Role of oleic acid coating in the heterogeneous uptake of dimethylamine by ammonium sulfate particles

Reactive uptake by ammonium (NH₄⁺) salts is one of the major pathways for the gas-to-particle partitioning of alkyl amines. Recent studies using particles of individual ammonium salts and mixtures with hydrophilic organics have revealed that the degree of amine uptake depends on the phase state of ammonium salts, the particulate water contents and particle viscosity. The role of hydrophobic organic compounds, another important category of particulate organics commonly detected in the ambient atmosphere, in amine uptake remains unknown. Here we report the uptake of dimethylamine (DMA) by ammonium sulfate (AS) particles coated with fresh or ozone-aged bulk oleic acid (OA) at 60%, 30%, and <5% relative humidities (RHs) using an electrodynamic balance coupled with Raman spectroscopy. OA and DMA were selected to represent hydrophobic organics and alkyl amines, respectively. Over 74% of the original NH₄⁺ ions were displaced due to DMA uptake, except those conditioned at <5% RH. On the other hand, the fresh or aged bulk OA coating retarded DMA uptake by preventing the particle surface from effectively accommodating gaseous DMA molecules. Judging from the estimated DMA uptake coefficients, the retardation gradually intensified as the weight percentage of coating increased before leveling off, likely when the particle surface was completely covered by fresh or aged bulk OA. We propose that the accommodation of DMA on the particle coating is the rate-limiting step of DMA uptake. Intensive aging of the OA coating had little effect on the equilibrium particle-phase compositions but retarded DMA uptake.

© 2017 American Association for Aerosol Research     

Thermodynamic and kinetic behavior of glycerol aerosol

Glycerol and propylene glycol mixtures are common carrier solutions in electronic cigarettes. Aerosols produced from these mixtures will evaporate quickly in a dry environment due to their high volatility. In a humid environment, such as the lungs, the kinetics of evaporation and hygroscopic growth determine the evolution of aerosol plume glycerol. Here, we apply a temperature and relative humidity-controlled hygroscopicity/volatility tandem differential mobility analyzer system to study the growth and evaporation kinetics of glycerol aerosol over a wide range of temperature, relative humidity, and residence times. Results show that at dry conditions glycerol aerosols evaporate within seconds at temperatures warmer than 20°C and that the accommodation coefficient of glycerol vapor on dry glycerol particles is 0.8. Under humidified conditions, the mutual depression of vapor pressures of the aqueous glycerol/water solution slows the glycerol evaporation rate consistent with thermodynamic and kinetic model predictions. Model calculations show that water vapor aided condensation of glycerol can occur at high relative humidity for glycerol vapor concentrations that result in glycerol particle evaporation under dry conditions. The combined results will help with constraining computational modules that model the evolution of glycerol-containing aerosols along a prescribed thermodynamic trajectory.

Copyright © 2016 American Association for Aerosol Research     

Instant acquisition of high resolution mobility spectra in a differential mobility analyzer with 100 independent ion collectors: Instrument calibration

Abstract

A parallel plate differential mobility analyzer (DMA) having 100 independent current collectors is calibrated to relate the axial distances L_n between the inlet slit and the detector position to the particle mobility Z at given voltage difference V and sheath gas flow rate Q. Calibrating species are tetraheptylammonium bromide clusters (THABr) and polyethylene glycol (PEG35k, 5?nm in diameter), generated by a bipolar electrospray source, and purified in a cylindrical DMA. Gaussian fitting of the raw discrete mobility spectra in the form of ion current I_n versus collector position L_n , I_n (L_n ), yield the mean value L_o of the collector position maximizing the signal for a given ion. The many (Z,V,L_o ) triads obtained at given Q from many different DMA voltages and standard mobilities collapse into a single 1/(Z_iV_j ) vs L_o curve when slight adjustments are made to the Z_i . For different flow rates, Q/(Z_iV_j ) vs. L_o curves collapse also, as long as the peaks are moderately narrow. However, for sufficiently small Q/Z, the THABr cluster peaks become broad, and the curves Q/(Z_iV_j ) vs. L_o cease to collapse precisely. In contrast, the data for PEG show that this behavior is not a low-Q (Reynolds number) effect from the growth of the two lateral boundary layers, but is rather due to the broad and non-Gaussian peak shapes obtained at low Q or high Z. The calibration is accordingly unaffected by the Reynolds number. This simplicity was unexpected, given the three-dimensional flow in this DMA with growing lateral boundary layers.

