Evaluation of the Alphasense optical particle counter (OPC-N2) and the Grimm portable aerosol spectrometer (PAS-1.108)

We compared the performance of a low-cost (～$500), compact optical particle counter (OPC, OPC-N2, Alphasense) to another OPC (PAS-1.108, Grimm Technologies) and reference instruments. We measured the detection efficiency of the OPCs by size from 0.5 to 5 µm for monodispersed, polystyrene latex (PSL) spheres. We then compared number and mass concentrations measured with the OPCs to those measured with reference instruments for three aerosols: salt, welding fume, and Arizona road dust. The OPC-N2 detection efficiency was similar to the PAS-1.108 for particles larger than 0.8 µm (minimum of 79% at 1 µm and maximum of 101% at 3 µm). For 0.5-µm particles, the detection efficiency of the OPC-N2 was underestimated at 78%, whereas PAS-1.108 overestimated concentrations by 183%. The mass concentrations from the OPCs were linear (r ≥ 0.97) with those from the reference instruments for all aerosols, although the slope and intercept were different. The mass concentrations were overestimated for dust (OPC-N2, slope = 1.6; PAS-1.108, slope = 2.7) and underestimated for welding fume (OPC-N2, slope = 0.05; PAS-1.108, slope = 0.4). The coefficient of variation (CV, precision) for OPC-N2 for all experiments was between 4.2% and 16%. These findings suggest that, given site-specific calibrations, the OPC-N2 can provide number and mass concentrations similar to the PAS-1.108 for particles larger than 1 µm.

Copyright © 2016 American Association for Aerosol Research     

Characterization of a Herrmann-type high-resolution differential mobility analyzer

Aerosol instrument characterization and verification for nanometer-sized particles requires well-established generation and classification instruments. A precise size selection of sub-3-nm charged aerosol particles requires a differential mobility analyzer (DMA), specially designed for the sub-3-nm size range. In this study, a Herrmann-type high-resolution DMA developed at Yale University was characterized in various operation conditions. A relation between sheath flow rate and tetraheptylammonium ion (C₂₈H₆₀N⁺, THA⁺, 1.47 nm, mobility equivalent diameter) was established. The maximum particle size that the DMA was able to classify was 2.9 nm with the highest sheath flow rate of 1427 liters per minute (Lpm), and 6.5 nm with the lowest stable sheath flow rate of 215 Lpm, restricted by the maximum and minimum flow rates provided by our blower. Resolution and transmission of DMA are reported for tetrapropylammonium (C₁₂H₂₈N⁺, TPA⁺, 1.16 nm), THA⁺, and THA₂Br⁺ (1.78 nm) ions measured with two different central electrodes and five different sheath flow rates. The transmission varied between 0.01 and 0.22, and the resolution varied between 10.8 and 51.9, depending on the operation conditions.

Copyright © 2016 American Association for Aerosol Research     

Design,simulation, and characterization of a radial opposed migration ion and aerosol classifier (ROMIAC)

We present the design, simulation, and characterization of the radial opposed migration ion and aerosol classifier (ROMIAC), a compact differential electrical mobility classifier. We evaluate the performance of the ROMIAC using a combination of finite element modeling and experimental validation of two nearly identical instruments using tetra-alkyl ammonium halide mass standards and sodium chloride particles. Mobility and efficiency calibrations were performed over a wide range of particle diameters and flow rates to characterize ROMIAC performance under the range of anticipated operating conditions. The ROMIAC performs as designed, though performance deviates from that predicted using simplistic models of the instrument. The underlying causes of this non-ideal behavior are found through finite element simulations that predict the performance of the ROMIAC with greater accuracy than the simplistic models. It is concluded that analytical performance models based on idealized geometries, flows, and fields should not be relied on to make accurate a priori predictions about instrumental behavior if the actual geometry or fields deviate from the ideal assumptions. However, if such deviations are accurately captured, finite element simulations have the potential to predict instrumental performance. The present prototype of the ROMIAC maintains its resolution over nearly three orders of magnitude in particle mobility, obtaining sub-20 nm particle size distributions in a compact package with relatively low flow rate operation requirements.

