Examination of atmospheric bioaerosol particles with radii > 0.2 μm

Besides their effects on air hygiene and health, bioaerosol particles play an important role in cloud physics, for example, some bacteria are able to accumulate water and act as ice nuclei. For sampling aerosol particle impactors were used. The larger particles were sampled size fractionated with a free wing impactor while for the smaller ones an isokinetic two-stage impactor was constructed. The bioaerosol particles of the coarse fraction were stained with a protein dye and could be distinguished from the non-dyed particles using an optical light microscope. The small particles were examined in a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDX). Three criteria were used to characterize the particles: morphology, elemental composition and behaviour during EDX. Literature and the results of our own experiments with test aerosols showed that biological particles have a special morphology together with a special elemental composition and also some of them change their form during EDX. Based on these criteria a scheme was developed dividing the atmospheric aerosol into six groups, each of them representing biological or non-biological particles.     

Method and test system for evaluation of bioaerosol samplers

A method and test system have been developed for the laboratory evaluation of the performance of bioaerosol samplers. The method differentiates between the overall physical sampling efficiency (which reflects the inlet and collection efficiencies) and the biological sampling efficiency (which reflects the survival of the test microorganisms during the sampling process). The number concentrations of laboratory-generated bioaerosol particles are measured with an aerosol size spectrometer up- and downstream of the bioaerosol sampler being tested. In a bioaerosol impactor, which was specially designed for testing microbiological aspects of bioaerosol sampling, the inlet and collection efficiencies are differentiated by measuring downstream of the collection surface location with and without the collection surface in place. The number of recovered particles is counted as microcolonies with a microscope after sampling the bioaerosol particles into agar and culturing them. The total recovery of these bioaerosol particles is determined as a ratio of the number of viable microorganisms recovered to the number of bioaerosol particles present in the air sampling volume upstream from the sampler. This total recovery is a measure of the ratio of culturable to non-culturable bacteria present in the air. By measuring physical and microbiological aspects simultaneously, information is gained on aspects of bioaerosol sampling that cannot be determined by either of these branches of science alone. This is exemplified by tests on the influence of relative humidity and desiccation time on colony count.

The newly-developed system can be used to test any bioaerosol sampler. A special single-stage impactor was designed, built and used to study how different sampling and analysis variables affect the total recovery of bioaerosol particles. The designed impactor was calibrated using PSL particles. Its inlet sampling efficiency was found to be within the range of 96–99.5%, depending on the sampling conditions and particle size, if the latter is less than 8 μm (this range represents single bacteria, bacterial agglomerates, and fungi). The collection efficiency was found to be about 100% when collecting PSL particles larger than 0.7 μm in diameter at 201 min⁻¹ or higher air flows.

The total recovery of microorganisms measured under these conditions is characterized only by the “survivability” of microorganisms during their sampling. It was found that relative humidity had a pronounced effect on total Pseudomonas fluorescens recovery. Experimental data also showed that the sampling time may be limited due to bacterial desiccation and subsequent loss in viability of collected microorganisms.     

On changes in bed-material particles from a 550 MWth CFB boiler burning coal, bark and peat

This paper presents our observations on coating build up, morphology and the elemental composition of bed-material particles collected from a 550 MWth CFB boiler burning coal, bark and peat fuel/fuel mixture. The special focus was on the changes of the elemental composition of coating layer on bed-material particles when different fuels were burned. The results were obtained using a scanning electron microscope coupled with an energy depressive X-ray analyser (SEM/EDX). The results clearly show that properties of bed-material particles are a result of complex interaction between the fuels burned previously, and the fuels used at the time of sampling.     

Laser microprobe mass analysis of fog droplet residues

In order to investigate air particulate matter incorporated in the aqueous phase the residues resulting from evaporated fog droplets are compared to single particles of the interstitial aerosol. A counterflow virtual impactor (CVI) is used to separate the fog droplets from both the gaseous and the particulate aerosol components. The fog droplets sampled evaporate and the residues are collected on thin organic films by means of a cascade impactor. Simultaneously a second impactor device (size range about 0.1 to 10 μm) of the same type is exposed to collect the particles of the interstitial aerosol. A special impactor head rejects the larger fog droplets. Mainly the laser microprobe mass analysis (LAMMA -500, Leybold AG) is used to get informations about the chemical composition of both the fog droplet residues and the interstitial particles. Preliminary results indicate that the two particle types show significant differences in their chemical composition. E.g. sulfate and nitrate containing carbonaceous particles preferably appear in the fraction of the interstitial aerosol. In contrast the residues dominantly consist of sulfate species only. Due to the in-fog scavenging of gases the formation of hydroxy methane sulfonate is observed.     

