Tandem Differential Mobility Analyzer Technique Research Articles

Derivation of particle-size changes from polydisperse size distribution measurements: numerical and experimental verification

Measurements of particle-size transformations due to changes in temperature, relative humidity, and other environmental factors are essential for understanding aerosol properties, such as its hygroscopicity and volatility. The commonly used measurement techniques, such as tandem differential mobility analyzer (TDMA) and single-particle levitation techniques, allow measurements of only one initial size at a time, which makes their applications to highly dynamic aerosol systems problematic. In this paper, we evaluate the Heisler method developed in the second half of 1970s that uses measurements of size distributions of a polydisperse aerosol before and after it undergoes a growth or evaporation. The Heisler method is based on preservation of particle number concentration and allows simultaneous determination of particle-size changes across the whole measured size distribution. In this paper, we evaluated the Heisler method using numerical techniques and compared it with TDMA measurements during experiments with adipic acid aerosol. We show that the Heisler method is sensitive to counting statistics and that significant errors (over 50%) can occur if there are particles that can grow or evaporate in or out of the measured size range. Otherwise, the Heisler method gives results within 5% of those obtained with TDMA measurements, providing a faster alternative to the TDMA technique.

Aerosol Properties Observed in the Subtropical North Pacific Boundary Layer

AbstractThe impact of anthropogenic aerosol on climate forcing remains uncertain largely due to inadequate representation of natural aerosols in climate models. The marine boundary layer (MBL) might serve as a model location to study natural aerosol processes. Yet source and sink mechanisms controlling the MBL aerosol number, size distribution, chemical composition, and hygroscopic properties remain poorly constrained. Here aerosol size distribution and water uptake measurements were made aboard the R/V Hi'ialakai from 27 June to 3 July 2016 in the subtropical North Pacific Ocean. Size distributions were predominantly bimodal with an average integrated number concentration of 197 ± 98 cm−3. Hygroscopic growth factors were measured using the tandem differential mobility analyzer technique for dry 48, 96, and 144 nm particles. Mode kappa values for these were 0.57 ± 0.12, 0.51 ± 0.09, and 0.52 ± 0.08, respectively. To better understand remote MBL aerosol sources, a new algorithm was developed which decomposes hygroscopicity distributions into three classes: carbon‐containing particles, sulfate‐like particles, and sodium‐containing particles. Results from this algorithm showed low and steady sodium‐containing particle concentrations while the sulfate‐like and carbon‐containing particle concentrations varied during the cruise. According to the classification scheme, carbon‐containing particles contributed at least 3–7%, sulfate‐like particles contributed at most 77–88% and sodium‐containing particles at least contributed 9–16% to the total aerosol number concentration. Size distribution and hygroscopicity data, in conjunction with air mass back trajectory analysis, suggested that the aerosol budget in the subtropical North Pacific MBL may be controlled by aerosol entrainment from the free troposphere.

Experimental evidence for the sintering of primary soot particles

Sintering behavior and the contribution of soot produced in the absence of oxygen are experimentally investigated. A tandem differential mobility analyzer technique equipped with thermophoretic sampling was used to reveal the change in the mobility diameter of soot aggregates, which is proportional to the surface area. The mobility diameter decreased with an increase in reheating temperature. The decrease in mobility diameter clearly shows a decrease in the aggregate surface area. SEM images also clearly showed the decrease in the surface area and the simplification after heat treatment. These are experimental evidence that sintering of carbonaceous particle occurs in the absence of oxygen. Acetylene, which is considered to promote surface growth, is also added to size-selected soot particles before heat treatment to investigate the effect of surface growth on simplification of aggregate structure. The addition of acetylene, which is relevant to the surface growth via the hydrogen-abstraction/carbon-addition (HACA) mechanism, did not affect the mean mobility diameter at low temperature around 1200K. These results showed that simplification of soot aggregates would not be affected by surface growth caused via the HACA mechanism.