© 2017 American Association for Aerosol Research     

A practical set of miniaturized instruments for vertical profiling of aerosol physical properties

In situ atmospheric aerosol measurements have been performed from a Manta unmanned aircraft system (UAS) using recently developed miniaturized aerosol instruments. Flights were conducted up to an altitude of 3000 m (AMSL) during spring 2015 in Ny-Ålesund, Svalbard, Norway. We use these flights to demonstrate a practical set of miniaturized instruments that can be deployed onboard small UASs and can provide valuable information on ambient aerosol. Measured properties include size-resolved particle number concentrations, aerosol absorption coefficient, relative humidity, and direct sun intensity. From these parameters, it is possible to derive a comprehensive set of aerosol optical properties: aerosol optical depth, single scattering albedo, and asymmetry parameter. The combination of instruments also allows us to determine the aerosol hygroscopicity.

Copyright © 2017 American Association for Aerosol Research     

Experimental evaluation of miniature plate DMAs (mini-plate DMAs) for future ultrafine particle (UFP) sensor network

Two iPhone-sized differential mobility analyzers (DMAs) in the parallel-plate configuration (i.e., mini-plate DMAs) were designed and their performance was calibrated in this study in order to gain the instructive knowledge for the future mini-plate DMA design and to have a well-calibrated mini-plate DMA for the ultrafine particle (UFP) sensor network. The performance of mini-plate DMAs was calibrated using the tandem DMA (TDMA) technique. The experimental transfer functions of prototypes at different particle sizes and under various combinational conditions of aerosol and sheath flow rates were derived from the TDMA data. It is concluded that mini-plate DMAs performed reasonably well for UFP sizing. It was also found that the sizing resolution of mini-plate DMAs is closer to the aerosol-to-sheath flow rate ratio when the percentage of aerosol slit opening in length was increased (relative to the width of aerosol classification zone). A new concept of “effective sheath flow rate” was introduced to better interpret the experimental observation on the area and FWHM (full width at half maximum) data of measured DMA transfer functions. Based on the experimental data, we proposed a modified equation for mini-plate DMAs to better calculate the voltage required to size particles of a given electrical mobility.

Copyright © 2016 American Association for Aerosol Research     

Optimized detection of particulates from liquid samples in the aerosol phase: Focus on black carbon

Operational parameters for a single particle soot photometer (SP2) and a CETAC Marin-5 nebulizer were optimized for detection of particulates aerosolized from liquid samples. The sensitivity of nebulization efficiency on nebulizer input gas pressure, liquid sample flow rate, and alcohol doping of the sample were explored. The nebulization efficiency of the Marin-5 was found to be roughly independent of applied gas pressure once above a minimum pressure. The nebulization efficiency changed by ～50% for an order of magnitude change in liquid sample flow. Doping the sample with isopropyl alcohol at a 1:1 ratio results in a ～50% relative increase in nebulization efficiency over a broad range of liquid flows. These results should apply to all particulate materials in the size range studied. SP2 operational parameters including sheath and sample flow were explored to optimize detection of refractory black carbon (rBC) specifically via coupling to the nebulizer. The SP2 tested samples up to 5 cc s?1 with 100% detection of rBC in its size range of detection, with increased sample jet spread and corresponding lack of detected rBC in the air at higher flows, leading to a total undetected rBC mass fraction of ～15% at 16 cc s?1. Varying sheath flow does not improve this result, which is significant because under reasonable Marin-5 operating conditions, the SP2 only samples a fraction of the total air flow out of the nebulizer. Recommended operational parameters for cases of sample with low rBC loadings are presented: first, when very little liquid sample is available; second, when considerable sample is available.

© 2017 American Association for Aerosol Research     

Using a new inversion matrix for a fast-sizing spectrometer and a photo-acoustic instrument to determine suspended particulate mass over a transient cycle for light-duty vehicles

Integrated particle size distribution (IPSD) is a promising alternative method for estimating particulate matter (PM) emissions at low levels. However, a recent light-duty vehicle (LDV) emissions study showed that particle mass estimated using IPSD (M_IPSD) with the TSI Engine Exhaust Particle Sizer (EEPS) Default Matrix was 56–75% lower than mass derived using the reference gravimetric method (M_Grav) over the Federal Test Procedure (FTP). In this study, M_IPSD calculated with a new inversion matrix, the Soot Matrix, is compared with M_Grav and also photoacoustic soot mass (M_Soot), to evaluate potential improvement of the IPSD method for estimating PM mass emissions from LDVs. In addition, an aerodynamic particle sizer (APS) was used to estimate mass emission rates attributed to larger particles (0.54–2.5 µm in aerodynamic diameter) that are not measured by the EEPS. Based on testing of 10 light-duty vehicles over the FTP cycle, the Soot Matrix significantly improved agreement between M_IPSD and M_Grav by increasing slopes of M_IPSD/M_Grav from 0.45–0.57 to 0.76–1.01 for gasoline direct injected (GDI) vehicles; however, for port-fuel injection (PFI) gasoline vehicles, a significant discrepancy still existed between M_IPSD and M_Grav, with M_IPSD accounting for 34 ± 37% of M_Grav. For all vehicles, strong correlations between M_IPSD and M_Soot were obtained, indicating the IPSD method is capable of capturing mass of soot particles. The discrepancy between the M_IPSD and M_Grav for PFI vehicles, which have relatively low PM emissions (0.22 to 1.83 mg/mile), could be partially due to limited size range of the EEPS by not capturing larger particles (0.54–2.5 µm) that accounts for ～0.08 mg/mile of PM emission, uncertainties of particle effective density, and/or gas-phase adsorption onto filters that is not detected by in situ aerosol instrumentation.