Automated Spore Measurements Using Microscopy,Image Analysis,and Peak Recognition of Near-Monodisperse Aerosols

Rapid detection of airborne fungal and bacterial spores would enable public agencies to respond quickly and appropriately to intentional releases of hazardous aerosols. Automated analysis of microscope images and automated detection of near-monodisperse peaks in aerosol size distribution data offer complementary approaches to traditional methods for the identification and counting of fungal and bacterial spores. First, spores of the fungus Scopulariopsis brevicaulis were aerosolized in a chamber and then collected with a slit impactor; later, digital microscope images were analyzed manually to determine spore cluster distributions. The images also were analyzed with ImageJ, a program that automatically outlined objects and measured Feret's diameter, area, perimeter, and circularity. These characteristics were used to identify spore clusters automatically using two data analysis methods. Second, a computer program was developed to discriminate near-monodisperse bioaerosol peaks from those for polydisperse ambient particulate matter (PM) and was successfully tested using simulated and real aerosol mixtures. The observed agreement between manual and automated spore counts and the ability to detect spore peaks suggest that it may be possible to develop a system to recognize intentional releases rapidly through examination of particle morphology and size distributions. The peak detection procedure is potentially the fastest technique when used with real-time instrument data, but assumes that intentional releases would consist of large numbers of uniformly sized particles in the respirable size range.

Copyright 2012 American Association for Aerosol Research     

Relationship Between Biologically Fluorescent Aerosol and Local Meteorological Conditions

Time-resolved characterization of biological aerosol is important both for understanding environmental processes that affect biological aerosols and for determining realistic test conditions for the evaluation of bioaerosol detection systems. Very little work has been done to develop an understanding of the temporal fluctuations in bioaerosol concentration. During an experiment from 1–10 November 2008 ambient biological aerosol and meteorological data were collected. A FLIR/ICx/S3I Instantaneous Bioaerosol Analysis and Collection sensor was used to count both the biological and nonbiological aerosol in two size bins. The data indicate that the ambient relative humidity affects the optically observable concentration of biological aerosol with higher relative humidity generally associated with higher biological aerosol concentrations. The short timescale over which these correlations exist implies an aerosol process, rather than a change in aerosol source.

© 2013 American Association for Aerosol Research     

湿法烟气脱硫系统对细颗粒脱除性能的实验研究

针对石灰石-石膏法、双碱法、氨法等典型湿法烟气脱硫（WFGD）工艺,在喷淋脱硫塔中实验研究了WFGD系统对细颗粒的脱除作用;采用电称低压冲击器、场发射扫描电镜、X射线衍射仪等对WFGD系统前后烟气中细颗粒的浓度、粒径分布、形态、元素及物相组成进行了测试分析,考察了脱硫剂、液气比对WFGD系统脱除细颗粒性能的影响,并进行了利用蒸汽相变原理促进细颗粒凝结长大并高效脱除的实验研究。结果表明,脱硫剂对WFGD系统脱除细颗粒的性能具有重要影响,由于形成无机盐气溶胶细颗粒,采用CaCO₃、NH₃·H₂O脱硫剂时,WFGD系统对细颗粒的脱除效果明显不及Na₂CO₃脱硫剂和水洗涤,且颗粒形貌特征及元素组成发生明显变化;除NH₃·H₂O脱硫剂外,液气比对WFGD系统脱除细颗粒的影响不明显;在脱硫塔进口烟气、塔内脱硫液进口上方添加适量蒸汽建立蒸汽相变所需的过饱和水汽环境可显著促进细颗粒的脱除。     

