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Aerosol Measurement: Principles, Techniques, and Applications, Third Edition
Abstract
Introduction Unipolar Diffusion Chargers Faraday Cage Electrometer Detection Diffusion Charging-Based Sensors Mobility Spectrometers with Electrical Detection Electrical Low Pressure Impactors List of Symbols References
... Charged droplets of positive polarity were generated in a home-built positive coronawire unipolar aerosol charger. 20,39,40 As described in the next paragraph, we determined the approximate charge and size distribution of the unaltered droplets and the charged droplets with a Scanning Mobility Particle Sizer (SMPS, model 3938 by TSI) (see Supporting Information for further information about the SMPS and related quantities such as electrical mobility and mobility radius). The "neutral" droplets were obtained by removing charged droplets from the aerosol flow in a home-built electrostatic precipitator. ...
... The "neutral" droplets were obtained by removing charged droplets from the aerosol flow in a home-built electrostatic precipitator. 20,39,41 By applying the voltage up to 4 kV, charged droplets of both polarities were lost by deposition on the precipitator walls. ...
It has recently been reported that reactions can occur faster in microdroplets than in extended condensed matter. The electric charge of droplets has also been suggested as a possible cause of this phenomenon. Here, we investigate the influence of electric charges on the photodegradation of single, optically trapped oleic acid aerosol droplets in the absence of other reactive species. The temporal evolution of the chemical composition and the size of droplets with charge states ranging from 0 to 104 elementary charges were retrieved from Raman spectra and elastic light scattering, respectively. No influence of the droplet charge was observed, either on the chemical composition or on the kinetics. Based on a kinetic multilayer model, we propose a reaction mechanism with the photoexcitation of oleic acid into an excited state, subsequent decay into intermediates and further photoexcitation of intermediates and their decay into nonvolatile and volatile products.
... This can be compared with traditional filter-based diffusion charging instruments: Based on instrument noise alone, the lower limit of detection in the miniDiSC is about 0.5 μm 2 /cm 3 , and for the TSI NSAM about 0.8 μm 2 /cm 3 (Dhaniyala et al. 2011). An additional error may occur if the electrometer zero offset drifts, which is not the case in our device. ...
... Particle charging, and thus the calibration constant, also depend on particle material and morphology, but only weakly (Dhaniyala et al. 2011). Nevertheless, a standardized calibration procedure should also specify these particle properties for a better comparability between instruments of different manu- facturers. ...
We introduce a new electrical measurement technique for aerosol detection, based on pulsed unipolar charging followed by a non-contact measurement of the rate of change of the aerosol space charge in a Faraday cage. This technique, which we call “aerosol measurement with induced currents,” has some advantages compared to the traditional method of collecting the charged particles on either an electrode or with a particle filter. We describe the method and illustrate it with a simple and miniature (shirt-pocket-sized) instrument to measure lung-deposited surface area. Aerosol measurement by induced currents can also be applied to more complex devices.Copyright 2014 American Association for Aerosol Research
... Wang et al. used the combination of a diffusion charging device with, among others, a CPC to characterize particles in terms of morphology, thus making use of the influence of morphology on the charging efficiency. Useful information on the performance of charge-based instruments can be found in Dhaniyala et al. (Dhaniyala, Fierz, Keskinen, & Marjamäki, 2011). Fierz et al. (Fierz et al., 2016) compared the performance of the diffusion charging device that HEPaC is based on using NaCl and soot particles as test aerosols. ...
... Wang et al. used the combination of a diffusion charging device with, among others, a CPC to characterize particles in terms of morphology, thus making use of the influence of morphology on the charging efficiency. Useful information on the performance of charge-based instruments can be found in Dhaniyala et al. (Dhaniyala, Fierz, Keskinen, & Marjamäki, 2011). Fierz et al. (Fierz et al., 2016) compared the performance of the diffusion charging device that HEPaC is based on using NaCl and soot particles as test aerosols. ...
... The TEM captures a visual representation of the particulates. In contrast, DMS measurements are based on electrical mobility diameters that can highly depend on the particulates' morphology, size, and composition [45] . As such, elements other than carbon in the observed ultrafine particulates-as apparent by the crystalline structurescould affect the particle charge model and thus skew the PSD. ...
Sub-23 nm particulate emissions from internal combustion engines have become a topic of interest for research and legislative regulations in recent years. Many studies focused on electrical mobility measurements of soot particles, but few works employed additional techniques that do not rely on equivalent diameters. In this work, exhaust-sampled soot from a 1.0 L gasoline direct injection engine was analysed by transmission electron microscopy (TEM). Three operating conditions were assessed: 1500 rpm fast-idle, 1500 rpm with 40 Nm and 1750 rpm with 20 Nm brake torque. A distinct mode of sub-10 nm particulates was found equally distributed on some sections of the TEM grids for all three conditions. These particulates appeared to be stable under the electron beam, suggesting a non-volatile nature. Differential mobility spectrometer measurements with and without catalytic stripper suggested the presence of volatiles but also indicated high levels of solid sub-23 nm particulates. Furthermore, more fractal soot agglomerates consisting of several primary particles were also observed by TEM. The nanostructure of primary particles exhibited mainly core-shell nanostructures for all operating conditions. An additional amorphous layer was observed surrounding primary particles for 1500 rpm fast-idle. Amorphous particulates and crystalline regions in agglomerates were identified for 1750 rpm with 20 Nm brake torque. Fringe analysis of the nanostructures was conducted for all three samples, with preliminary findings indicating similar fringe lengths of ca. 1.04 nm and tortuosity values of around 1.16.
... The TEM captures a visual representation of the particulates. In contrast, DMS measurements are based on electrical mobility diameters that can highly depend on the particulates' morphology, size, and composition [279]. As such, elements other than carbon in the observed sub-23 nm particulatesas apparent by the crystalline structures-could affect the particle charge model. ...
Understanding the intricacies of particulate emissions from internal combustion engines is important due to their harmful impact on human health and the environment, as well as their contribution to engine wear and performance. Particulate emissions were historically associated with diesel engines which are predominantly using direct-injection systems. Thus, diesel soot is widely covered in the literature. However, the increasing market share of vehicles with gasoline direct-injection (GDI) engines over recent years raises the question of how GDI soot compares to diesel soot and if existing knowledge can be applied. The aim of this work was to close this gap in knowledge between GDI and diesel engine soot.
Fringe analysis was assessed as a tool for quantifying graphitic nanostructures from transmission electron microscopy (TEM) images. A considerable influence of the processing parameters on the produced metrics was demonstrated, and optimised parameters were proposed. Moreover, the importance of the TEM focus point and the role of image quality was outlined. Subsequent thermogravimetric analysis of soot-in-oil samples suggested that the soot deposition rate into the lubricating oil is similar for GDI engines compared to diesel engines, even though their exhaust particulate emissions are generally considered to be one order of magnitude lower. Direct comparison of GDI and diesel engine soot samples and a carbon black identified primary particles with similar core-shell nanostructures in TEM images. However, for the GDI samples, also particles with surrounding amorphous layer were observed along with entirely amorphous particulates and traces of wear and oil chemistry. Fringe analysis revealed that fringes of GDI soot were distinctly shorter compared to the other soot types. This finding was confirmed by Raman spectroscopy, indicating that GDI soot is more disordered.
Electrical mobility measurements of particulate emissions were acquired for a GDI engine with a differential mobility spectrometer (DMS). As additional processing is required to compare the detailed particle size distributions to the regulatory solid particle number (SPN), different methods were assessed. While lognormal function fitting can be sufficient for SPN23 measurements, modelling of counting efficiencies by applying digital filtering functions is required for measurements below 23 nm. A new function was designed to match the proposed counting efficiencies for SPN10 of upcoming regulations. Measurements with the DMS combined with a catalytic stripper showed an increase of up to 11.2% using this new function compared to the closest previous sub-23 nm function. However, the results are highly dependent on the shape of the particle size distribution. For a matrix of test conditions, the shift from SPN23 to SPN10 was observed to result in increases of 27% to 390%.
Furthermore, soot particulates were sampled from the exhaust gas on TEM grids for three operating conditions. Core-shell primary particles were observed for all conditions. In addition, some particles at 1500 rpm fast-idle exhibited a surrounding amorphous layer. For 1500 rpm with 40 Nm brake torque, crystalline features within agglomerates and entirely amorphous/crystalline particulates could be found. Fringe analysis of the graphitic primary particle nanostructures did not find significant differences between the operating conditions; however, longer fringes than for the soot-in-oil samples were identified. An additional feature observed in all samples were separate sub-10 nm particulates of non-volatile nature. The average diameter of these particulates was below the lower detection size limit of the DMS.