Copyright © 2016 American Association for Aerosol Research     

Measurement of size-dependent dynamic shape factors of quartz particles in two flow regimes

Understanding and modeling the behavior of quartz dust particles, commonly found in the atmosphere, requires knowledge of many relevant particle properties, including particle shape. This study uses a single particle mass spectrometer, a differential mobility analyzer, and an aerosol particle mass analyzer to measure quartz aerosol particles mobility (d_m), vacuum aerodynamic, and volume equivalent diameters, mass, composition, effective density, and dynamic shape factor as a function of particle size, in both the free molecular and transition flow regimes. The results clearly demonstrate that dynamic shape factors can vary significantly as a function of particle size. For the quartz samples studied here, the dynamic shape factors increase with size, indicating that larger particles are significantly more aspherical than smaller particles. In addition, dynamic shape factors measured in the free-molecular (χ_v) and transition (χ_t) flow regimes can be significantly different, and these differences vary with the size of the quartz particles. For quartz, χ_v of small (d_m < 200 nm) particles is 1.25, while χ_v of larger particles (d_m ～ 440 nm) is 1.6, with a continuously increasing trend with particle size. In contrast, χ_t of small particles starts at 1.1 increasing slowly to 1.34 for 550 nm diameter particles. The multidimensional particle characterization approach used here goes beyond determination of average properties for each size, to provide additional information about how the particle dynamic shape factor may vary even for particles with the same mass and volume equivalent diameter.

© 2016 American Association for Aerosol Research     

Development of portable aerosol mobility spectrometer for personal and mobile aerosol measurement

We describe development of a portable aerosol mobility spectrometer (PAMS) for size distribution measurement of submicrometer aerosol. The spectrometer is designed for use in personal or mobile aerosol characterization studies and measures approximately 22.5×22.5×15 cm and weighs about 4.5 kg including the battery. PAMS uses electrical mobility technique to measure number-weighted particle size distribution of aerosol in the 10–855 nm range. Aerosol particles are electrically charged using a dual-corona bipolar corona charger, followed by classification in a cylindrical miniature differential mobility analyzer. A condensation particle counter is used to detect and count particles. The mobility classifier was operated at an aerosol flow rate of 0.05 L/min, and at two different user-selectable sheath flows of 0.2 L/min (for wider size range 15–855 nm) and 0.4 L/min (for higher size resolution over the size range of 10.6–436 nm). The instrument was operated in voltage stepping mode to retrieve the size distribution in approximately 1–2 min. Sizing accuracy and resolution were probed and found to be within the 25% limit of NIOSH criterion for direct-reading instruments. Comparison of size distribution measurements from PAMS and other commercial mobility spectrometers showed good agreement. The instrument offers unique measurement capability for on-person or mobile size distribution measurement of ultrafine and nanoparticle aerosol.     

Capture of submicrometer particles in a pressurized electrostatic precipitator

This study investigated the influence of gas pressure on the submicrometer particle capture performance of an electrostatic precipitator (ESP). Current-voltage characteristics and particle capture performance of the ESP were studied in air and in simulated flue gas (SFG) under 1, 2, and 3 atm. Using negative corona and air as the feed gas, the penetration of most particles of 40–400 nm in diameter decreased from 8 × 10^?4 ? 2 × 10^?2 to 2 × 10^?4 ? 1 × 10^?2 as pressure increased from 1 atm to 3 atm at constant current; and increased from 3 × 10^?5 ? 1 × 10^?3 to 2 × 10^?4 ? 1 × 10^?2 as pressure was elevated when the voltage was held roughly constant. Similar type of disparity under different pressures was also observed for positive corona and for SFG. Experiments set up to capture fly ash in the ESP showed that with constant current, higher pressure resulted in a higher initial charge fraction of the particles from the furnace, which could facilitate the penetration of fly ash particles. A semiempirical model was developed based on the Deutsch–Anderson equation and experimental data under 1, 2, and 3 atm to calculate the particle penetrations under high pressure. The total charge number on a particle (n') is calculated by incorporating the effects of current (I) and pressure (P) on relative weights of the diffusion charging number (n_diff) and field charging number (n_field), that is, n' = B₁(I,P)n_diff + B₂(I,P)n_field, where B₁(I,P) and B₂(I,P) are both empirical coefficients dependent on current and pressure. Experimental penetrations under 1.5 and 2.5 atm validated this model over the particle diameter range in 100–400 nm.