Single-Particle Fluorescence Spectrometer for Ambient Aerosols

A fluorescence particle spectrometer (FPS) for real-time measurement of the fluorescence spectra of aerosol particles in the size range 1-10 w m diameter is reported. The prototype FPS has a sufficiently high sample rate (from 5 to 28 l/min for 3.5 w m to 11 w m diameter particles) to measure aerosol within buildings at practical rates (from 1 up to 600 particle fluorescence spectra per minute). Previously reported bioaerosol prototype detectors for measurement of single particle spectra (Pan et al., Opt. Lett ., 24, 116-118 (1999); Hill et al., Field Anal. Chem. Tech ., 3, 221-239 (1999)) were unable to sample the ambient environment; air containing particles had to be forced under pressure into a sample cell. In addition, sample rates were so small (less than 0.01 l/min) as to be impractical for most applications. The present design overcomes these deficiencies by the use of an airtight cell that highly concentrates micrometer-sized particles. A virtual impactor first concentrates aerosol particles, which are then drawn under negative pressure through an aerodynamic focusing nozzle in the inlet of the instrument, through the sample region, providing further concentration. The rate of particle spectra measured by the FPS increases significantly when the particle inlet is within a few meters of some common sources of indoor biological particles, e.g., a person coughing, sneezing, or rubbing his skin, or the presence of a dog. The spectra obtained have a variety of spectral shapes. The FPS may be useful in a variety of areas, e.g., in studying and monitoring airborne particles that cause diseases or allergies.     

OPTICAL PROPERTIES OF AEROSOLS

ABSTRACT

The evolution of small aerosol particles accompanying the combustion of straw for energy production is investigated. A sampling equipment specially designed for field measurements is described and characterized. The aerosol is studied by low-pressure cascade impactor and scanning mobility particle sizer, the particle morphology by transmission electron microscopy, and the chemical composition by energy dispersive x-ray analysis. The combustion gas contains 3–500 mg/Nm³ of submicron particles with a mean diameter of approximately 0.3 μm. The particles consist of almost pure potassium chloride and sulphate. The formation mechanism is analyzed by a theoretical simulation of the chemical reactions and the aerosol change during cooling of the flue gas. It is concluded that some sulphation of KC1 occurs in the gas phase although the sulphate concentration is much lower than predicted by an equilibrium assumption. The theoretical simulation proves that the fine mode particles can be formed by homogeneous nucleation of either KCl or K2SO₄ as the first step and further growth occurs by coagulation and diffusive condensation of both KC1 and K₄SO₄ on existing particles.     

PIXE and PDMS Methods Applied to Aerosols Analysis

The aim of this study was to apply the PIXE (Particle Induced X-ray Emission) and PDMS (Plasma Desorption Mass Spectrometry) techniques to characterize airborne dust particles containing metals. Aerosols generated at a mineral-sand processing plant were characterized in this study. The aerosol samples were collected at a plant that processes mineral sands to obtain rutile, ilmenite, zircon, and monazite concentrates. A cascade impactor with six stages was used to collect mineral dust particles with aerodynamic diameters in the range of 0.64 to 19.4 _mum. The particles impacted on each stage of the cascade impactor were analyzed by PIXE, which permits the determination of the elemental mass air concentration and the MMAD (Mass Median Aerodynamic Diameter). The chemical compositions of the aerosol samples were identified by PDMS analysis. This study shows that, by using PIXE and PDMS techniques, it is possible to determine the chemical compounds in which the elements are associated in the aerosol particles. Based on the results of the PIXE analysis, the elemental mass concentrations and the MMADs were determined.     

Size Distributions of Species in Fine Particles in Denver Using a Microorifice Impactor

Size-fractionated fine particles in Denver, Colorado, were collected by a microorifice impactor and analyzed for mass and elemental composition by a β-gauge and an X-ray fluorescence spectrometer. A least-squares procedure was used to fit a lognormal distribution function to the data in order to determine values and uncertainties for the mass median aerodynamic diameter and geometric standard deviation. The validity of the uncertainties so determined was tested by comparing them to uncertainties derived from Monte Carlo simulations.     

Collection efficiencies of aerosol samplers for virus-containing aerosols

Collection efficiencies of four bioaerosol samplers (Andersen impactor, AGI-30 impinger, gelatin filter, and nuclepore filter) were evaluated for virus-containing aerosols. Four different bacteriophages were used as surrogates for the mammalian viruses. Results showed that the collection efficiency was significantly affected by the morphology of the virus particles. For hydrophilic viruses, the collection efficiencies of the Andersen impactor, impinger, and gelatin filter were 10 times higher than that of the nuclepore filter. For hydrophilic viruses, the collection efficiencies of all four samplers were 10–100 times higher than hydrophobic viruses. The infectivity of the virus in collected samples was also evaluated for an AGI-30 impinger. Results showed that the viruses retained more infectivity when the samples were refrigerated (up to 1 day) during storage than when stored at room temperature (up to 8 h). Therefore, even when refrigerated, airborne virus samples collected using an impinger should be processed as soon as possible to avoid loss of virus infectivity.     