Determination of the Mean Mobility of Aerosol Nanoparticles Classified by Differential Mobility Analyzers

MonteCarlo simulations of diffusive particle trajectories, as well as Stolzenburg's model calculations, have shown that the mean mobility of the particles classified by a differential mobility analyzer (DMA) at a given applied voltage may differ from the theoretical one inferred from the Knutson–Whitby equation if the particles are withdrawn from the tails of the particle mobility distribution. In this case, the true mean mobility, defined as the mean mobility of the particles classified at the specified voltage, can be precisely measured by a second DMA operating in series with the first one (tandem DMA). However, if particles are extracted from the central part of the distribution, their mobility can be correctly measured with a single DMA. Besides showing the importance of the usage of the tandem DMA technique for accurate measurements of mobility, this article provides an analytical expression which, if the mobility distribution of the polydisperse aerosol fed to the DMA is known, allows an accurate estimation of the true (mean) mobility of the classified particles.Copyright 2014 American Association for Aerosol Research

Evaluation of the performance of a soft X-ray charger for the bipolar charging of nanoparticles

The use of soft X-rays in a neutralizer represents an alternative technique that could replace conventional radioactive sources. In this study, we evaluated the charging characteristics of a soft X-ray aerosol neutralizer. In addition, the results from the evaluation of the soft X-ray charger were compared with results obtained using a neutralizer incorporating an 241Am radioactive source. The tandem differential mobility analyzer technique was used previously to determine the size-dependent positive, negative, and neutral charge fractions of a soft X-ray neutralizer. This technique was used to show that the neutral fractions obtained using the soft X-ray charger agreed well with the predictions of bipolar diffusion charging theory, and that the soft X-ray charger could be used as a neutralizer for a scanning mobility particle sizer system.

Accurate Determination of Aerosol Activity Coefficients at Relative Humidities up to 99% Using the Hygroscopicity Tandem Differential Mobility Analyzer Technique

Aerosol water content plays an important role in aqueous phase reactions, in controlling visibility, and in cloud formation processes. One way to quantify aerosol water content is to measure hygroscopic growth using the hygroscopicity tandem differential mobility analyzer (HTDMA) technique. However, the HTDMA technique becomes less reliable at relative humidity (RH) >90% due to the difficulty of controlling temperature and RH inside the second DMA. For this study, we have designed and implemented a new HTDMA system with improved temperature and RH control. Temperature stability in the second DMA was achieved to ±0.02°C tolerance by implementing active control using thermoelectric heat exchangers and PID control loops. The DMA size resolution was increased by operating high-flow DMA columns at a sheath:sample flow ratio of 15:0.5. This improved size resolution allowed for improving the accuracy of the RH sensors by interspersing ammonium sulfate reference scans at high frequency. We present growth factor d...

Production of Residue-Free Nanoparticles by Atomization of Aqueous Solutions

It has been difficult to produce physically and chemically pure nanoparticles from a particle solution in deionized (DI) water by atomization due to residue from impurities in the DI water. In this study, we show that further purification of DI water by reverse osmosis (RO) membrane filtration under high pressure (900 psi) substantially reduces solvent impurities when submicrometer and nanosized particles are atomized. Comparison of size distributions among several types of DI water showed that the production of residual particles significantly decreased by a factor of ∼11 (by number) after the RO filtration of regular DI water from commercially available DI water. Mixing states of aerosolized particles produced from particle solutions were examined by using the hygroscopicity tandem differential mobility analyzer technique and transmission electron microscopy/energy dispersive spectroscopy. We found that residual particles were externally mixed with particles of interest during the aerosolization and drying process and that impurities in DI water can beadded onto the target particles (i.e., internally mixed). By removing impurities in DI water by the RO filtration, the effect of impurities for the production of clean nanoparticles was significantly reduced, suggesting that the atomization method can still be a promising tool to produce pure nanoparticles in a simple and inexpensive way. Copyright 2012 American Association for Aerosol Research

Enhancement of Extrinsic Charging Efficiency of a Nanoparticle Charger with Multiple Discharging Wires

A unipolar charger with multiple discharging wires has been developed and investigated to enhance the extrinsic charging efficiency of nanoparticles by using sheath air near the wall of the charger. The applied voltage of the charger ranged from +4.0 to +10 kV, corresponding to corona current from 0.02 to 119.63 μA. Monodisperse NaCl particles of 10 ∼ 50 nm and Ag particles of 2.5 ∼ 10 nm in diameter were produced to test the performance of the charger with multiple discharging wires and to investigate the particle loss at different sheath flow rates, corona voltages and sheath air velocities. Results showed that the optimal efficiency in the charger was obtained at +9 kV applied voltage, 10 L/min aerosol flow rate and 20 L/min sheath air flow rate. The extrinsic charging efficiency increased from 2.86% to 86.3% in the charger as the particle diameter increasing from 2.5 to 50 nm. The TDMA (tandem-differential mobility analyzer) technique was used to investigate the charge distribution, and the charge dis...