Copyright © 2016 American Association for Aerosol Research     

Characteristics of particles deposited on a single free-fall charged droplet

Particles deposited on a free-fall charged droplet were experimentally studied. A droplet, charged under 40% Rayleigh limit, fell through the particle chamber to capture particles by electrostatic attractions. The velocity of the droplet was smaller than 2.1 m/s. The particle-laden droplet eventually spread on a glass slide, which was further analyzed using optical microscope. It was found that the equivalent number of particles captured by the charged droplet were larger than that of uncharged ones by one order of magnitude at least. Remarkably, particles on the charged droplet agglomerated into a large cluster, which indicates that the agglomerated cluster can be actively precipitated due to the gravity force if the droplet completely evaporates. The front side of the charged droplet was the predominant region to capture the particles. However, the actual area of capture was smaller than hemispheric surface.

Copyright © 2017 American Association for Aerosol Research     

Preparation and characterization of polycaprolactone microspheres by electrospraying

The ability to reproducibly produce and effectively collect electrosprayed polymeric microspheres with controlled morphology and size in bulk form is challenging. In this study, microparticles were produced by electrospraying polycaprolactone (PCL) of various molecular weights and solution concentrations in chloroform, and by collecting materials on different substrates. The resultant PCL microparticles were characterized by optical and electron microscopy to investigate the effect of molecular weight, solution concentration, applied voltage, working distance, and flow rate on their morphology and size. The work demonstrates the key role of a moderate molecular weight and/or solution concentration in the formation of spherical PCL particles via an electrospraying process. Increasing the applied voltage was found to produce smaller and more uniform PCL microparticles. There was a relatively low increase in the particle average size with an increase in the working distance and flow rate. Four types of substrates were adopted to collect electrosprayed PCL particles: a glass slide, aluminium foil, liquid bath, and copper wire. Unlike 2D bulk structures collected on the other substrates, a 3D tubular structure of microspheres was formed on the copper wire which could find application in the construction of 3D tumor mimics.     

Triboelectric charging of fungal spores during resuspension and rebound

The triboelectric charging of fungal spores was experimentally characterized during rebound and resuspension. A fungal spore source strength tester (FSSST) was used as a primary aerosol generator for spores of three fungal species and two powders (silicon carbide and silver). The critical velocity of rebound was determined using a variable nozzle area impactor (VNAI), and the charging state of particles after resuspension and rebound was measured using the FSSST, different impactor setups, electrometers, and optical particle counters. In the impactor setups and the FSSST, five different surface materials relevant for indoor environments were used (steel, glass, polystyrene, paper, and polytetrafluoroethylene). The critical velocity of rebound was determined to be 0.57 m/s for fungal spores, which is relatively low compared to silicon carbide and previous results for micron-sized aerosol particles. Based on the rebound impactor measurements, we were able to define the crucial parameters of charge transfer for different particle–surface material pairs. A contact charge parameter, which describes the triboelectric charging during rebound, was found to have a negative correlation with the charging state of the particles after the resuspension from an impactor. This connects the triboelectric charging during rebound and resuspension to each other. Based on the contact charge parameter values, quantified triboelectric series could be formed. The results of this work show that fungal spores can be charged both positively and negatively during rebound and resuspension depending on the fungal species and surface material.

