A MAGIC concept for self-sustained,water-based,ultrafine particle counting

A self-sustaining, motion-tolerant, water-based condensation particle counter (CPC) has been designed, fabricated, and tested. Referred to as “MAGIC” for moderated aerosol growth with internal water cycling, the particle size response is similar to the 5-nm cut-point commercial CPCs. MAGIC is a laminar-flow instrument with three temperature stages: cool, warm, and cool. The middle warm-walled stage initiates the condensational growth and the final cool-walled stage maintains supersaturated conditions while recovering water vapor. By using a continuous wick throughout all three stages, the system recharges itself through a combination of water condensate from the sampled airstream and recovery of water vapor from the peak supersaturation region. A reservoir-less prototype system based on this concept was built and tested. Experiments show equal performance in any orientation, upright or inverted, and tolerance to tipping, shaking and vibrational shocks up to 5?g. Under mild ambient conditions, it provided multi-week operation without replenishing the wick.

Copyright © 2018 American Association for Aerosol Research     

Water-based single-flow mixing condensation particle counter

A novel water-based condensation particle counter has been developed using a patented, single-flow mixing (SFM) condenser that permits a conventional thermal approach of using a hot saturator followed by a cold condenser to activate and grow particles for counting with an optical detector. A computational fluid dynamics (CFD) model of the internal flow, temperature, and vapor profiles was used to predict the effectiveness of the SFM condenser. Using the results from the CFD model, the counting efficiency was numerically calculated for pure water droplets, and the CPC cut-point (i.e., 50% counting efficiency) was predicted to be 8.3 nm. The experimental performance of the new CPC was measured with differential mobility analyzer-classified, monodisperse particles. The measured cut-points were 8.2 nm for Ag particles and 3.9 nm for NaCl particles. The reduction in the cut-point for NaCl is the result of a compound effect: water uptake by NaCl particles, which increases their size before entering into the growth section (condenser), and the reduction of the equilibrium vapor pressure of water over NaCl-water droplets, resulting in a decrease of the activation diameter.

Copyright © 2016 American Association for Aerosol Research     

Moderated,Water-Based,Condensational Particle Growth in a Laminar Flow

Presented is a new approach for laminar-flow water condensation that produces saturations above 1.5 while maintaining temperatures of less than 30°C in the majority of the flow and providing an exiting dew point below 15°C. With the original laminar flow water condensation method, the particle activation and growth occurs in a region with warm, wetted walls throughout, which has the side-effect of heating the flow. The “moderated” approach presented here replaces this warm region with two sections—a short, warm, wet-walled “initiator,” followed by a cool-walled “moderator.” The initiator provides the water vapor that creates the supersaturation, while the moderator provides the time for particle growth. The combined length of the initiator and moderator sections is the same as that of the original, warm-walled growth section. Model results show that this new approach reduces the added heat and water vapor while achieving the same peak supersaturation and similar droplet growth. Experimental measurements confirm the trends predicted by the modeling.

Copyright 2014 American Association for Aerosol Research     

Preliminary investigation of a water-based method for fast integrating mobility spectrometry

A water-based condensational growth channel was developed for imaging mobility-separated particles within a parallel plate separation channel of the Fast Integrated Mobility Spectrometer (FIMS). Reported are initial tests of that system, in which the alcohol condenser of the FIMS was replaced by a water-based condensational growth channel. Tests with monodispersed sodium chloride aerosol verify that the water-condensational growth maintained the laminar flow, while providing sufficient growth for particle imaging. Particle positions mapped onto particle mobility, in accordance with theoretical expectations. Particles ranging in size from 12 nm to 100 nm were counted with the same efficiency as with a butanol-based ultrafine particle counter, once inlet and line losses were taken into account.