Generation of Submicron Mass/Volume-Monodisperse Aerosols with a Nebulizer-Impactor-Electrostatic Classifier System

A study was undertaken to investigate the mass size dispersion of particles classified according to their electrical mobilities. This is of primary concern in experiments that measure a concentration dependent property of the classified particles. Initially, the mass size distribution of particles produced by the 3-jet Collison, 6-jet Collison, and Misty-Ox nebulizers was measured. A 0–2 stage impactor was placed after the nebulizers and the size mass distribution was measured again. The polydisperse particle stream was then used to generate “size classified” aerosol and the mass size distribution of the equal mobility particles was calculated and measured. It was found that without an impactor in line, the aerosol stream contained a significant mass fraction of multiply charged particles. When an impactor was inserted directly after the nebulizers, the multiply charged particles were effectively removed and the particles were nearly monodisperse.     

Antimicrobial Air Filtration Using Airborne Sophora Flavescens Natural-Product Nanoparticles

We investigated nanoparticle generation from a natural plant extract using the aerosol technique of the nebulization-thermal drying process, and tested its usefulness for antimicrobial air filtration. Sophora flavescens Ait. ethanolic extract was prepared as an antimicrobial natural-product suspension. Suspension droplets were generated using a single-jet Collison nebulizer, passed through an active carbon absorber to remove ethanol, and mixed and dried with sheath air. For drying, natural-product particles were exposed to 200°C for ～1 s. Finally, particles were introduced into a scanning mobility particle sizer, and their size distribution and morphology were analyzed. For application of natural-product particles to antimicrobial air filtration, the nanosized particles generated were deposited continuously onto air filter medium at various times. Physical characteristics (filtration efficiency, pressure drop, and fiber morphology by scanning electron microscopy), and biological characteristics (antimicrobial tests against Staphylococcus epidermidis, Bacillus subtilis, and Escherichia coli bioaerosols) were then evaluated. We also analyzed the chemical composition of particles deposited on the filter surface. The results showed that the nanoparticles generated were spherical and demonstrated a polydisperse size distribution, ranging from several tens to several hundred nanometers. Although the filter pressure drop increased with the amount of nanoparticle on the filter, the bioaerosol filtration efficiency and antimicrobial activity were enhanced. In particular, the S. flavescens natural-product nanoparticle-deposited filters were more effective for removal of Gram-positive than Gram-negative bioaerosols. These results are promising for the implementation of this new technology for control of air quality against hazardous bioaerosols.     

A Field Intercomparison of Three Cascade Impactors

ABSTRACT

Cascade impactors separate aerosol particles inertially and collect them for later analysis. While laboratory calibrations typically indicate performance close to design specifications, during field operation impactors are subject to a number of sampling artifacts, including particle bounce, inlet and internal losses, and particle size changes as pressure drops within the impactor.

To test the vulnerability of some commonly used impactors to these problems under Held conditions, we participated in a shipboard intercomparison off the coast of Washington state between a micro-orifice uniform deposit impactor (MOUDI), a Berner low-pressure impactor, and a Sierra high-volume slotted impactor. Since there were some inconsistencies in the results, a second intercomparison was performed at Bellows Beach, Hawaii, between two MOUDIs and the Berner impactor.

Impactor samples were analyzed for soluble inorganic ions including Na⁺, K⁺, Cl^?, and NO^? ₃, primarily from large (>1 μm) sea salt particles and NH⁺ ₄, nonsea salt sulfate (NSS), and methanesulfonate (MS^?), found primarily in smaller aerosols.

The Sierra collected sea salt particles far more efficiently than the other impactors, which had severe inlet losses for 7 μm and larger particles. The MOUDI and Berner showed insignificant differences in the mass median diameter of accumulation mode particles (～0.34 μm), whereas the Sierra indicated almost twice the diameter (0.58 μm) of the others.     