... The measurement scheme in diffusion charging-based sensors involves the use of attachment of unipolar ions to particles by diffusion followed by detection of particle current. Ions undergoing Brownian motion attach to particle surface, imparting an electrical charge to the particles (Dhaniyala et al., 2011). Subsequently, the charged particles are ...
We compare different approaches to measure surface area of aerosol agglomerates. The objective was to compare field methods, such as mobility and diffusion charging based approaches, with laboratory approach, such as Brunauer, Emmett, Teller (BET) method used for bulk powder samples. To allow intercomparison of various surface area measurements, we defined ‘geometric surface area’ of agglomerates (assuming agglomerates are made up of ideal spheres), and compared various surface area measurements to the geometric surface area. Four different approaches for measuring surface area of agglomerate particles in the size range of 60–350 nm were compared using (i) diffusion charging-based sensors from three different manufacturers, (ii) mobility diameter of an agglomerate, (iii) mobility diameter of an agglomerate assuming a linear chain morphology with uniform primary particle size, and (iv) surface area estimation based on tandem mobility–mass measurement and microscopy. Our results indicate that the tandem mobility–mass measurement, which can be applied directly to airborne particles unlike the BET method, agrees well with the BET method. It was also shown that the three diffusion charging-based surface area measurements of silver agglomerates were similar within a factor of 2 and were lower than those obtained from the tandem mobility–mass and microscopy method by a factor of 3–10 in the size range studied. Surface area estimated using the mobility diameter depended on the structure or morphology of the agglomerate with significant underestimation at high fractal dimensions approaching 3.
It has become evident that additional metrics along the particle mass concentration, together with dense air quality monitoring networks within cities, are needed to understand the most efficient ways to tackle the health burden of particulate pollution. Particle lung-deposited surface area (LDSAal) is a metric to estimate particle exposure in the lung alveoli, and it has gained interest as a parameter for air quality monitoring as it is relatively easy and cost-efficient to measure with electrical particle sensors. Also, various studies have indicated its potential as a health-relevant metric. In addition to the electrical particle sensors, the LDSAal can be measured with various size distribution methods. However, different LDSAal measurement methods have fundamental differences in their operation principles, e.g., related to the measurement size ranges, size classification or conversion from the originally measured quantity into the LDSAal. It is not well understood how these differences affect the accuracy of the measurement in ambient conditions, where especially the particle effective density and hygroscopicity can considerably change the particle lung deposition efficiencies. In this study, the electrical particle sensor measurement (Partector) and two size distribution approaches (ELPI+ and DMPS/SMPS) were compared in road traffic environments with different environmental conditions in Helsinki and Prague. The results were compared by utilising the general assumptions of the LDSAal measurement (spherical hydrophobic particles with the standard density) and by evaluating the effects of the particle effective density and hygroscopicity. Additionally, the Partector and ELPI+ approaches were compared in various urban environments near road traffic, airports, river traffic and residential wood combustion. The results show that the comparison of different LDSAal measurement methods can be complicated in ambient measurements. The challenges were especially related to the accumulation mode particles roughly larger than 200–400 nm for which the dominant deposition mechanism in the lung changes from diffusion to impaction and the particle effective density and hygroscopicity tend to increase. On the other hand, the results suggest that the differences between the methods are reasonably low when considering only ultrafine and soot particles, which have an effective density closer to the standard (1.0 g cm-3) and are more hydrophobic, highlighting the suitability of the LDSAal as a monitored metric when estimating the spatial differences in the particulate pollution within cities.
In this work, we investigated the influence of different types of soot aerosol on the counting efficiency (CE) of instruments employed for the periodic technical inspection (PTI) of diesel vehicles. Such instruments report particle number (PN) concentration. Combustion aerosols were generated by a prototype bigCAST, a miniCAST 5201 BC, a miniCAST 6204 C, and a miniature inverted soot generator (MISG). For comparison purposes, diesel soot was generated by a Euro 5b diesel test vehicle with by-passed diesel particulate filter (DPF). The size-dependent counting efficiency profile of six PN–PTI instruments was determined with each one of the aforementioned test aerosols. The results showed that the type of soot aerosol affected the response of the PN–PTI sensors in an individualised manner. Consequently, it was difficult to identify trends and draw conclusive results about which laboratory-generated soot is the best proxy for diesel soot. Deviations in the counting efficiency remained typically within 0.25 units when using laboratory-generated soot compared to Euro 5b diesel soot of similar mobility diameter (∼ 50–60 nm). Soot with a mobility diameter of ∼ 100 nm generated by the MISG, the lowest size we could achieve, resulted in most cases in similar counting efficiencies as those generated by the different CAST generators at the same particle size, showing that MISG may be a satisfactory – and affordable – option for PN–PTI verification; however, further optimisation will be needed for low-cost soot generators to comply with European PN–PTI verification requirements.
div class="section abstract"> Particle Number (PN) measurement testing has for long been conducted by using Condensation Particle Counter (CPC) based technology. While accurate at low concentrations, CPC has nevertheless several drawbacks for in-field use, such as the use of a working fluid, the need for dilution, the delicate optical components and the sensitivity to contamination. Diffusion Charging (DC) based particle counting technologies have often been disregarded as a valid alternative to CPC based methods due to their intrinsic particle size dependent counting efficiency and lower sensitivity. However, Dekati’s novel ePNC PN technology has brought DC technology to the next level. Due to its patented technology, the Dekati ePNC’s particle counting efficiency is nearly size independent, turning DC as a competing technology for CPC, especially for demanding field applications, such as Periodic Technical Inspection (PTI), Portable Emission Measurement Systems (PEMS) for Real Driving Emissions (RDE), and brake and tire wear measurements. These applications require the ability to continuously measure elevated particle concentrations with sufficient accuracy in often harsh environments such as garages or during on-road testing. In this study we will look closer into the ePNC technology and show the results of recent measurement campaigns that prove the suitability of the Dekati ePNC’s DC technology as an alternative to CPC methods. For example, during vehicle type approval style chassis dynamometer measurement ePNC technology was found to produce comparable results against a PMP reference (7% difference in emission factor).
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In this article we present a method that aims towards more ideal number concentration response of a diffusion charger and diffusion collector-based particle sensor (Dekati ePNC) by utilizing reduced pressure conditions in the sensor. A model is derived for the charger and diffusion collector stage of the sensor as well as for the total particle number concentration response. The derived model is validated with experimental characterization measurements in different operation pressures. The experimental results show that reduced operation pressure makes the sensor response less dependent on particle size, approaching pure particle number response. The obtained results improve the feasibility of the diffusion-charging based method in the measurement of particle number concentration.
The data inversion algorithm of the Dekati electrical low pressure impactor (ELPI+) is the procedure to convert the electrical currents measured in each impactor stage into a particle size distribution. If a particle is collected in the incorrect stage, either due to particle bounce or premature collection, the erroneously induced current will cause an error to propagate through the data inversion. In this work, it is examined how this error propagation will modify the particle number size distribution. It is shown that particle bounce can contribute considerably to an erroneous particle distribution as one large bouncing particle can be misinterpreted as more than 10,000 small particles. This indicates that efforts should be made to avoid particle bounce in the ELPI+ as particle bounce thoroughly modifies the obtained particle number size distribution. On the other hand, it appears that the ELPI+ is quite robust against the effects of premature particle collection.
Fine mode particulate matter (PM) is known to be a major risk for human health as it can cause respiratory and cardiovascular diseases. Whether produced from anthropogenic or natural processes, fine mode PM is often electrically charged with positive, negative and/or bipolar charges. These pre-existing charges can, in some cases, significantly affect measurements from sensors which involve a charging stage prior to measurement by e.g. electrometry. In this work we demonstrate quantitatively the impact of pre-existing charges ( q = 0 , ± 1 , ± 2 ) on three different unipolar diffusion charging (DC) configurations using both simulations and experiments. The DC configurations include traditional steady state DC and pulsed modes of DC, where in all configurations a positive corona charger is used. We show that the impact of pre-existing charges on steady state (SS) and modulated precipitation (MP) DC is comparable. Positively pre-charged particles cause a larger deviation for SS and MP DC, in contrast to modulated diffusion charging (MDC), where negatively pre-charged particles have a greater impact. In addition, we show that the ion concentration in the charging region can be increased to significantly reduce the resulting error caused by the pre-charges for both SS DC and MP DC, in contrast to MDC. These results show that the influence of unipolarly pre-charged particles is very differently pronounced for the different modes of DC. The impact strongly depends on the polarity of the pre-charges, characteristics of the charger and the DC configuration itself. These insights are particularly interesting for the design and the application of diffusion charging based sensors.