Copyright © 2017 American Association for Aerosol Research     

Effect of gas temperature on the capture of charged particles by oppositely charged water droplets

This work reports experimental results on the effects of temperature (25, 45, and 65°C at different relative humidity) on the scrubbing of charged submicron particles by means of cold (25°C) droplets charged with opposite polarity. The aim of the study is to experiment how the capture of particles is influenced by the simultaneous presence of electrostatic and phoretic forces related to the occurrence of thermal and water vapor gradients close to the droplet surface. This information plays an important role in the development of wet electrostatic scrubbing (WES), an emerging technology for submicron and ultrafine particle capture. Tests were performed in a lab-scale system in which the particle laden-gas was scrubbed by a train of identic droplets. Particles were charged by a corona source while droplets are generated by electrospraying. Experiments revealed that for particles larger than about 250–300 nm, there were higher removal efficiencies in nonisothermal conditions, with limited differences between 45 and 65°C tests. For particles finer than about 150 nm, we sometimes observed lower removal efficiencies for higher gas temperatures, probably due to the difficulties in controlling particle charging for these particles. The experiments were interpreted with a consolidated stochastic model that predicted successfully the data at isothermal conditions, but was less effective for tests at higher gas temperatures. In our opinion, this discrepancy relies on synergies among the fluid dynamic field induced by droplet evaporation/condensation, the phoretic and the electrostatic forces, which are not considered in the model.

Copyright © 2016 American Association for Aerosol Research     

Emission of 1.3–10 nm airborne particles from brake materials

Operation of transport vehicle brakes makes a significant contribution to airborne particulate matter in urban areas, which is subject of numerous studies due to the environmental concerns. We investigated the presence and number fractions of 1.3–10 nm airborne particles emitted from a low-metallic car brake material (LM), a non-asbestos organic car brake material (NAO) and a train brake cast iron against a cast iron. Particles were generated by a pin-on-disc machine in a sealed chamber and analyzed using a nano condensation nucleus counter, a CPC, and an FMPS. It was found that 1.3–4.4 nm particles are emitted during the friction. For the pairs with the LM and NAO, 1.3–4.4 nm particles predominate in number at temperatures above 160°C. The emission of the 1.3–4.4 nm particles precedes the emission of above 4.4 nm particles. For the cast iron pair, the number of 1.3–4.4 nm particles is smaller than the number of 4.4–10 nm particles. The findings suggest that brake materials produce a significant number of 1.3–4.4 nm airborne particles, and these particles should not be neglected in environmental and tribological studies.

Copyright © 2017 American Association for Aerosol Research     

Control of the radial distribution of chemical components in spray-dried crystalline microparticles

The process of particle formation from evaporating droplets containing more than one solute was studied. Two-component microparticles were produced using a piezoceramic dispenser with an inner diameter of 30 µm. Initial droplets had a diameter in the range of 70–85 µm and contained sodium nitrate and potassium nitrate in different molar ratios of 30:70, 50:50, and 70:30, corresponding to weight ratios of 26.5:73.5, 45.7:54.3, and 66.2:33.8, in the form of aqueous solutions with initial concentrations of 1 or 10 mg/ml. The monodisperse droplets were dried in a dry laminar gas flow with temperatures of 50°C or 100°C. Different initial conditions affected the particle formation process and the particle morphology. The diameter of the final dried microparticles ranged from 4 to 10 µm. Their density varied from 1250 to 1950 mg/ml. The formulation and process conditions determined the distribution of chemical components in the dried microparticles, especially their surface composition as determined by energy-dispersive X-ray spectroscopy. The distribution of the chemical components was theoretically explained using characteristic times for the crystallization kinetics of the drying process. It was shown that the solute that reached supersaturation first formed most of the outer shell of the microparticles.