Roadside aerosol study using hygroscopic, organic and volatility TDMAs: Characterization and mixing state

Traffic-related aerosol particles are ubiquitous in the urban atmosphere. As they are produced at ground level, they can also cause adverse health effects to urban dwellers. However, knowledge of the formation, transformation and chemically resolved size distribution of urban ultrafine particles is incomplete. Thus, more of these measurements are needed for better assessment of ambient air quality and its potential health effects. The particle number concentration, aerosol black carbon (BC) concentration and size distribution of traffic-related aerosols were measured near two major roads in Kuopio, Finland, from 16 June to 5 July, 2004. Furthermore, the properties of roadside aerosol particles were examined with the Tandem Differential Mobility Analyzer technique (TDMA). A suite of TDMA instruments relying on water (hygroscopic TDMA) and ethanol (organic TDMA) condensation as well as heating (volatility TDMA) were deployed to study the composition of the nucleation and Aitken mode particles ( D p = 10–50 nm) formed from vehicle exhaust. The results show that a simple three-component model was able to reproduce characteristic insoluble, organic and water-soluble volume fractions. Insoluble constituents were dominant in the Aitken mode particles, whereas organic compounds dominated the nucleation mode sizes. On average, only a small volume fraction was water-soluble, but a clear external mixing was observed particularly when enough time was allowed after the tail pipe emissions. The contribution of the insoluble material was seen to increase as a function of particle size, being typically less than 10% at 10 nm and between 20 and 50% at 50 nm, in contrast to the organic fraction, which decreased from about 80% at nucleation mode size range to 50–60% at 50 nm.

Hygroscopic behavior of partially volatilized coastal marine aerosols using the volatilization and humidification tandem differential mobility analyzer technique

Coastal marine nucleation, Aitken, and accumulation mode aerosol particles with back trajectories indicative of marine origin were examined using a volatilization and humidification tandem differential mobility analyzer (VHTDMA) to reveal the volatilization temperatures of the various component species. The diameter hygroscopic growth factors of the residue particles were continually examined throughout the volatilization process. In each of the three modes the dominant particle type appeared to be composed of the same four physicochemically distinct species though in different ratios. These species exhibited volatility and hygroscopic behavior consistent with combinations of a volatile organic species, sulfuric acid, ammonium sulfate or bisulfate, iodine oxide, and an insoluble nonvolatile residue. The Aitken and accumulation mode aerosols contain large fractions of the insoluble, volatile, organic‐like material, and the volatilization of this species results in a distinct increase in the water volume uptake of the particles. Of the four distinct species, only the sulfuric acid‐like species constituted an increasing volume fraction with decreasing particle size. This finding indicates that nucleation mode particles constitute an acidic seed and as such would undergo acid‐catalyzed secondary organic aerosol growth at a faster rate. The lack of a nonvolatile hygroscopic residue consistent with sea salt in these particles implies that the aerosolization of seawater is not the dominant production mechanism for these submicrometer coastal marine aerosols.

Time-resolved growth behavior of acid aerosols in ethanol vapor with a tandem-DMA technique

We have studied the growth behavior of citric acid, tartaric acid, adipic acid and ammonium bisulfate particles in ethanol vapor with an organic tandem differential mobility analyzer technique (OTDMA). Time dependence of the change in the particle size due to interaction with ethanol vapor was determined. The results show that adipic acid particles have normal deliquescence-hysteresis type of growth behavior, whereas the growth of the other particle types is strongly time dependent due to chemical reactions with timescales on the order of seconds. Citric acid and tartaric acid obviously undergo auto-protonation catalyzed esterification due to their favorable molecular structure, whereas adipic acid has unfavorable structure for intramolecular catalysis. As a rather strong acid, ammonium bisulfate can transfer a proton to ethanol and hence a sulfate esterification reaction takes place. We suggest that time dependent growth behavior can be used for detection of acidic species in atmospheric aerosols.

Characterization of diesel particles: effects of fuel reformulation, exhaust aftertreatment, and engine operation on particle carbon composition and volatility.