In this article, we present a novel method for the elemental analysis of airborne aerosol particles using electrodynamic balance (EDB) trapping followed by laser-induced breakdown spectroscopy. The setup consists of a newly designed corona-based aerosol charger, double-ring electrodynamic balance trap and optical arrangement for the spectroscopy. Experimental laboratory measurements using the method show that the minimum particle size for successful analysis is 1 µm in diameter, and the minimum airborne concentration is of the order of 1 particle/cm³. In addition to the method, we will present results on the charging efficiency of the developed charger and novel stability analysis of the EDB at the charge region. The results from the stability analysis will ease the way towards analyzing submicron particles with the technique.
Copyright © 2020 American Association for Aerosol Research
The non-volatile Particle Number (PN) emissions of late technology diesel Heavy-Duty Vehicles (HDV) are very low due to the introduction of Diesel Particulate Filters (DPF). Nevertheless, a large fraction (50%) of particles below the current lower regulated size (23 nm) was recently reported. Moreover, large differences between laboratory and PN Portable Emission Measurement Systems (PN-PEMS) have been observed. In order to better understand such differences, the physical properties of the exhaust aerosol from two Euro VI technology diesel heavy duty engines were studied. It was found that urea injection leads to formation of non-volatile particles. The produced particles covered a wide size range spanning from below 10 nm to above 100 nm. As such, they contribute to the regulated PN emissions, with measured concentrations corresponding to as high as 2 × 10¹¹ #/kWh over a World Harmonized Transient Cycle (WHTC). However, a large fraction of them was undetected owing to their small particle size. Low-cut off size (10 nm) Condensation Particle Counters (CPCs) (which are under discussion to be included in the regulations) measured up to twice as high concentrations. Considering the large particle losses in the sampling systems at this size range, the true concentrations can be two times higher from what the low-cut-off CPCs reported. When the temperature of the SCR system exceeded a threshold of 300 °C, the produced particles were found to be positively charged, increasing the average exhaust aerosol charge up to +3 elementary charges per particle. Scanning Mobility Particle Sizer (SMPS) measurements of non-neutralized samples revealed that even the smallest of them can carry more than one positive charges. The findings of this study can explain the differences reported between PEMS and laboratory systems and especially those based on diffusion charging. They also provide insight for a refinement of technical requirements prescribed in the European PEMS regulation to more accurately quantify the PN emissions from such technologies.
Copyright © 2019 American Association for Aerosol Research
Particulate matter in the atmosphere is known to affect Earth’s climate and to be harmful to human health. Accurately measuring particles from emission sources is important, as the results are used to inform policies and climate models. This study compares the results of two ELPI + devices, two PM10 cascade impactors and an eFilter, in combustion emission measurements. The comparison of the instruments in a realistic setting shows what types of challenges arise from measuring an emission aerosol with unknown particle morphologies and densities, different particle concentrations and high temperature. Our results show that the PM10 cascade impactors have very good intercorrelation when the collected mass is greater than 150 µg, but below that, the uncertainty of the results increases with decreasing mass. The raw signals of two ELPI + devices were nearly identical in most samples, as well as the particle number concentrations and size distributions calculated from raw signals; however, transforming the current distributions into mass distributions showed variation in the mass concentration of particles larger than 1 µm. The real-time time signal measured by eFilter was similar to the total current measured by ELPI+. The eFilter and PM10 cascade impactors showed similar particle mass concentrations, whereas ELPI + showed clearly higher ones in most cases. We concluded that the difference is at least partially due to volatile components being measured by ELPI+, but not by the mass collection measurements.
A new diffusion charging-based aerosol instrument design is presented, intended to be a starting point for a low-cost particle concentration sensor. The aim in the design is to minimize instrument response dependence on the sample flow rate. The operation principle, response functions of the components, and performance of a prototype instrument are reported. Based on the performance evaluation, the instrument response remained constant within ±15% over a wide sample flow rate range of 3 to 10 lpm. While in the design some sensitivity is sacrificed to minimize flow rate dependency, the detection limit is still sufficient for a practical sensor application.
© 2018 American Association for Aerosol Research
Sonic jet chargers have originally been used in aerosol measurement devices for particle charging and neutralization. Here, our goal was to study if this charger type could be used in particle control devices in which particle concentrations and gas volumes are much higher. The study includes charging efficiency tests in a laboratory and with a commercial 20 kW wood pellet burner. Actual particle removal efficiency was tested with a laboratory scale parallel plate electrostatic collector. The results show that sonic jet-type chargers also have potential in filtering applications.
In Chaps. 6 and 7, it was suggested that diamond and silicon films grow by CNPs formed in the gas phase. With the assumption that CNPs are liquid-like, the film growth by CNPs explains many puzzling phenomena. Then, a question arise, “How about other CVD systems?” Would the non-classical crystallization by the building block of CNPs be applicable only to exceptional cases such as diamond and silicon or a general growth mechanism of CVD? In generalizing this new paradigm of the thin film growth, the most important work to do would be to confirm the generation of CNPs in other CVD processes.
Particle charging by indirect photoemission may be an alternative to charging methods based on corona discharge or on bipolar charging by radioactive ion sources. An indirect charger using a low energy UV radiation is introduced and characterized in detail. Depending on the carrier gas, photoelectrons or ions formed by electron-attachment charge the particles by diffusion charging. The achieved charging efficiency is in the range of 20–70% for particle sizes of 20 to 100 nm. It can easily be modulated by a small voltage applied to the photoemitter. To achieve stable electron emission, glassy carbon with its very inert surface is used as photoemitter. Particle losses are very small. The charge distribution has been measured by a tandem DMA setup. The experimental results are compared with the theory of unipolar diffusion charging based on the Fuchs’ combination probability of ions with the particles. The charger characterization has been performed by carbon particles in a size range of 20–100 nm, produced by a spark discharge generator.
The effect of aerosol particle size on the performance of an N95 filtering facepiece respirator (FFR) and a surgical mask (SM) was evaluated under different breathing conditions, including breathing frequency and mean inspiratory flow (MIF) rate. The FFR and SM were sealed on a manikin headform and challenged with charge-equilibrated NaCl aerosol. Filter penetration (P filter) was determined as the ratio of aerosol concentrations inside and outside the FFR/SM size-selectively (28 channels) within a range of 20 to 500 nm. In addition, the same models of the FFR and SM were donned, but not sealed, on an advanced manikin headform covered with skin-like material. Total inward leakage (TIL), which represents the total particle penetration, was measured under conditions identical to the filter penetration experiment. Testing was conducted at four mean MIFs (15, 30, 55, and 85 L/min) combined with five breathing frequencies (10, 15, 20, 25, and 30 breaths/min). The results show that SM produced much higher P filter and TIL values, and thus provide little protection against aerosols in the size range tested. P filter was significantly affected by particle size and breathing flow rate (P P filter as a function of the particle size exhibited more than one peak under all tested breathing conditions. The effect of breathing frequency on P filter was generally less pronounced, especially for lower MIFs. For the FFR and SM, TIL increased with increasing particle size up to about 50 nm; for particles above 50 nm, the total penetration was not significantly affected by particle size and breathing frequency; however, the effect of MIF remained significant.Copyright 2013 American Association for Aerosol Research
The Cambustion DMS500, a novel aerosol sizing instrument with fast time resolution, was first employed to sample jet engine particulate matter emissions during Project APEX. This paper compares the performance of the DMS500 to that of traditional aerosol instruments for sampling jet engine exhaust aerosol under field conditions during this campaign. The observed geometric mean diameter with respect to the particle number (D(g)) ranged from 15 to 45 nm, and with respect to the mass (third moment) distribution (D(gM)) from 21 to 112 rim, the geometric standard deviation (sigma(g)) ranged from 1.22 to 1.90 and the total number concentration (N) ranged from 6 x 10(3) to 3.3 x 10(5)/cm(3) (after dilution). On average, the D(g), D(gM), sigma(g), and N of the DMS500 size distributions differed by -9, -7, +1, and +30% from the reference values of the traditional instruments. Compared with the reference values, both D(g) and sigma(g) of the DMS500 showed a small but statistically significant decrease with increasing particle size. Effects due to particle shape appeared to be the most likely explanation for the observed size-related trends. The 30% disagreement in concentration measurements is reasonable when the sensitivity of the 3022 condensation particle counter to pressure fluctuations encountered during measurements at the engine exhaust nozzle is taken into account.