© 2016 American Association for Aerosol Research     

Characterization of the TSI model 3086 differential mobility analyzer for classifying aerosols down to 1 nm

Measurement systems for particle sizing starting at 1 nm are used to bridge the gap between mass spectrometer measurements and traditional aerosol sizing methods, and thus to enable measurement of the complete size distribution from molecules and clusters to large particles. Such a measurement can be made using a scanning mobility particle sizer equipped with a diethylene glycol growth engine (e.g., TSI Model 3777 Nano Enhancer) along with a condensation particle counter, and a differential mobility analyzer (DMA) appropriate for such small sizes. Previous researchers have used high-resolution DMA (HRDMA) and also the TSI Nano-DMA (Model 3085) in such a scanning mobility particle sizer system. In this study, we evaluate the performance of the recently introduced TSI 1 nm-DMA (Model 3086). The transfer function was characterized using 1–2 nm monomobile molecular ion standards. The same measurements were repeated on a TSI Nano-DMA, with good agreement to previously published values. From the measured transfer function, the resolution of each DMA model was determined as a function of particle size and sheath flow rate. Resolution of the TSI 3086 in the 1–2 nm range was 10–25% higher than the TSI 3085. Measured resolutions of the TSI 3086 were 10–20% lower than theoretically predicted values, whereas those of the Model 3085 were 0–10% lower.

Copyright © 2018 TSI Inc.     

A water-based fast integrated mobility spectrometer (WFIMS) with enhanced dynamic size range

A water-based fast integrated mobility spectrometer (WFIMS) with enhanced dynamic size range is developed. The WFIMS builds on two established technologies: the fast integrated mobility spectrometer and laminar flow water-based condensation methodology. Inside WFIMS, particles of differing electrical mobility are separated in a drift tube and subsequently enlarged through water condensation. Particle size and concentration are measured via digital imaging at a frame rate of 10 Hz. By measuring particles of different mobilities simultaneously, the WFIMS resolves particle diameters ranging from 8 to 580 nm within 1 s or less. The performance of WFIMS was characterized with differential mobility analyzer (DMA) classified (NH₄)₂SO₂ particles with diameters ranging from 8 to 265 nm. The mean particle diameters measured by WFIMS were found to be in excellent agreement with DMA centroid diameters. Furthermore, detection efficiency of WFIMS was characterized using a condensation particle counter as a reference and is nearly 100% for particles with diameter greater than 8 nm. In general, measured and simulated WFIMS mobility resolutions are in good agreement. However, some deviations are observed at low particle mobilities, likely due to the non-idealities of the WFIMS electric field.

Copyright © 2017 American Association for Aerosol Research     

Investigation of vehicle exhaust sub-23 nm particle emissions

A solid particle number limit was applied to the European legislation for diesel vehicles in 2011. Extension to gasoline direct injection vehicles raised concerns because many studies found particles below the lower size limit of the method (23 nm). Here we investigated experimentally the feasibility of lowering this size. A nano condensation nucleus counter system (nCNC) (d_50% = 1.3 nm) was used in parallel with condensation particle counters (CPCs) (d_50% = 3 nm, 10 nm and 23 nm) at various sampling systems based on ejector or rotating disk diluters and having thermal pre-treatment systems consisting of evaporation tubes or catalytic strippers. An engine exhaust particle sizer (EEPS) measured the particle size distributions. Depending on the losses and thermal pre-treatment of the sampling system, differences of up to 150% could be seen on the final detected particle concentrations when including the particles smaller than 23 nm in diameter. A volatile artefact as particles with diameters below 10 nm was at times observed during the cold start measurements of a 2-stroke moped. The diesel vehicles equipped with the Diesel Particulate Filter (DPF) had a low solid sub-23 nm particles fraction (<20%), the gasoline with direct injection vehicles had higher (35–50%), the gasoline vehicles with port fuel injection and the two mopeds (two and four-stroke) had the majority of particles below 23 nm. The size distributions peaked at 60–80 nm for the DPF equipped vehicles, at 40–90 nm for the gasoline vehicles with a separate nucleation mode peak at approximately 10 nm sometimes. Mopeds peaked at sizes below 50 nm when their aerosol was thermally pre-treated.