Diesel exhaust particles are the major constituent of urban carbonaceous aerosol being linked to a large range of adverse environmental and health effects. In this work, the effects of fuel reformulation, oxidation catalyst, engine type, and engine operation parameters on diesel particle emission characteristics were investigated. Particle emissions from an indirect injection (IDI) and a direct injection (DI) engine car operating under steady-state conditions with a reformulated low-sulfur, low-aromatic fuel and a standard-grade fuel were analyzed. Organic (OC) and elemental (EC) carbon fractions of the particles were quantified by a thermal-optical transmission analysis method and particle size distributions measured with a scanning mobility particle sizer (SMPS). The particle volatility characteristics were studied with a configuration that consisted of a thermal desorption unit and an SMPS. In addition, the volatility of size-selected particles was determined with a tandem differential mobility analyzer technique. The reformulated fuel was found to produce 10-40% less particulate carbon mass compared to the standard fuel. On the basis of the carbon analysis, the organic carbon contributed 27-61% to the carbon mass of the IDI engine particle emissions, depending on the fuel and engine operation parameters. The fuel reformulation reduced the particulate organic carbon emissions by 10-55%. In the particles of the DI engine, the organic carbon contributed 14-26% to the total carbon emissions, the advanced engine technology, and the oxidation catalyst, thus reducing the OC/EC ratio of particles considerably. A relatively good consistency between the particulate organic fraction quantified with the thermal optical method and the volatile fraction measured with the thermal desorption unit and SMPS was found.

Even−Odd Alternation of Evaporation Rates and Vapor Pressures of C3−C9 Dicarboxylic Acid Aerosols

Aliphatic straight-chain dicarboxylic acids have been identified as common water-soluble organic components of atmospheric aerosols. To model the partitioning of such compounds between gas and particle phase in the atmosphere, information about their vapor pressures is essential. In this work, vapor pressures of C3−C9 dicarboxylic acids are derived from measured evaporation rates of submicron aerosols over the temperature range of 290−314 K using the tandem differential mobility analyzer technique. Vapor pressures obtained from the experimental data were as follows: log( , Pa) = − 4822 K/T + 12.9, log( , Pa) = − 7196.8 K/T + 19.8, (296 K) = 6.7 × 10-4 Pa, log( , Pa) = −8065.0 K/T + 22.2, log( , Pa) = −7692.8 K/T + 21.8, log( , Pa) = −9629.4 K/T + 26.5, and log( , Pa) = −7968.7 K/T + 21.7. Vapor pressures of C3−C9 dicarboxylic acids are shown to alternate strongly with the parity of the number of carbon atoms. Higher vapor pressures of the odd acids fit the less stable crystal structure, the propensity of...

A differential mobility analyzer and a Faraday cup electrometer for operation at 200–930 Pa pressure

We have developed a differential mobility analyzer (DMA) based on the DMA devised by Seto et al. (1997) and a Faraday cup electrometer for measurement of nanometer-sized particles at a few hundred Pa and examined the operating characteristics of the DMA using the tandem DMA technique. The tandem DMA calibration establishes that the DMA successfully classifies particles in the 200–930 Pa pressure range. It was also found that the transfer function of the DMA follows the triangular transfer function and the resolution of the DMA is close to that given for an ideal case. As a standard of a minimum pressure that may be probed with the present DMA system, 400 Pa is estimated when the DMA operates with a 3 nl min −1 sheath flow and a 1 nl min −1 aerosol flow rate.

Growth of monodisperse, submicron aerosol particles exposed to SO2, H2O2, and NH3

The growth of monodisperse particles (0.07 to 0.5 µm) exposed to SO2 (0–860 ppb), H2O2 (0–150 ppb) and sometimes NH3 (0–550 ppb) in purified air at 22 °C at relative humidities ranging from 25 to 75% were measured using the Tandem Differential Mobility Analyzer technique. The experiments were performed in a flow reactor with aqueous (NH4)2SO4 and Na2SO4 droplets. For (NH4)2SO4 droplets the fractional diameter growth was independent of size above 0.3 µm but decreased with decreasing size below that. When NH3 was added the fractional growth increased with decreasing size. Measurements were compared with predictions of a model that accounts for solubility of the reactive gases, the liquid phase oxidation of SO2 by H2O2, and ionic equilibria. Agreement between measured and predicted droplet growth is reasonable when the ionic strength effects are included. Theory and experiments suggest that NH3 evaporation is responsible for the decrease in relative growth rates for small aqueous ammonium sulfate particles. The observed droplet growth rates are too slow to explain observed growth rates of secondary atmospheric sulfate particles.