A method to find particle effective density and the fractal dimension, based on simultaneous size distribution measurements with SMPS and ELPI, is introduced. A fitting procedure is used to find the particle density as a function of particle size and the fractal dimension. The method was tested by simulation and by experimental measurements of particles with varying morphology. For fractal dimension values between 2.2 and 3.0, fractal dimension was measured with an accuracy of 0.1, and effective density was measured with 15% relative accuracy.
Vehicle exhaust aerosol characterisation requires real-time instruments to record particle concentration over transient tests. This work demonstrates the application of a real-time prototype diffusion charger to characterise exhaust aerosol. The output signal of the diffusion charger is first cross-compared with different particle properties and is shown to closely correlate with the active surface. The instrument is then calibrated using monodisperse diesel exhaust particles. The calibrated signal is shown to precisely estimate the particle surface area obtained from integration of different size distributions, including nucleation mode particles, in a range of more than five orders of magnitude. The mean particle diameter is also estimated in real time, combining the diffusion charger signal with aerosol number concentration provided by a particle counter. Finally, we demonstrate the potential to use the active surface for establishing vehicle exhaust particle emission levels, similarly to the regulated gravimetric procedure applied today.
A size-segregated chemical composition of atmospheric aerosols was investigated in May 2004 at the SMEAR II station, southern Finland. Aerosols were collected using two 12-stage low pressure impactors (SDI) and two virtual impactors (VI). The samples were analyzed for mass, inorganic ions and organic (OC) and elemental carbon (EC). By comparing the gravimetric mass and the results from the chemical analyses, a chemical mass closure was constructed. In addition to the impactors an Electrical Low Pressure Impactor (ELPI), Differential Mobility Particle Sizer (DMPS) and Aerodynamic Particle Sizer (APS) were used to measure the mass size distribution continuously. The chemical composition of fine particles (particle diameter < 1 μm) was very similar over the whole measurement campaign with 40% of mass composed of ammonium sulfate, 35% of OC and 5% of EC. In the submicron range the chemical mass closure of the collected samples was reached within a few percent on average. The chemical mass to gravimetric mass ratio was 0.98 ± 0.10 and 1.05 ± 0.13 (average ± S.D.) for the VI and SDI, respectively. Also, quite a good agreement was obtained between the mass size distributions measured with the ELPI and that measured with the DMPS-APS combination. When the total mass concentration of the fine particles was calculated, the mass concentration of the ELPI was found to be larger than that of the SDI and VI (ELPI/VI ratio 1.11 ± 0.13). This may be due to the semivolatile components lost in impactors. For the SDI and DMPS-APS the concentration of the fine particles was smaller than that of the VI with the SDI/VI and DMPS-APS/VI ratios of 0.70 ± 0.11 and 0.92 ± 0.08, respectively. For the DMPS and APS the mass concentration was calculated from the number concentration by estimating the particle density. The particle density was assessed in two ways; from the chemical composition of the particles (composite density) and by comparing the mass obtained from the DMPS-APS combination with the VI mass concentration (gravimetric density). The densities obtained for fine particles were 1.49 ± 0.03 and 1.66 ± 0.13 g cm-3 for the composite and gravimetric density, respectively.
Even for a bipolar charging based device (such as the SMPS), differences in particle charging can cause inaccuracies when agglomerates are sampled with a device calibrated for spherical aerosols (Lall & Friedlander, 2006). A comparison with DMA cut engine agglomerates reveals some differences in size classification (for Dme>100 nm) between a unipolar diffusion charging electrical mobility instrument (e.g. DMS) and the bipolar charging DMA.
These differences are consistent with agglomerates gaining more charge than would be expected for spheres. This is also reflected in the gain for agglomerate particles larger than 100 nm. The DMS’s lognormal data inversion allows for separate calibrations for each mode in the spectrum to account for these effects. For example, for Diesel emissions, the assumed spherical nucleation mode and the agglomerate accumulation mode can have different calibrations.
The charged fraction of ultrafine silver particles (5–50 nm) suspended in high-purity helium (99.998% grade) was measured as a function of particle size under different charging conditions (i.e., ion concentration and charging time). The charger used is a modified version of the one described in a previous study (Romay et al., 1991). Two radioactive sources of 250 μCi Po-210 are located in a cavity at the charger inlet. The cavity configuration is designed to control the range of the α rays. The modified charger provides a uniform electric field in the axial direction to separate the positive ions from the negative ions and electrons produced by the ionizing radiation. Thus, the aerosol particles are first exposed to a small region of electrons and positive ions, and are subsequently charged by positive ions in a much longer section of the charger. The ion concentration is estimated from the ion current collected in the exit electrode and measured with a picoammeter. The charging time is estimated from the length of the unipolar section of the charger and from the flow velocity profile in the charger. Experiments were performed to measure the fraction of uncharged particles by using a condensation particle counter equipped with a multichannel analyzer. From the measured uncharged fraction, ion concentration and charging time, the combination coefficient between positive ions and neutral particles was determined. The Knudsen number of the experimental data ranged from 2.4 to 24. The results were compared with the combination coefficient calculated using several available theories. It is concluded that Fuchs' limiting-sphere theory (1963), using ion properties of He2 , gives the best agreement with the experimental results. This is Particle Technology Laboratory Publicati No. 807.
An Ultrafine Water-based Condensation Particle Counter (UWCPC), a Scanning Mobility Particle Sizer (SMPS) incorporating an UWCPC, and a Fast Mobility Particle Sizer (FMPS) were deployed to determine the number and size distribution of ultrafine particles. Comparisons of particle number concentrations measured by the UWCPC, SMPS, and FMPS were conducted to evaluate the performance of the two particle sizers using ambient particles as well as lab generated artificial particles. The SMPS number concentration was substantially lower than the FMPS (FMPS/SMPS = 1.56) measurements mainly due to the diffusion losses of particles in the SMPS. The diffusion loss corrected SMPS (C-SMPS) number concentration was on average 15% higher than the FMPS data (FMPS/C-SMPS = 0.87). Good correlation between the C-SMPS and FMPS was also observed for the total particle number concentrations in the size range 6 nm to 100 nm measured at a road-side urban site (r2 = 0.91). However, the particle size distribution measured by the C-SMPS was quite different from the size distribution measured by the FMPS. An empirical correction factor for each size bin was obtained by comparing the FMPS data to size-segregated UWCPC number concentrations for atmospheric particles. The application of the correction factor to the FMPS data (C-FMPS) greatly improved the agreement of the C-SMPS and C-FMPS size distributions. The agreement of the total particle concentrations also improved to well within 10% (C-FMPS/C-SMPS = 0.95).
Electrical low pressure impactor ELPI was modified to measure particles below 30 nanometers in aerodynamic diameter. This was accomplished by adding a filter stage to collect and measure nanoparticles. The charging unit of the instrument was modified to increase the charging efficiency of the smallest, nanometer sized, particles. The modified charging unit was calibrated and the new construction of the ELPI was tested in laboratory and in vehicle dynamometer test cell. Measurements performed in the engine test cell showed that modifications improve the size range and measurement capability of the ELPI for engine emissions.
Diesel particulate mass emissions and their corresponding size distributions have been investigated on a diesel passenger car at steady state conditions using standard filters and a cascade impactor. These tests have been carried out at two different engine operating conditions (2100 rpm, 2.7 and 13.3 kW, respectively) corresponding to low and high exhaust gas temperatures. Two diesel fuels differing in their sulfur content (150 ppm and 2500 ppm S) have been used for these investigations. The particulate size distribution after diesel oxidation catalyst was found to be affected by the sulfur content of the diesel fuel and by the exhaust gas temperature. Interpretations of these results on a mechanistic basis are given. The diesel particulate emission studies have been extended to dynamic vehicle tests. The structural changes of the particulates using a single and a double-brick catalyst system have been determined during the MVEG-A test in order to identify the influence of catalyst volume. The development of the diesel particulates (size distribution and composition) along the exhaust system have also been studied. The particulate size distribution seems to be shifted to smaller particulate sizes with increasing catalyst volume. However, the primary particles remain unchanged by the diesel oxidation catalyst. Therefore, the apparent shift to smaller particulate sizes is explained by the increased removal of the Soluble Organic Fraction with higher catalyst volume. The adhesive responsible for an enlargement of the particulates is missing and therefore smaller particulate sizes have been observed. Hydrocarbon and sulfat condensation seems to be the key parameters responsibile for changes in particulate sizes. Experimental results therefore must be interpreted carefully, because they are affected strongly by the experimental arrangement. A detailed discussion of the effects of analytical conditions (e.g. the dilution ratio) on the observed results is given.