Method to Determine the Number of Bacterial Spores Within Aerosol Particles

We describe methodology to reveal the number of microbial spores within aerosol particles. The procedure involves visualization under differential- interference-contrast microscopy enhanced by high-resolution photography and further analysis by computer-assisted imaging. The method was used to analyze spore of Bacillus globigii in aerosols generated by a small (pressured metered-dose inhaler type) generator. Particles consisting in 1 or 2 spores accounted for 85% of all generated particles. This percentage rose to 91% when the same aerosol was collected on an Andersen cascade impactor that collected particles larger than 0.65 μm and was even higher (96%) when particles larger than 3.3 μm were also eliminated. These results demonstrate that the imaging analysis of aerosol particles collected on glass slides is sensitive to even relatively small changes in aerosol particle composition. The accuracy of the enhanced microscopic method described herein (differences between visual and computer analysis were approximately 3% of the total particle counts) seems adequate to determine the spore composition of aerosols of interest in biodefense.     

Characterization of a method for aerosol generation from heavy fuel oil (HFO) as an alternative to emissions from ship diesel engines

This work describes a laboratory method to synthesize aerosols with properties similar to those emitted by ocean going ships. In this method, an oxy-hydrogen flame burner nebulizes and combusts heavy fuel oil (HFO). The oil was fed to the burner via a syringe pump at a maximum rate of 15 ml/h. Adjusting the feed temperature of the oil and the use of a quenching ring in the burner, it is possible to obtain an aerosol with a mode diameter of about 11 nm. This is close to the reported 5–8 nm for the nano-mode of ship emissions. Filter samples were also analyzed for elemental carbon, organic carbon and anion composition. No elemental carbon mass was detected and only a few sulfur containing compounds were present. A chemical equilibrium model was applied for both oxy-hydrogen flame and 2-stroke ship diesel engine combustion conditions to predict equilibrium concentrations, chemical formula and phase of vanadium and nickel containing compounds. The model confirmed that the real-world ship diesel engine and the oxy-hydrogen flame burner combustion processes produced the same vanadium, nickel and sulfur particulate matter (PM) products in terms of chemical formula and phase. Both the 5–8 nm particles from real-world ship emissions and the laboratory synthesized particles contain transition metals. Transmission electron microscope (TEM) images of laboratory synthesized particles show similar morphology to those sampled from a ship. Cloud condensation nuclei (CCN) measurement indicates that neither laboratory generated nor ship emitted aerosol is hygroscopic. To our knowledge, this is the first time the 5–8 nm particles emitted from ships have been aptly synthesized on a laboratory scale.     

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     

Procedures for Chemical Characterization of Sized Particles in Stationary Source Emissions

Methodology for the determination of the chemical composition of sized particulate emissions from sources is described. Emphasis is on the determination of the elemental content with X-ray fluorescence analysis (XRF) the principal analytical technique employed. Devices and procedures developed for the collection of sized particles from stationary sources are presented. Included are a sampler for size classification of submicrometer particles based on diffusion and a special in-stack cascade impactor that was developed to collect samples compatible for XRF analysis. Representative data on the chemical content of sized emission samples collected with these devices at several stationary sources are discussed.     

System design and test method for measuring respirator filter efficiency using mycobacterium aerosols

Recently, the protection of health care workers from tuberculosis-containing aerosols has been the subject of considerable debate. An experimental apparatus and test protocol were developed to measure the collection efficiency of surgical mask and respirator filter media using a microbial aerosol challenge. Mycobacterium chelonae (M. chelonae), used as a surrogate for Mycobacterium tuberculosis, was generated from liquid suspension using a Collison nebulizer. Upstream and downstream concentrations of viable aerosol particles were measured using Andersen cascade impactors, while total particle concentrations were measured with an aerodynamic particle sizer (APS). A monodisperse polystyrene latex (PSL) sphere aerosol (0.804 μm) was used in separate experiments to measure filter efficiency; concentrations were determined with the APS. The mycobacterial aerosol ranged in size from 0.65 to 2.2 μm when measured with the cascade impactor. A similar size range was found with the APS, yielding a count median diameter of about 0.8 μm. Samples of the mycobacterial aerosol were collected on glass slides, stained M. chelonae, as determined by environmental scanning electron microscope, were found to be rod shaped with an average length of 2 μm and average width of 0.3 μm. To evaluate the apparatus over a range of filter efficiencies (10–100%), different layers of fiberglass filter paper were tested for penetration using a 0.12 μm dioctyl phthalate (DOP) aerosol measured with a light scattering photometer, in addition to the mycobacterial and PSL aerosols. For the range of efficiencies tested it was shown that filter collection of DOP was linearly related to that of both mycobacterial and PSL sphere aerosols (r² = 0.99), demonstrating that an inert aerosol may be used to predict the collection of biological aerosols by such filter media.