Copyright © 2020 American Association for Aerosol Research     

Experimental study of oil particle emission rate and size distribution during milling

Metalworking fluids (MWFs) used in milling generate oil particles through impaction, action of centrifugal forces and evaporation/condensation mechanisms. The oil particles suspended in the factory atmosphere can affect the health of the labor force. In order to study the emission properties of these oil particles, this work investigates the oil particle emission rate and size distribution during milling using an environmental chamber method. Two commonly used operating modes for MWFs were selected, the minimum quantity lubrication (MQL) mode (40?ml/h) and the cooling mode (1 m³/h). The cooling mode without cutting was studied separately for comparison with the cooling mode with cutting. The results show that the oil particle emission rate in milling ranges from 7.2 to 641?mg/h, and the size distribution ranges from 0.265 to 12.5?µm. Evaporation/condensation is the main mechanism in the MQL mode. The majority of oil particles formed by evaporation/condensation are in the range of 0.265 to 1.8?µm. As the tool rotation speed increases, the particle emission rate increases, while the mass mean diameter (MMD) and the sauter mean diameter (SMD) decrease. Oil particles are mainly generated by the action of centrifugal force in the cooling mode, and mainly distributed in the range of 1.8 to 12.5?µm. The particle emission rate increases with the tool rotation speed, and the particle MMD and SMD increase with the tool rotation speed only in the cooling mode without cutting. The particle emission rate ranging from 1.8 to 12.5?µm, as well as PM5 and PM10, are a polynomial function of the square of tool rotation speed during the cooling mode. The coefficient of determination (R²) is above 0.99.

© 2018 American Association for Aerosol Research     

Numerical modeling of the performance of high flow DMAs to classify sub-2 nm particles

While there are several computational studies on differential mobility analyzers (DMA), there is none for high flow DMA to classify nanoparticles less than 3?nm. A specific design of a high flow DMA, a half mini DMA, is investigated to predict its performance through numerical modeling in the incompressible flow regime. The governing equations for flow field, electric field and aerosol transport are solved using COMSOL 5.3. The transfer function of the half mini DMA is compared with that of a nano DMA (TSI 3085). The results show that both the height of the transfer function and resolution (R) of the half mini DMA are much better than those of nano DMA in sub-2?nm particle size range. Finally, the transfer function of half mini DMA is evaluated for different values of aerosol flow rate to the sheath flow rate (q/Q). Comparison of the simulated transfer function with existing models from Knutson–Whitby and Stolzenburg is also elucidated. It is found that the former model overestimates the resolution; whereas the latter is close to the simulation results for q/Q above 0.067. This work provides a useful method to study the flow regimes and transfer function of a high flow DMA.

Copyright © 2018 American Association for Aerosol Research     

Modification of the TSI 3081 differential mobility analyzer to include three monodisperse outlets: Comparison between experimental and theoretical performance

Differential mobility analyzers (DMAs) are widely used to determine the size of aerosol particles, and to probe their size-dependent physicochemical properties when two are employed in tandem. A limitation of tandem DMA (TDMA) systems is their long measuring cycle when the properties of more than one monodisperse population of particles need to be probed. In this work, we propose a simple modification of the classical cylindrical DMA by including three monodisperse-particle outlets in its central electrode (namely, the 3MO-DMA), with the objective of using it as the first DMA in TDMA systems for reducing their measuring cycle. The performance of the 3MO-DMA at different flow conditions was evaluated using laboratory-generated aerosol particles, and compared with theoretical predictions. The theory predicted accurately (i.e., within 3%) the geometric mean diameters of the three distinct populations, as well as the resolutions of the first and the third outlet, under all experimental conditions. For the second outlet, the resolution was 10% to 74% lower than that predicted theoretically depending on the sheath-to-aerosol flow ratio. Nevertheless, the geometric standard deviation of the monodisperse aerosol from all the outlets was less than 1.09, which is sufficient for using the 3MO-DMA designed and tested in this work as a first DMA to produce a monodisperse aerosol flow containing three distinct particle populations in TDMA systems.

Copyright © 2016 American Association for Aerosol Research     

New approaches to calculate the transfer function of particle mass analyzers

Abstract

This article provides an overview of methods to evaluate transfer functions for the Couette centrifugal particle mass analyzer (CPMA) and aerosol particle mass analyzer (APM). The work first considers finite difference approaches to solving the partial differential equation governing particle motion, which represents an accurate but computationally-demanding approach to evaluating the transfer function. This is used as a baseline to compare to particle tracking methods, which have been shown to yield closed form expressions for the transfer function. In this work, we extend on previous treatments by presenting a generalized framework that allows us to consider a range of representations of the particle migration velocity. As a result, we derive new closed form expressions for the exact representation of the particle migration velocity under APM conditions and provide significant improvements in the accuracy of the transfer function for CPMA conditions. In the latter case, for a CPMA, particle migration effects dominate, which makes the transfer function easier to approximate. We also show that Taylor series approximations to the particle migration velocity should be taken about the centerline radius rather than the equilibrium radius as was done previously. We end by extending the particle tracking approach and derive new closed form expressions for the transfer function that include diffusion.