Number concentration of particles emitted by combustion engines has recently attracted attention, due to the fact that particles of the size range found in tail pipe emissions are suspected of being hazardous to human health. This paper describes the application of an Electrical Low Pressure Impactor (ELPI) to the measurement of number concentrations of diesel exhaust particles. The size distribution of particles as fine as 30 nm is determined using the aerodynamic diameter as the characteristic dimension. Results were obtained on both the engine and chassis dynamometer, in real-time, for steady state and transient tests. Swedish Environmental Class 1 diesel fuel was used, having a sulfur content of less than 10 ppm wt. A scheme for the calculation of particle losses in the sampling system was developed, showing high penetration of particles under the conditions examined. An estimate of particle density was produced by sampling monodisperse fractions of the exhaust aerosol to the ELPI through a Differential Mobility Analyzer (DMA) under steady state conditions. The effective density values as a function of particle diameter were compared with values obtained by direct comparison of ELPI and DMPS measured size distributions. The values of effective density decreased with increasing particle diameter. The size distribution measurement results present an accumulation mode peak in the range of 30 to 100 nm aerodynamic diameter, depending on the engine operation conditions. The shift in particle size could be explained by coagulation effects, and possibly HC adsorption in the dilution tunnel. Tests in various transient and steady state driving cycles showed considerable differences in size distribution.
In 1984, the International Commission on Radiological Protection (ICRP) appointed a task group of Committee 2 to review and revise, as necessary, the ICRP Dosimetric Model for the Respiratory System. The model was originally published in 1966, modified slightly in Publication No. 19, and again in Publication No. 30 (in 1979). The task group concluded that research during the past 20 y suggested certain deficiencies in the ICRP Dosimetric Model for the Respiratory System. Research has also provided sufficient information for a revision of the model. The task group's approach has been to review, in depth, morphology and physiology of the respiratory tract; deposition of inhaled particles in the respiratory tract; clearance of deposited materials; and the nature and specific sites of damage to the respiratory tract caused by inhaled radioactive substances. This review has led to a redefinition of the regions of the respiratory tract for dosimetric purposes. The redefinition has a morphologic and physiological basis and is consistent with observed deposition and clearance of particles and with resultant pathology. Regions, as revised, are the extrathoracic (E-T) region, comprising the nasal and oral regions, the pharynx, larynx, and upper part of the trachea; the fast-clearing thoracic region (T[f]), comprising the remainder of the trachea and bronchi; and the slow-clearing thoracic region (T[s]), comprising the bronchioles, alveoli, and thoracic lymph nodes. A task group report will include models for calculating radiation doses to these regions of the respiratory tract following inhalation of representative alpha-, beta-, and gamma-emitting particulate and gaseous radionuclides. The models may be implemented as a package of computer codes available to a wide range of users. This should facilitate application of the revised human respiratory tract model to worldwide radiation protection needs. (C)1989Health Physics Society
The objective was to characterize diesel exhaust aerosols on road and to duplicate the results in the laboratory without altering the physical characteristics of the nuclei mode. On-road emissions from four, heavy-duty diesel truck engines were measured. The same engines were reevaluated in the manufacturers’ laboratories. For highway cruise and acceleration conditions, all engines produced bimodal size distributions with the nuclei mode ranging in size from 6 to 11 nm and the accumulation mode from 52 to 62 nm. On-road size distribution measurements nearly always showed a nuclei mode while laboratory measurements showed a nuclei mode under many, but not all conditions. Laboratory studies showed that nuclei mode particles consisted mainly of heavy hydrocarbons. More than 97% of the volume of 12 and 30 nm particles disappeared on heating to 400 °C. The volatility resembled that of C24–C32 n-alkanes implying a significant contribution from lubricating oil.
This paper presents experimental results of unipolar diffusion charging of monodisperse combustion aerosols in the size range of 10–300 nm. A Hewitt-type corona charger was designed for the purpose of the investigation and tested in a Tandem-DMA experimental configuration with particles produced by a combustion aerosol generator (CAG). Measurements of the mean charge and the actual charge distribution of the monodisperse samples have been performed, while the apparatus allowed for the investigation of the effect of pre-existing charge of the aerosols. Results are compared with Fuchs' limiting-sphere theory and good agreement is found in most cases. Calculations with the birth-and-death method, using combination coefficients determined by the limiting-sphere theory, show that a Gaussian-type distribution fits well the experimental results. It is shown that pre-existing charge on the particles, as long as it is lower than the mean charge dictated by the operating conditions (NitNit product) of the charger, does not affect the final charging state of the aerosol. Using the classification column of a differential mobility spectrometer (DMS) we performed similar measurements at sub-atmospheric pressures. Variation of the pressure in the range of 250–1000 mbar appears to have an effect on the average number of charges acquired by the particles, although the differences among the measurements lie within the uncertainty levels of the experimental arrangement.
If moisture (or another impurity) is present in an aerosol stream undergoing unipolar or bipolar charging, charge-transfer or clustering will occur, so that the masses of neutral molecules and ions actually involved in the diffusion charging of the aerosol will differ. In the case of bipolar charging, the masses of ions of different polarity will be different. Calculations are carried out using four different charging theories to estimate the ion mass effects on bipolar and unipolar charging, Approximate relationships are used to obtain ion mean free paths and electrical mobilities. In the case of unipolar charging the effect of increasing the ion mass is to slow down the charging. In the case of bipolar charging a larger difference in the masses of positive and negative ions leads not only to a greater asymmetry, but causes the degree of asymmetry to become a stronger function of particle size. Refs.
p within 2.4% error. The response of the EAD is almost proportional to diameter, D p .AP AS2000CE (Photoelectric Aerosol Sensor man- ufactured by EcoChem) gave both size and composition-dependent responses. For diesel particles produced at high engine loads, the response was nearly proportional to Fuchs surface area. However, at lighter engine loads, the response dropped sharply with decreas- ing D p. Light engine loads are associated with high fractions of volatile particles that may suppress the photoemission response. The secondary purpose of this study is to investigate the difference in charging rate between singlets (NaCl particles) and agglomer- ates (diesel particles) by using diffusion chargers. Agglomerates (diesel particles at engine load 75%) acquire more charge than singlets (NaCl particles) by 15 and 17% for LQ1-DC and EAD, respectively.
Recently, Park et al. [(2007). Development and performance test of a unipolar diffusion charger for real-time measurements of submicron aerosol particles having a log-normal size distribution. Journal of Aerosol Science, 38, 420–430] have introduced a methodology for performing simple and fast measurements of submicron aerosol particles having a log-normal size distribution, using a unipolar diffusion charger, an electrometer, and a condensation particle counter (CPC). The methodology can be applied to particles of 30–700nm and requires an assumption of their geometric standard deviation in size. In this paper we propose a much cheaper but faster method which involves substituting a unipolar field charger and another electrometer for the CPC. With the data obtained using this dual-charger system, we developed a data inversion algorithm and estimated the particle size distribution by minimizing the differences between the measured aerosol currents and the calculated values. To compare the size distribution with the data measured using a scanning mobility particle sizer (SMPS), sodium chloride (NaCl) particles smaller than 0.1μm in diameter, and dioctyl sebacate (DOS) particles with a diameter of 0.1–0.7μm, were used. The estimated results for the NaCl and DOS particles were within 10% of the data measured with the SMPS, while a 33% deviation from the SMPS results was obtained in Park et al. [(2007). Development and performance test of a unipolar diffusion charger for real-time measurements of submicron aerosol particles having a log-normal size distribution. Journal of Aerosol Science, 38, 420–430]. Furthermore, the detection time obtained with the use of our dual-charger system was faster (3s) than the 5s obtained by Park et al. [(2007). Development and performance test of a unipolar diffusion charger for real-time measurements of submicron aerosol particles having a log-normal size distribution. Journal of Aerosol Science, 38, 420–430].
