranged from κ eff 8 × 10 − 4 to κ eff = 0.09 as a function of OH exposure. Condensation of hydrophilic organic or inorganic coatings (produced from oxidation of gas-phase precursors introduced to the flow reactor) on soot particles speeds up the CCN activation by a factor of 6–50 depending on the nature and thickness of the coating. The results suggest that CCN activation of atmospheric BC-containing particles is primarily due to secondary coatings.

A Computational Fluid Dynamics model is presented to calculate the particle collection efficiency in electrostatic precipitators in terms of the interactions among gas-particle flow, dust resistivity and electric field. Various material properties and operation parameters are included to extend the model capability for process optimization. The current paper focuses on the effect of gas temperature for a cold-side precipitator. It is found that, as temperature increases in the range 90–120 °C considered, the operation voltage drops significantly, due to increased dust resistivity. Behaviors of fine particles in the range from 0.05 to 25 μm in diameter are simulated in a circular wire-plate configuration. A minimum efficiency is predicted for particle size around 0.5 μm. Below this critical size, the increased collection efficiency results from relatively large surface charge density due to diffusion charging, and reduced inter-phase drag. For moist gas, temperature has significant non-linear effect on the fly ash resistivity. It is demonstrated that a same change in temperature from 120 °C to 90 °C is more effective than from 150 °C to 120 °C, as far as the collection efficiency is concerned.

We investigated changes in particle detection performance of a condensation particle counter (CPC) for automobile exhausts (model 3790, TSI Inc.) during an accelerated durability test with high particle loading. The CPC was exposed to carbon particles for over 100 h with a mode diameter of 80 nm at a number concentration of 106 cm−3, i.e., 100 times more than the maximum of the designed concentration range of the CPC specified by the manufacturer. This was equivalent to exposure at the designed maximum concentration (i.e., 104 cm−3) for five years. The detection efficiencies of the CPC evaluated regularly at 23, 41, and 55 nm during the test remained unchanged, which indicated that the particle loading was less than the level at which the performance of the CPC would be degraded by contamination of the interior of the CPC. Based on this observation, it was concluded that the changes in the detection performance of our customers׳ CPCs, which we observed occasionally during annual maintenance and recalibration, were due to causes other than particle loading within the designed concentration range. The unchanged detection efficiencies in our test of exposure equivalent to five years suggest that yearly calibration should be sufficient for maintaining the measurement accuracy of a CPC if it is operated under normal conditions.

We examine the formation of particulate films via Langevin dynamic simulations of monodisperse particle deposition onto flat substrates. In simulations, we consider particle inertia, thermal motion, and a constant advective force directing particles towards the substrate. These conditions effectively mimic the formation of dust cakes during filtration, as well as the production of nanostructured films at the outlet of gas phase synthesis reactors. Parameterization of the equations of motion shows that the movement of each particle depends only upon Kn D (the diffusive Knudsen number, proportional to the particle׳s mean persistence distance to its radius) and χ F (the ratio of the particle׳s translational energy at the advective velocity to the thermal energy). Use of these ratios is advantageous because one dimensionless number ( Kn D ) is independent of the advective velocity, while the second ( χ F ) is independent of background gas pressure. This is in contrast with the traditionally employed Peclet and Stokes numbers, both of which are dependent upon the advective velocity and gas pressure. The film structures resulting from simulations are quantified in terms of their porosities, and results are reported for simulations over a wide range of Kn D (10 −3 –10 1 ) and χ F (10 −4 –10 3 ), deposition areas 1000×1000 particle radii in size, and with more than 10 6 deposited particles. For each simulated deposit, three distinct regions are observed: (I) a region close to the substrate (within 50 primary particle radii), wherein the porosity begins at a minimum and increases to constant value; (II) a region where this constant porosity is maintained (which is dependent on Kn D and χ F ), constantly increasing in height and encompassing the majority of the film after sufficient deposition; and (III) the outer region, in which the film is still growing, and the porosity increases from the region II value to unity, spanning less than 100 particle radii in height in most circumstances. A regression equation is provided to describe the film porosity as a function of Kn D and χ F in region II, facilitating a priori predictions of film porosities from deposition parameters.

The resuspension of a monolayer of spherical glass and polypropylene particles from a channel floor by a dry and turbulent airflow was investigated. Special attention was given to the influence of the particle size, the particle and wall material, the wall surface roughness and the critical friction velocity. The experiments were performed in an air-driven small-scale test facility and the channel floor was made of interchangeable glass and steel wall segments. The turbulent channel flow was recorded using a planar Particle Image Velocimetry system. Prior to the experiments the spherical particles were classified using Scanning Electron Microscopy techniques. The particles on the channel floor were detected and classified by means of an optical microscope combined with a digital camera. A statistically sufficient particle monolayer was generated on the channel floor by dispersing the particles into the flow during a pure deposition regime. Afterwards, particle resuspension was induced by stepwise increase of the fluid velocity. The resuspension was quantified by the fraction of remaining particles against the friction velocity for a particle diameter range between 3 µm and 45 µm. It was found that particles instantly resuspend once a critical friction velocity is exceeded. Larger particles require lower fluid velocities for the removal than smaller particles. The wall surface roughness seems to scatter the resuspension process with respect to the friction velocity.

A calibration setup has been established to calibrate or validate UN-ECE Regulation 83 compliant CPCs. A spark discharge generator (GFG 1000) is used to produce carbon particles. For charge conditioning an indirect photoelectric charger is combined with an electrostatic precipitator instead of the very common radioactive bipolar charger. The particle number concentration is determined with a Faraday cup electrometer. Detailed studies have been conducted to validate the calibration setup. The particle size classification was checked by a screen-type diffusion battery. Moreover the effect of multiple charged particles was investigated. Three independent measurements of the counting efficiency as well as the linearity of a typical regulation compliant counter were performed. The calibration setup was also compared with a similar system using a radioactive bipolar charger. The obtained results demonstrate that the counting efficiency can be measured in a particle size range from 13 nm to 60 nm with a high repeatability. A linearity check at a particle size of 55 nm is possible in a concentration range of 1000–10 000 cm 3 . The effect of multiple charged particles produced by the indirect photoelectric charger was found to be negligible. Additionally, the difference between a CAST aerosol and a GFG aerosol as a calibration material is discussed.