Copyright © 2019 American Association for Aerosol Research     

Design and evaluation of an aerodynamic focusing micro-well aerosol collector

Aerosol sampling and identification is vital for the assessment and control of particulate matter pollution, airborne pathogens, allergens, and toxins and their effect on air quality, human health, and climate change. In situ analysis of chemical and biological airborne components of aerosols on a conventional filter is challenging due to dilute samples in a large collection region. We present the design and evaluation of a micro-well (µ-well) aerosol collector for the assessment of airborne particulate matter (PM) in the 0.5–3 µm size range. The design minimizes particle collection areas allowing for in situ optical analysis and provides an increased limit of detection for liquid-based assays due to the high concentrations of analytes in the elution/analysis volume. The design of the collector is guided by computational fluid dynamics (CFD) modeling; it combines an aerodynamic concentrator inlet that focuses the aspirated aerosol into a narrow beam and a µ-well collector that limits the particle collection area to the µ-well volume. The optimization of the collector geometry and the operational conditions result in high concentrations of collected PM in the submillimeter region inside the µ-well. Collection efficiency experiments are performed in the aerosol chamber using fluorescent polystyrene microspheres to determine the performance of the collector as a function of particle size and sampling flow rate. The collector has the maximum collection efficiency of about 75% for 1 µm particles for the flow rate of 1 slpm. Particles bigger than 1 µm have lower collection efficiencies because of particle bounce and particle loss in the aerodynamic focusing inlet. Collected samples can be eluted from the device using standard pipettes, with an elution volume of 10–20 µL. The transparent collection substrate and the distinct collection region, independent of particle size, allows for in situ optical analysis of the collected PM.

© 2017 American Association for Aerosol Research     

Effects of exponentially decaying and growing concentrations on particle size distribution from a scanning mobility particle sizer

Abstract

A scanning mobility particle sizer (SMPS) is one of the most widely used instruments to obtain size distribution for atmospheric particles. In an SMPS measurement, a voltage scanning process on a differential mobility analyzer is required, and it typically takes 30?s to 120?s to obtain one entire size distribution. A size distribution obtained by an SMPS measurement might have significant deviations from actual values due to the scanning process when the measured particle concentrations change over time. In this study, we introduce an analytical approach for estimating particle size distribution under exponentially decaying and growing particle concentrations. The analytical SMPS results are validated by performing experiments using exponentially decaying particle concentrations under the same conditions. Furthermore, the effects of a decay parameter, initial size distribution, and scan time are evaluated, and the deviations from actual (real or true) size distributions obtained by an exact solution are analyzed. Geometric mean diameters and standard deviations of the size distributions from SMPS results increase or decrease with exponentially decaying or growing concentrations, respectively, and total concentrations estimated by the analytical SMPS approach are significantly underestimated or overestimated compared to real total concentrations. While SMPS measurements have been widely employed in various applications such as atmospheric particle characterization in highly variable particle concentrations versus time, very few studies on the influence of changing concentrations on SMPS measurements have been conducted. Therefore, the introduced analytical approach and findings provide valuable insight into the importance of accurate SMPS measurements with changing particle concentrations.

Copyright © 2020 American Association for Aerosol Research     

Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of n-butane using LII,HIM, and 1 nm-SMPS

Laser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a so-called nucleation flame obtained at a threshold equivalence ratio Φ = 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at Φ = 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex situ analysis by helium-ion microscopy (HIM) of particles sampled thermophoretically and (2) online size distribution analysis of microprobe-sampled particles using a 1 nm-SMPS. The size distributions of the incipient soot particles, found in the nucleation flame and in the early soot region of the Φ = 1.95 flame, derived from time-resolved LII signals are in good agreement with HIM and 1 nm-SMPS measurements and are in the range of 2–4 nm. The thermal and optical properties of incipient soot were found to be not radically different from those of mature soot commonly used in LII modeling. This explains the ability of incipient soot particles to produce continuous thermal emissions in the visible spectrum. This study demonstrates that LII is a promising in situ optical particle sizing technique that is capable of detecting incipient soot as small as about 2.5 nm and potentially 2 nm and resolving small changes in soot sizes below 10 nm.

© 2017 American Association for Aerosol Research