Unipolar diffusion charging of nonspherical particles was investigated for various particle shapes. We researched with TiO2 agglomerates produced by the thermal decomposition of titanium tetraisopropoxide (TTIP) vapor. TTIP was converted into TiO2 in the furnace reactor and was subsequently introduced into the sintering furnace. Increasing the temperature in the sintering furnace, aggregates were restructured into higher fractal dimensions. The aggregates were classified according to their mobility using a differential mobility analyzer. The projection area and the mass fractal dimension of particles were measured with an image-processing technique performed by using transmission electron microscope (TEM) photographs. The selected aggregates were charged by the indirect photoelectric charger, and the average number of charges per particle was measured by an aerosol electrometer and a condensation particle counter. For particles of the same mobility diameter, our results showed that the particle charge quantity decreases as the sintering temperature increases. This result is understandable because particles with lower fractal dimension have larger capacitance and geometric surface area.
The spatial variability of highly time resolved size distributions was investigated in a narrow valley which provides the opportunity to study the impact of different sources on ambient particle concentrations during summer and winter time. The measurements were performed with a Fast Mobility Particle Sizer (FMPS) from TSI, Inc. on a mobile laboratory in Southern Switzerland. The results indicate enhanced number concentrations (between 150 000 and 500 000 cm−3) along the busy highway A2 which is the main transit route through the Swiss Alps connecting the northern and southern part of Switzerland. Especially the nanoparticles with diameters lower than 30 nm showed strongly increased number concentrations on the highway both in summer and winter. In winter time, high aerosol volume concentrations (PM0.3) were found in villages where wood burning is often used for heating purposes. Both traffic and wood burning were found to be important sources for particulate mass which accumulates during temperature inversions in winter time. Traffic was the dominant and wood burning a minor source for the nanoparticle number concentration. This is important regarding health impacts and its attribution to different sources because wood burning might contribute most to particulate mass whereas at the same time and place traffic contributes most to particulate number. In addition, during summer time volatility measurements were performed with the FMPS showing that the nucleation mode prevalently seen on the highway was removed by more than 95% by thermal treatment.
Early studies of atmospheric electricity suggested that the electrical conductivity of the atmosphere should be sufficient to dissipate the charge on the surface of the earth in a matter of minutes. Efforts to understand how substantial electric fields could be maintained globally in spite of the high dissipation rates were propelled into the forefront of physics research at the turn of the century when it was observed that the newly discovered X-rays produced ions that behaved much like those in the atmosphere. Many of the approaches that are now employed in electrical measurements of aerosols were first conceived during the first three decades of this century. Initially the focus was on gas ions, but they were found to consist of charged clusters of water molecules that exhibited a number of distinct mobilities that were substantially lower than those that resulted after long efforts to dry the gas. The coaxial condenser mobility analyzer, introduced by McClelland in 1898 and enhanced by Zeleny in 1900, was used to measure atmospheric ions as early as 1901 by Ebert. Based upon atmospheric measurements with this device in 1905, Langevin reported on the existence of ions with mobilities 3000 times lower than those observed in the laboratory studies. These so-called large ions correspond to particles in what we now know as the accumulation mode of the atmospheric aerosol. The aspiration condenser dominated measurements of atmospheric ions for six decades even though Erikson developed a differential mobility analyzer by 1921, and Rohmann produced a differential mobility sampler in 1923. Only after electronics was improved in the 1950s and 1960s were these instruments reintroduced. It was based upon condenser measurements of atmospheric “ions” that Junge first described the structure of the ultrafine particle size distribution in 1955.The development of the Whitby Aerosol Analyzer in 1966 built upon earlier developments, including the Faraday cup electrometer that was used by both Zeleny and McClelland at the turn of the century, and a long history of mobility analyzers. This instrument represented a breakthrough nonetheless since it was the first mobility aerosol analyzer that was sufficiently refined and robust to be commercially produced. That instrument was refined into the electrical aerosol analyzer (EAA) that became the primary tool for characterizing ultrafine aerosol particles in the atmosphere for a number of years. At the same time that the EAA was developed, the differential mobility analyzer was reintroduced in a form that quickly became the standard for production of submicron calibration aerosols. Early efforts to transform that instrument from a calibration tool to a measurement device met with limited success due to the lack of a suitable detector. The introduction of a continuous flow condensation nucleus counter was followed quickly by the development of computerized differential mobility analysis of particle-size distributions and later accelerated through the introduction of scanning mode operation. Efforts to extend differential mobility analysis to smaller and larger particle sizes than have been accessible with the instruments developed during the 1970s continue to produce enhancements in the instrumentation and the understanding of its performance.
Experimental data are reported for unipolar diffusion charging of NaCl and Ag aerosols in the 0.004- to 0.075-μm-diameter range. Monodisperse, uncharged particles were exposed to unipolar positive ions produced by a corona discharge. The nt product was varied between 3 × 10 and 1 × 10 (ions/cm) (s), where n is the unipolar ion concentration and t is the charging time. The resulting aerosol charged fraction was measured by using a single-particle-counting condensation nucleus counter in conjunction with an electrostatic condenser. From the measured charged fraction and the known charging parameters, the combination coefficient between the neutral particles and the positive ions are obtained. Measurements are then compared with the available charging theories. It is found that the theory of Marlow and Brock best predicts charging rates in the ultrafine particle size range. Above 10 nm, the data approach the theory of Fuchs consistent with the experimental observation of Adachi et al. *Presented at Seminar on Ultrafine Aerosols: Properties and Behavior, University of Duisburg, Federal Republic of Germany, September 29, 1986.
Several method have been proposed to accelerate the measurements made with differential mobility analyzers (DMA), including the Scanning Electrical Mobility Spectrometer (SEMS) and the Scanning Mobility Particle Sizer (SMPS). Wang and Flagan (1990) developed a data analysis procedure that accounts for the migration of the particles through a time-varying electric field and the delay associated with transport from the analyzer column outlet to the detection point. Experience using a variety of detectors and scan rates has indicated that the instrument response depends on the plumbing configuration if a condensation particle counter (CPC) is used as a detector. When a sharply peaked distribution is analyzed, the apparent breadth of the measured distribution depends on the detector used and on the scan rate. Size distributions measured with an increasing voltage scan exhibit a tail toward large particle sizes, while decreasing voltage scans produce a more pronounced tail on the small particle end of the distribution. The reduction in peak height with increasing scan rate can be attributed to particle retention in the plumbing between the outlet of the DMA analyzer column and the point in the CPC where the particles are detected optically. This paper examines the smearing of the transfer function of the SEMS as a result of flow non-idealities in the system. A model has been developed to predict the distortion of the transfer function in terms of the particle residence time distribution within the instrument. Results of this model are compared with calibration data as a function of the detector employed and the scan rate. We include laboratory observations of the phenomenon and examine the use of data inversion techniques to retrieve the true size distribution.
The purpose of this experimental study was to determine if corona-enhanced chemical vapor deposition of silicone found in personal care products can cause silicon-oxide to grow on the discharge wires of electrostatic air cleaners. To test the hypothesis, a wire-cylinder precipitator was operated with a positive corona discharge for 180 hours with an air/octamethylcyclotetrasiloxane mixture. The 30.5 cm-long precipitator has a 200 μm-diameter tungsten wire suspended in a 3.3 cm-diameter aluminum tube. The silicone, called cyclomethicone in product ingredient lists, is commonly found in deodorants, hair care products, lotions, and cosmetics, and it is volatile at room temperature. Experiments were conducted at current and voltage levels typical of the charging section of commercial indoor air cleaners. Concentration of the cyclomethicone vapor was approximately 1000 ppm. Results confirm that the presence of this silicone vapor in positive corona discharge creates amorphous silicon-oxide deposits on the wire. The extent and composition of the deposit were determined with scanning electron microscopy and energy dispersive spectroscopy. As the silicon-oxide deposit grew in thickness, the normally uniform corona became sparsely spaced tufts. Current was reduced until the corona was completely suppressed. After 180 hours, the deposit was 79 μm thick. Current was reduced 95% from 0.09 mA to 0.004 mA at an operating voltage of 7.5 kV. In an indoor air cleaner, such a decrease in the magnitude and uniformity of current would reduce particle charging and collection efficiency.