© 2017 American Association for Aerosol Research     

Clogging of Industrial High Efficiency Particulate Air (HEPA) Filters in Case of Fire: From Analytical to Large-Scale Experiments

The Institute of Radioprotection and Nuclear Safety (IRSN in French) is conducting research on the impact of a fire on the behaviour of containment devices such as high efficiency particulate air (HEPA) pleated filters for radioactive materials. This work aims to study the clogging of HEPA filters in case of fire involving realistic materials (polymers making up gloves boxes, waste treatment solvent, hydraulic oil, solid material mixtures making up a trash bin, electrical cables, and cabinets) used in nuclear facilities, from the medium to large scale. The clogging kinetics of industrial pleated HEPA filters is monitored by measuring the pressure drop of the filters and the filtered air temperature at a given filtration velocity (from 0.23 to 2.1 cm/s). Upstream HEPA filters, combustion aerosols are characterized in terms of size distribution, mass concentration, composition, and particle morphology using, respectively, a DMS500 (Cambustion^LTD), glass fiber filter sampling, and transmission electron microscope analysis of particles deposited on TEM grids. Particles emitted denote well-known fractal morphology, are composed of carbonaceous primary particles with diameters ranging from 31 nm to 48 nm and showing an high clogging efficiency. An empirical relationship has been successfully applied to the obtained results for a larger range of fuels, filtration velocities and fire conditions.

Finally, experiments have been performed on a large-scale facility, using full-scale fire scenarios (electrical cabinet, constant, and variable filtration velocity) and a reasonable agreement was observed with our empirical relationship. At this scale, particles appear to be compact, with a complex composition and diameters close to 220 nm with a lower clogging efficiency.

Copyright 2014 American Association for Aerosol Research     

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

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

Copyright © 2018 American Association for Aerosol Research     

Design and evaluation of a low flow personal cascade impactor

A very compact cascade impactor with 2 L/min sampling flow rate has been developed. Its dimensions are 8.5 cm L x 5.0 cm W x 11.4 cm H, and it weighs 0.27 kg, with ten impaction stages with aerodynamic cutpoints in the range of 60 nm to 9.6 μm. The top eight stages, collecting particles down to 170 nm in aerodynamic diameter, can be used as a stand-alone impactor with a portable, battery-powered pump. Particle collection efficiencies were obtained with two types of commonly used substrates, aluminum foil and glass fiber filters. Impactor cutpoints with aluminum foil substrates agree well with conventional impactor theory. The efficiency curves are sharp with minimum overlap between them. Thus, the compact impactor design does not compromise its performance, making it suitable for general purpose applications where a lower sampling flow rate provides adequate mass collection. With glass fiber filter substrates, impactor cutpoints are smaller and the efficiency curves are less steep, in particular for the last stages. Also, the collection efficiency curves do not drop to near zero at small Stokes numbers. Instead, excess particle collection efficiency of around 10% is observed for the top six stages, and becomes higher for the last four stages. This is due to the collection of particles by filtration as the impinging jets penetrate the filter substrate. Thus, using glass fiber filter substrates should generally be avoided due to the non-ideal effect on the impactor collection efficiency curves, especially for the last two stages.

Copyright © 2018 American Association for Aerosol Research     

Evaluation of metallic filter media for sub-micrometer soot particle removal at elevated temperature