Copyright © 2016 American Association for Aerosol Research     

Evaluation of portable dilution system for aerosol measurement from stationary and mobile combustion sources

This study presents the emission factor of PM_2.5, elemental carbon (EC), organic carbon (OC), and water-soluble ions for biomass-fired-induced downdraft gasifier and light duty diesel vehicle (LDDV). A portable dilution system (PDS) developed for on-field measurement of aerosol and their precursors from combustion sources were used for quenching of aerosol at near-atmospheric condition before collection on filters. PDS consists of a heated duct and particle sampling probe, dilution tunnel, zero air assembly, and a power supply unit. PDS was evaluated under controlled conditions in laboratory for gasifier cookstove and LDDV over wide range of dilution ratios to understand the effect of dilution on mixing, particle formation, and loss. The invariability in CO₂, recorded along the length and at radial distances of cross-section of dilution tunnel, confirmed the rapid and homogenous mixing inside the dilution tunnel. The particle loss and nucleation inside the dilution tunnel accounted for 6–20% at different dilution ratios (30:1–90:1). PM_2.5 emission factors for wood combustion in gasifier cookstove showed mild decrease (13%) with increasing dilution ratio from 75:1 to 108:1. However, a considerable decrease of 37% (221–139 mg km^?1) was observed for LDDV with increase in dilution ratio from 39:1 to 144:1. Similar decrease in particulate organic carbon emission rates were observed indicating scarcity of sorptive organics, and insufficient residence time for condensation limited the particle formation from vapor phase organic compounds at high dilution ratios.

© 2016 American Association for Aerosol Research     

Atmospheric particle composition-hygroscopic growth measurements using an in-series hybrid tandem differential mobility analyzer and aerosol mass spectrometer

The ability of atmospheric particles to absorb water has extensive climate, atmospheric chemistry, and health implications, and considerable effort has gone into determining relationships between particle composition and hygroscopicity. Parallel techniques, in which co-located composition and hygroscopicity measurements are combined to infer composition-hygroscopicity relationships, may not detect the influence of external mixtures. Previous in-line measurements have been limited to single-particle composition or a limited analyte range, and are often non-quantitative and/or offline. Here, we present for the first time in-series, online, quantitative hygroscopicity-composition measurements using a Brechtel Manufacturing, Inc. Hybrid Tandem Differential Mobility Analyzer and an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer. This technique is first verified using laboratory-generated external particle mixtures, then extended to ambient measurements at a seaside sampling side at the Hong Kong University of Science and Technology. The technique successfully separated laboratory-generated particles of differing hygroscopicities and showed promise for atmospheric particles, though high mass attenuation endemic to the HTDMA dual size selection limits application to environments with at least ～14–41 μg/m³ of particulate mass, depending on composition.

Copyright © 2017 American Association for Aerosol Research     

A light-weight,high-sensitivity particle spectrometer for PM2.5 aerosol measurements

A light-weight, low-cost optical particle spectrometer for measurements of aerosol number concentrations and size distributions has been designed, constructed, and demonstrated. The spectrometer is suitable for use on small, unmanned aerial vehicles (UAVs) and in balloon sondes. The spectrometer uses a 405 nm diode laser to count and size individual particles in the size range 140–3000 nm. A compact data system combines custom electronics with a single-board commercial computer. Power consumption is 7W at 9–15 V. 3D printing technology was used in the construction of the instrument to reduce cost, manufacturing complexity, and weight. The resulting Printed Optical Particle Spectrometer (POPS) instrument weighs about 800 g with an approximate materials cost of 2500 USD. Several POPS units have been constructed, tested in the laboratory, and deployed on UAVs. Here we present an overview of the instrument design and construction, laboratory validation data, and field engineering data for POPS.     

Inversion to obtain aerosol size distributions from measurements with a differential mobility analyzer

To measure size distributions of submicrometer aerosols with an electrical differential mobility analyzer (DMA) requires an inversion procedure. The Knutson (1976) and the Hoppel (1978) inversion procedures were numerically investigated for the case of log-normal aerosol size distributions. It was found that the Hoppel procedure converges to the same result as that given by the Knutson procedure. The computational range for geometric mean diameter ( _g) was 0.025-0.25 μm, and for geometric standard deviation (σ_g) was 1.1–2.4. The inversion error was found to be greater than 10% in certain “forbidden zones” of _g and σ_g values. For the case of an ideal DMA having no lower mobility limit, only one forbidden zone exists, this consisting of small σ_g values. The boundary of this forbidden zone intercepts the computational range boundaries at σ_g = 1.25, and σ_g = 1.62, . These results also apply to an actual DMA when the size distribution of particles larger than the DMA singly charged mobility limit is available a priori. If such information is not available, the concentration of these larger particles is assumed to be zero in performing the inversion. This assumption adds a second forbidden zone, consisting of large σ_g values and having the intercepts σ_g = 2.44, and σ_g = 1.50, . The first forbidden zone remains nearly the same.