Design and theory of a new compact ultrafine particle sizing instrument, called the miniature electrical-mobility aerosol spectrometer (MEAS), was recently introduced [Ranjan, M., & Dhaniyala, S. (2007). A new miniature electrical spectrometer: Theory and design. Journal of Aerosol Science, 39, 950–963]. In the MEAS, electrostatic precipitation technique is used for both generation of sheath flow and classification of particles based on their electrical mobility. An electrometer-array, connected to the collection electrodes in the classifier section, is used to measure the number of particles collected in the different mobility channels, and these data are inverted using MEAS transfer functions to obtain particle number size distributions. Design of a prototype MEAS and the experimental approach to validate the performance of the individual components of the instrument are presented. Particle size distributions obtained from MEAS measurements compare well with those obtained using a scanning mobility particle sizer (SMPS; TSI 3936), validating theoretical calculations of instrument transfer functions. The operational limits of MEAS are determined from the calculation of error in the inverted size distribution as a function of total particle concentration. This analysis suggests that the designed MEAS can be used for applications such as personal and ambient monitoring under conditions of moderate to high particle concentrations.
A new modification of electrical low pressure impactor (ELPI) for the particle effective density measurement is presented. The system is capable of real-time operation and it is based on the serial measurement of mobility and aerodynamic diameter. In the studied configuration, a zeroth order mobility analyser is installed inside of the ELPI-instrument. The system is feasible for single modal distributions. For several particle materials and varying size distributions, the measured average density values were within 15% of the values obtained with a reference method.
Theory and design of a new electrical-mobility based instrument for measurement of aerosol particle size distributions in real-time is presented. Miniature electrical aerosal spectrometer (MEAS) has a rectangular cross-section with two main regions: the electrostatic precipitator (ESP) and classifier sections. The ESP section enables charged particle injection into the classifier section in a narrow range of streamlines at the desired location. The injected charged particles are then segregated based on their electrical mobility in the classifier section and collected on a series of plates that are connected to electrometers. Real-time particle size distribution measurements can be inferred from the electrometer signal strengths with the knowledge of the instrument transfer function. A theoretical approach is developed to calculate MEAS transfer function considering the non-uniformity in the electric and flow fields inside the instrument, and accounting for the instrument dimensions and its operating conditions. The theoretical predictions of size classification characteristics are seen to compare well with numerical results. The modeling results suggest that an optimal operational domain exists for MEAS.
The performance of the Electrical Low-Pressure Impactor (ELPI) has been evaluated using monodisperse aerosols. ELPI is a near real-time size analyzer consisting of an aerosol charger and a cascade impactor. Particle cut sizes of the cascade impactor and the charging efficiency of the charger were determined experimentally in the designed range of the instrument, i.e. 0.03–10μm. The Stokes numbers of 50% particle cut sizes of all the impactor stages were found to vary from 0.421 to 0.483 with an average of 0.456 and standard deviation of 0.017. The collection efficiency curves were found to be steep with an average steepness (ratio of 70–30% and 30% collection efficiency) of 1.19. The charging efficiency of the charger was close to that specified by the manufacturer for particles smaller than 2μm in diameter. For larger particles, a deviation from the manufacturer’s specifications was observed. It results from the high particle loss in the charger because of the small size of the charger, strong trapping electric field, and the perpendicular directions of the aerosol stream and the electric field in the charging zone. A comparison with SMPS for size distribution measurements was made in this study. The good agreement between the two measured size distributions shows the capability of the ELPI for near real-time particle size measurements.
Diesel soot overloads the ELPI-impactor rapidly if it is equipped with the standard flat-surface impactors. This non-ideal behaviour was studied recently (J. Aerosol Sci. 32 (2001) 1117). It was found that rapid overloading, or surface build-up, is a result of a fluffy bed of soot particles that covers the impactor surfaces and starts to filter airborne soot particles. This paper reports additional results with oil-soaked sintered impactors for the ELPI. It is demonstrated that (rapid) overloading is eliminated with oil-soaked sintered impactors. The maximum allowed mass load for the ELPI impactor is increased 50-fold.
The effect of particle shape on the diffusion charging of aerosols was investigated. The charging-equivalent sphere diameter dQE, found previously to be much larger than the mobility diameter dm, may be related to the uncharged fraction of particles leaving a bipolar diffusion charger. This fraction was measured for three types of particles classified by electrical mobility: polystyrene latex (PSL) spheres, ammonium sulfate spheres and TiO2 agglomerates of 10–20 nm primary particles. The uncharged fraction was ∼5% lower for the agglomerates than for spheres with the same mobility, for size range 100 < dm < 800 nm. This implies that dQE ∼ 1.1 dm for the agglomerates, which is a smaller difference between dQE and dm than reported by previous studies, but is consistent with the theoretical predictions (Laframboise and Chang, 1977, J. Aerosol Sci.8, 331–338).
The Diffusion Size Classifier (DiSC) is a new instrument to measure number concentration and average diameter of nanometer sized particles in the size range 10 - 200nm. It is small, easily portable and battery operated and therefore well suited for field measurements. The measurement range is suitable for ambient air concentrations (1000 - 500000 particles/cm 3 ); together with a diluter it can be used for emission measurements. The number concentrations measured with DiSC agree well with those measured with a condensation particle counter. The response time is short enough to measure transient engine operation. The DiSC is therefore a useful instrument for number concentration measurements in non-laboratory settings.
Particle size measurements using the electrical low pressure impactor (ELPI) and scanning mobility particle sizer (SMPS) are compared from the perspective of characterizing the particulate matter in motor vehicle exhaust. Both steady state vehicle operation and transient drive cycles are considered, and both gasoline and diesel fueled vehicle emissions are compared. Although the ELPI and SMPS measure different physical properties, respectively, the aerodynamic diameter and mobility diameter, the steady state particle size distributions are in close agreement, except for the 37 nm impactor stage of the ELPI which may overestimate particle number by up to a factor of two relative to the SMPS. This has little effect on the volume, or mass, weighted distribution. These, too, are generally in good agreement, though discrepancies appear at large particle size due to multiple charging effects in the SMPS and to electrometer offsets and the small particle loss correction in the ELPI. Selecting particles based on their electrical mobility with the SMPS, and then measuring their aerodynamic diameter with the ELPI, reveals that diesel particulate matter with well-specified mobility diameter exhibits a wide range in aerodynamic diameter and, therefore, also in effective density. Over transient drive cycles, the ELPI provides second by second particle distributions, whereas the SMPS must be run in a fixed particle size mode and size distributions constructed from repeated tests. The ELPI registers higher instantaneous PM emission rates during transients than the SMPS due to the faster time responses of the ELPI. The time integrated ELPI and SMPS size distributions, however, remain in good agreement. The relative merits of the two instruments for steady state and transient tests are discussed.
Response functions of the ELPI impactor with normal and porous substrates are presented together with fit functions to describe particle collection. In addition to primary collection efficiency, fits for the secondary collection mechanisms, diffusion, image charge force, and space charge field are presented. Charging efficiency for different configurations is also presented. Presented response functions can be used in data reduction and inversion of ELPI data.
A novel electrical-mobility-based technique to measure total particle number concentration over a selected size range is presented. Charged particles are condensed out onto an electrode that is shaped such that the product of its transfer function and the particle charging efficiency is a constant, independent of particle size. The resulting total current is then proportional to the number concentration of the sampled particles over the collected size range. The theoretical approach for the calculation of the electrode shape function is described. The extension of this technique for measurement of higher moments of the particle size distributions over a desired size range is briefly discussed.This concept is used to design a new instrument, called the tailored electrode concentration sensor (TECS). For validation of the theoretical concept, the collection electrode in the TECS instrument is designed for concentration measurements over a size range of 30–90nm. In the TECS, the collection section is located downstream of an electrostatic precipitator section, where the sampled flow is split into aerosol and sheath flows, similar to the design of the MEAS [Ranjan, M., & Dhaniyala, S., (2007), Theory and design of a new miniature electrical-mobility aerosol spectrometer, Journal of Aerosol Science, 38(9), 950–963]. This results in a compact, low pressure drop instrument. Experimental results confirm that the response of the optimally-shaped electrode in the TECS system is only proportional to total number concentration over the selected size range.
An on-line method is presented for simultaneous size distribution and particle density measurement, based on parallel measurements made by SMPS and ELPI. The measured SMPS number distribution is integrated with the ELPI response functions to produce calculated current response. By varying the density value, the best fit is sought between the calculated and measured current response. Simulation tests made showed relatively good stability against small uncertainties in the distribution and the response functions. Test measurements of well-defined aerosols with known density were conducted. Density values of 0.86, 1.1, and 1.9g/cm3 were measured for liquid particles of DOS, Santovac vacuum oil, and Fomblin vacuum oil, respectively. These values are within 8% of the accepted bulk values. For solid particles of NaCl, Zn, and Ag, slightly larger experimental errors in the range of 4–18.2% were found.