Soot particle removal performance of two types of metallic filter media, sintered metal powder and sintered metal fiber, is experimentally evaluated as potential improvements to conventional ceramic filtration media for gasoline direct injection (GDI) engine PM after-treatment application. Soot collection efficiency and flow resistance of several grades of metallic media are measured at temperatures of 25, 350, and 650°C and a range of representative filtration velocities for sub-micrometer soot particles generated from a propane flame. Theoretical collection efficiency based on single fiber efficiency theory shows good agreement with experimental data for nearly spherical KCl particles at 350°C. Improved collection efficiency is observed for soot particles in the interception-dominated size range above ～100 nm due to enhanced interception length. Soot collection is slightly enhanced at higher temperature, which is consistent with model predictions. Sintered metal fiber media are found capable of removing ～75% of soot particles by mass with an incremental flow resistance of less than 1.5 kPa under 10 cm/s and 350°C, which is promising for gasoline particulate filter (GPF) application. The media level figure of merit (FOM) is used to quantify the soot collection efficiency versus flow resistance tradeoff of all media tested. It is found that due to their more open structure (higher porosity) sintered metal fiber media have FOMs nearly one order of magnitude higher than those of sintered metal powder media, and by analogy those of conventional wall flow ceramic media. This suggests that sintered metal fiber media represents an attractive alternative to ceramic media for designing GPFs; however, further research into creating comparable surface area to the honeycomb structures used for wall flow filters is needed to extract the full potential of metal fiber media.

Copyright © 2017 American Association for Aerosol Research     

Spatial aerosol flow maldistribution: A design flaw confounding the proper calibration and data interpretation of stages “0” and “1” of the Andersen eight-stage nonviable cascade impactor

We introduced monodisperse calibrant particles into an eight-stage non-viable Andersen cascade impactor (ACI) operated at 28.3 L/min and separately quantified the particle mass captured under each of the four concentric rings of nozzles on stages 0 and 1, the entry and succeeding stages of this impactor. On both stages, we found that each ring of nozzles has a particle capture efficiency behavior that differs from the others, and the fraction of calibrant particles deposited under each of the individual rings of nozzles depended on the particle size. We believe this behavior derives primarily from a radial flow velocity non-uniformity associated with recirculation zones introduced by the 110° expansion angle of the inlet cone. Because of these recirculation zones, the inertia of particles larger than about 5 µm aerodynamic diameter will cause their point-wise local concentration to differ from the concentration at the inlet entry. This concentration maldistribution continues to stage 1 primarily because of the annular collection plate at stage 0. The influence of the inlet cone aerodynamics on the performance of both stages means that the size of particles deposited on these plates will be uncertain unless the aerosol transport entering the impactor associated with calibration using monodisperse particles exactly simulates the in-use aerosol flow conditions. The degree of realism necessary in the calibration method has heretofore not been discussed in published calibrations of the ACI, introducing uncertainty in the size interpretation of the particle mass collected on stages 0 and 1 in practical applications of this impactor.

Copyright © 2017 American Association for Aerosol Research     

Cu-Sn binary metal particle generation by spray pyrolysis

Cu-Sn binary particles were generated via spray pyrolysis from metal salt precursors with ethylene glycol as the co-solvent and reducing agent. The morphology, crystallinity, and elemental distribution of particles were tunable by changing the reaction temperature, residence time, and quench gas flow rate. Hollow porous particles were fabricated with a higher Sn concentration on the particle surface when the furnace set point was 500°C, while solid particles with a lower surface Sn concentration were generated when the furnace set point was 1000°C. Particles with spherical morphologies were obtained at long residence time conditions (4.5 s). Cu-Sn binary particles with irregular structures (e.g., pores on the particle surface, fragmented spherical particles, and lamellar fragments) were formed at short residence time conditions (0.92 s). A possible spray pyrolysis mechanism was proposed that incorporates chemical reaction steps and structural progression. By this mechanism, the metal salts are believed to sequentially undergo hydrolysis to metal hydroxides, decomposition to metal oxides, reduction to metals, and finally diffusion of Sn into the Cu matrix to generate the Cu-Sn solid solution.