Many theories have been offered in attempts to describe accurately the rate of charge accumulation of fine particles in a unipolar ion field. There has been little experimental data, however, for comparison with these new theories. In this study, the existing experimental data are reviewed and compiled, and additional particle charging experiments done to extend the quantity of data. Before this study was performed the majority of the experimental data which was available for comparison with particle charging theories was contained in a paper by G. Hewitt, published in 1957. Hewitt's experiments were carefully done, but the range of experimental variables was limited and his work was never independently verified. The experiments reported here followed Hewitt's procedures closely and were designed to extend the quantity of experimental data. There is some overlap between this and Hewitt's data, and in these instances, the agreement is good. Experimental results are presented in this paper for the charging rate of particles of 0.109–7 μm dia. at various values of electric field strength, ion density and charging time. A limited amount of data is also given which shows the important effects on the charging rate of variations in the particle dielectric constant and the polarity of the corona ion source.
Calculations of the ion—aerosol attachment coefficients are carried out for Fuchs' theory (as corrected in this paper) and for a theory which includes three body trapping. The resulting charge distributions agree quite well for particles with radii greater than about 0.007 μm. For smaller particles three-body trapping becomes increasingly important. Comparison of theoretically predicted charge distributions with recently measured charge distributions at radii smaller than 0.02 μm show good agreement. Asymmetric charging due to differences in the physical properties of positive and negative ions can result in large differences in the number of positively and negatively charged particles, particularly at larger radii. The asymmetric charge distribution is also shown to depend on the ionization rate. For the case when aerosol concentrations are comparable to the ion concentrations the effect of polydispersity on the charge distribution is difficult to predict. It is shown that a dominant size particle can establish a positive to negative ion ratio which, in turn, will determine a charge distribution at other sizes, different from that which would exist in the absence of the dominant species.
The first part of this article reports the analytical form of the Electrical Low Pressure Impactor (ELPI) kernel functions. In this latter part, the numerical quality of ELPI response matrices is studied and an example of an inversion algorithm is given. The ELPI assemblies with and without an electrical filter stage and with smooth or sintered impaction plates are studied and compared with basic impactor kernels and the kernels for the calculation of the aerosol mass distribution. It is shown that the ELPI assembly with the electrical filter stage and smooth impaction plates should be the best choice for the inversion of data if no bounce occurs. The comparison to a mass impactor shows that the devices are on par in data inversion. The inversion ELPI data is studied with a Bayesian algorithm assuming a bimodal lognormal size distribution of the aerosol. The algorithm includes a novel procedure for obtaining an initial guess of the distribution parameters. To our knowledge, it is also the first algorithm to use ELPI current readings as its input. Simulations and diesel emission measurements show that the proposed algorithm is a useful tool in the study of ELPI data.
The emissions of particles, and gaseous compounds, into the ambient air from biomass-fired moving grate boilers were characterized under different boiler operation conditions. The boilers had a thermal capacity of 1 MW. The flue gas cleaning systems consisted of multicyclones for the removal of coarse particles. Dry wood fuel that consisted of shavings, wood chips, and sawdust from a local wood industry and wood pellets were fired at two plants. The influence of boiler load on the emissions was characterized. An electrical low-pressure impactor (ELPI) was used to determine the particle number concentration with high time resolution. A low-pressure cascade impactor (LPI) was utilized for the mass size distribution and the size-differentiated chemical composition. Elemental analysis of the fly ash collected on impactor substrates was made by particle-induced X-ray emission (PIXE) analysis. The concentration of elemental carbon under different load conditions was also measured. In addition, emissions of polycyclic aromatic hydrocarbons (PAHs) from the boiler that was operating on dry wood fuel were compared with PAH emissions from two different biomass-fired boilers (one was operating on forest residues and the other on pellets). The boiler load had little influence on the particle mass concentration of submicrometer-sized particles, which was in the range of 50−75 mg/m3 (0 °C, 101.3 kPa, dry gas, 13% CO2). The total particle number concentration increased and the particle size decreased as the boiler load increased. The elemental analysis revealed that potassium and sulfur were the dominating components in the submicrometer size range, whereas potassium and calcium were major components in the coarse fraction. The PAH emissions between the three boilers varied by almost 3 orders of magnitude.
The electrical charging of aerosol particles by unipolar gaseous ions was studied theoretically and experimentally. The primary objective of the study was to make precise determinations of the aerosol particle charge under various conditions of charging and to compare the experimental results with those predicted by theory. Experiments were performed using monodisperse oleic acid aerosols generated by a vibrating orifice generator, monodisperse NaCl and DOP (di-octyl phthalate) aerosols generated by an electrostatic classification method, and sulfuric acid aerosols generated by the photo-chemical conversion of gaseous SO/sub 2/ in the smog chamber. The experiments covered a particle size range of 0.0075 ..mu..m to 5.04 ..mu..m diameter. The corresponding range in Knudsen number (Kn = lambda/sub i//a, lambda/sub i/ = mean free path of ions, a = particle radius) was from 0.0056 to 3.86. The charging parameter, n/sub 0/t, was varied between 2.56 x 10/sup 6/ to 5.1 x 10/sup 7/ ion-s/cc, where n/sub 0/ is the concentration of ions and t is the charging time. Comparisons of the results with available aerosol charging theories indicate that there is reasonable agreement between the theory and experiment in the continuum regime (Kn > 1) and the transition regimes (Kn approx. or equal to 1), where the ion mean free path is no longer small in comparison with particle size, there is considerable discrepancy between the experimental data and available charging theories. A semi-empirical equation was developed which agrees well with the experimental data over the entire range of particle size and charging parameters covered in the experiments. Theoretical results are presented showing the distribution of charges on the particles as a function of particle size and the charging parameter n/sub 0/t. 70 figures, 24 tables, 112 references. (auth)
There has been increased interest in obtaining size distribution data during transient engine operation where both particle size and total number concentrations can change dramatically. Traditionally, the measurement of particle emissions from vehicles has been a compromise based on choosing between the conflicting needs of high time resolution or high particle size resolution for a particular measurement. Currently the most common technique for measuring submicrometer particle sizes is the Scanning Mobility Particle Sizer (SMPSTM) system. The SMPS system gives high size resolution but requires an aerosol to be stable over a long time period to make a particle size distribution measurement. A Condensation Particle Counter (CPC) is commonly used for fast time response measurements but is limited to measuring total concentration only. This paper describes a new instrument, the Engine Exhaust Particle SizerTM (EEPSTM) spectrometer, which has high time resolution and a reasonable size resolution. The EEPS was designed specifically for measuring engine exhaust and, like the SMPS system, uses a measurement based on electrical mobility. Particles entering the instrument are charged to a predictable level, then passed through an annular space where they are repelled outward by the voltage from a central column. When the particles reach electrodes on the outer cylindrical (a column of rings), they create a current that is measured by an electrometer on one or more of the rings. The electrometer currents are measured multiple times per second to give high time resolution. A sophisticated realtime inversion algorithm converts the currents to particle size and concentration for immediate display.
We investigated the effect of particle pre-existing charges on unipolar charging. Particles carrying a defined number and polarity of pre-existing charges were used to study the unipolar charging process in a unipolar diffusion charger with positive ions. It was found that the particles initially carrying negative charges have almost the same amount of positive charges as the initially uncharged particles after passing the test charger; and the particles initially carrying more positive charges have more final charges. An analytical solution of a model for particle charge distribution of initially charged particles was provided for unipolar charging based on Fuchs' theory and the birth-and-death theory. The N ion t value used in this model was obtained by fitting the experimental data of average charge on particles for initially uncharged particles. The results from the analytical solution show very good agreements with experimental data regarding the relationship between the pre-existing charge and the final charge on particles (50–200 nm in this study). Experimental tests of the response of Nanoparticle Surface Area Monitor (NSAM) against initially charged particles demonstrated that NSAM could have a large response deviation (more than 20% in the tested charge level) depending on the particle size and the amount of pre-existing positive charges on particles. Modeling of NSAM response showed similar deviation and predicted that when pre-existing charge is high enough, the NSAM response can be as large as 5 and 9 times of the uncharged particle response for alveolar and tracheobronchial surface area concentration, respectively.