Copyright © 2017 American Association for Aerosol Research     

Characterization of nanosized silica size standards

Nanosized silica size standards produced with a sol–gel synthesis process were evaluated for particle size, effective density, and refractive index in this study. Particle size and effective density measurements were conducted following protocol from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. Particle sizes were measured via electrical mobility analysis using a differential mobility analyzer (DMA) at sheath flow rates (Q_sh) of 3.0 and 6.0 L/min and a constant aerosol flow rate (Q_a) of 0.3 L/min. The measured mean and mode diameters agreed well with the labeled sizes in the size range 40–200 nm, with differences ranging from 0.03% to 0.8%, well within the labeled expanded uncertainties (95% confidence intervals) of 1.8%–2.2%. The coefficient of variation (CV) of the size distribution was 0.012–0.027 for 40–200 nm. Particle sizes measured for 20 nm and 30 nm standards showed size differences with respect to the certified sizes of 1.7% and 8.3% at Q_sh = 6.0 L/min, but the size distributions were narrow, with CV = 0.047–0.064. The average effective density for the range 40–200 nm measured with an aerosol particle mass analyzer (APM) was 1.9 g/cm³. The real component of the refractive index measured with an optical particle counter (OPC) was 1.41 at a wavelength of 633 nm. All properties (size, effective density, and refractive index) were stable and could be measured with good repeatability. From these evaluations, it was found that the nanosized silica size standards have good characteristics for use as size standards and constitute a feasible alternative to PSL particles.

© 2017 American Association for Aerosol Research     

Effects of temperature on the formation of secondary organic aerosol from amine precursors

Aerosol formation is directly influenced by meteorological properties such as temperature and relative humidity. This study examines the influence of temperature on the physical properties and chemical composition of the aerosol produced from radical oxidation of aliphatic amines. Aerosol formation for temperatures ranging from 10 to 40°C was investigated in dual 90 m³ indoor atmospheric chambers. Further, chemical and physical responses of aerosol formed at one temperature and then raised/cooled to another were investigated in detail. Around two to three times more aerosol formation occurred at 10°C than at 40°C. This has important implications for locations influenced by amine emissions during the winter months. Significant aerosol formation occurred with the oxidation of amines with nitrate radical (100–600 μg/m³) and consisted largely of amine nitrate salts. These reactions are important contributors to aerosol formation during the nighttime hours, when nitrate radical is the dominant oxidant and temperatures tend to be cooler. Solid/gas partitioning of amine nitrate salt aerosol was consistent with literature results. A novel, temperature dependent, mechanism describing peroxy and hydroperoxy radical reactions was observed in the trimethylamine with hydroxyl radical oxidation experiments.

Copyright © 2016 American Association for Aerosol Research     

Modeling the thermodynamics and kinetics of sulfuric acid-dimethylamine-water nanoparticle growth in the CLOUD chamber

Dimethylamine (DMA) has a stabilizing effect on sulfuric acid (SA) clusters, and the SA and DMA molecules and clusters likely play important roles in both aerosol particle formation and growth in the atmosphere. We use the monodisperse particle growth model for acid-base chemistry in nanoparticle growth (MABNAG) together with direct and indirect observations from the CLOUD4 and CLOUD7 experiments in the cosmics leaving outdoor droplets (CLOUD) chamber at CERN to investigate the size and composition evolution of freshly formed particles consisting of SA, DMA, and water as they grow to 20 nm in dry diameter. Hygroscopic growth factors are measured using a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA), which combined with simulations of particle water uptake using the thermodynamic extended-aerosol inorganics model (E-AIM) constrain the chemical composition. MABNAG predicts a particle-phase ratio between DMA and SA molecules of 1.1–1.3 for a 2 nm particle and DMA gas-phase mixing ratios between 3.5 and 80 pptv. These ratios agree well with observations by an atmospheric-pressure interface time-of-flight (APi-TOF) mass spectrometer. Simulations with MABNAG, direct observations of the composition of clusters <2 nm, and indirect observations of the particle composition indicate that the acidity of the nucleated particles decreases as they grow from ～1 to 20 nm. However, MABNAG predicts less acidic particles than suggested by the indirect estimates at 10 nm diameter using the nano-HTDMA measurements, and less acidic particles than observed by a thermal desorption chemical ionization mass spectrometer (TDCIMS) at 10–30 nm. Possible explanations for these discrepancies are discussed.

Copyright © 2016 American Association for Aerosol Research