Development and qualification of a VH-TDMA for the study of pure aerosols

We create and qualify a Volatility and Hygroscopicity Tandem Differential Mobility Analyzer (VH-TDMA) for the study of aerosols. This VH-TDMA measures size distributions, volatility, and hygroscopicity and includes an auxiliary conditioner that allows quick connection to other external aerosol conditioners. The differential mobility analyzers are not temperature controlled, allowing the surrounding environment to influence the measurement conditions, and this is fully accounted for when measuring aerosol volatility and hygroscopicity. For the volatility conditioner, the VH-TDMA uses a 15?m coil of tubing in an oven to evaporate aerosol samples at elevated temperatures. We measured several single component model aerosols to qualify the differential mobility particle sizer (DMPS) channel and each of the conditioners: hygroscopicity and volatility. Due to insufficient power supply calibration in this study, the TDMA channel is limited to particle sizes greater than 70?nm. The DMPS channel was able to reproduce ammonium sulfate size distributions when compared to common scanning mobility particle sizers. For hygroscopicity, the standard deviation in the measured ammonium sulfate growth factors was 0.03 over a 4-h experiment. From this data, the TDMA has an observed relative humidity error of ±0.6% with manufacturer reported error of ±1.2% relative humidity. The volatility channel reproduced the previously published saw tooth pattern of room temperature saturation vapor pressures from atomized C3-C9 diacids. The maximum percent difference in room temperature saturation vapor pressure was approximately 80%. The enthalpy of sublimation derived from the diacids increased monotonically (except for suberic acid) and resembled measurements from mass effusion techniques.

Copyright © 2019 American Association for Aerosol Research     

Determination of Evaporation Coefficients of Ambient and Laboratory-Generated Semivolatile Organic Aerosols from Phase Equilibration Kinetics in a Thermodenuder

Accurately predicting formation and partitioning of ambient organic aerosols remains a challenge despite decades of sustained effort in this domain. A major source of uncertainty is the poorly characterized volatility of these aerosols. This uncertainty stems in large part from difficulty separating the overlapping effects of aerosol thermodynamic properties and evaporation coefficients in thermodenuder volatility studies. For lack of other information, it is commonly assumed that the evaporation coefficient is unity when interpreting thermodenuder data, leading to potentially large biases in inferred volatility of the sampled aerosol. In this paper, we present a novel thermodenuder-based approach for determining evaporation coefficients of pure compound and complex aerosols without knowledge of their thermodynamic properties. The method involves tracing the normalized dynamic response of an aerosol system to a step change in temperature as it flows through a heated tube. The approach is validated using pure compounds and a mixture of laboratory-generated dicarboxylic acids, and is applied to concentrated ambient aerosols sampled in Beirut, Lebanon. Three valid data sets were obtained from more than 200 h of ambient air sampling during the month of August 2010, yielding values of 0.34, 0.46, and 0.28 for an assumed binary gas diffusion coefficient of 7.8 × 10^?6 m²/s at 60°C.

Copyright 2012 American Association for Aerosol Research     

Computational Models for Simulating Multicomponent Aerosol Evaporation in the Upper Respiratory Airways

An effective model for predicting multicomponent aerosol evaporation in the upper respiratory system that is capable of estimating the vaporization of individual components is needed for accurate dosimetry and toxicology analyses. In this study, the performance of evaporation models for multicomponent droplets over a range of volatilities is evaluated based on comparisons to available experimental results for conditions similar to aerosols in the upper respiratory tract. Models considered include a semiempirical correlation approach as well as resolved-volume computational simulations of single and multicomponent aerosol evaporations to test the effects of variable gas-phase properties, surface blowing velocity, and internal droplet temperature gradients. Of the parameters assessed, concentration-dependent gas-phase specific heat had the largest effect on evaporation and should be taken into consideration for respiratory aerosols that contain high volatility species, such as n-heptane, at significant concentrations. For heavier droplet components or conditions below body temperatures, semiempirical estimates were shown to be appropriate for respiratory aerosol conditions. In order to reduce the number of equations and properties required for complex mixtures, a resolved-volume evaporation model was used to identify a twelve-component surrogate representation of potentially toxic JP-8 fuel based on comparisons to experimentally reported droplet evaporation data. Due to the relatively slow evaporation rate of JP-8 aerosols, results indicate that a semiempirical evaporation model in conjunction with the identified surrogate mixture provide a computationally efficient method for computing droplet evaporation that can track individual toxic markers. However, semiempirical methodologies are in need of further development to effectively compute the evaporation of other higher volatility aerosols for which variable gas-phase specific heat does play a significant role.     

A model for the relative humidity effect on the readings of the PM10 beta-gauge monitor

In the previous paper (Atmos. Environ. 15 (1981) 1087), we found that the PM₁₀ concentrations detected by the Wedding beta-gauge PM₁₀ monitor and those measured by the manual hi-vol PM₁₀ sampler were quite close when the ambient relative humidity (RH) was lower than the deliquescence RH (DRH) of aerosols. However, when the deliquescent point was exceeded, PM₁₀ concentrations of the beta-gauge were found to be higher and differences increased with an increasing ambient RH. In addition, theoretical water mass calculated by a thermodynamic model (ISORROPIA model, (Aquat. Geochem. 4 (1998) 123)) was found to be much higher than the actual values. In this study, models were developed to determine water evaporation loss from collected particles on the filter tape of the beta-gauge during sampling and in the monitoring room. Simulated results show that all absorbed water will evaporate completely at RH lower than about 85%. However, absorbed water does not evaporate completely at RH higher than about 85%, and remaining water in particles accounts for higher beta-gauge readings than the hi-vol concentrations. The simulated daily beta-gauge PM₁₀ concentrations are close to the actual beta-gauge readings obtained previously.     

Aerosol growth studies — IV. Phase transformation of mixed salt aerosols in a moist atmosphere

The phase transformation and subsequent droplet growth of the mixed salt aerosols NaCl—KCl and (NH₄)₂SO₄—H₂SO₄ were investigated in a continuous-flow apparatus at 25 and 30°C as a function of relative humidity. Monodisperse salt aerosols (d = ≈ 0.5 μm, OG = 1.07–1.13) were prepared and mixed with N₂ carrier gas at controlled humidities. The particle-size distribution of the aerosol before and after growth by water vapor condensation was continuously monitored with an optical particle counter. It was found that mixed salt aerosols were characterized by stage-wise growth when the relative humidity in the atmosphere was increased. The onset of growth took place at a specific deliquescence humidity determined by the water activity at the eutonic composition. Thus, mixed NaCl—KCl aerosols deliquesce at 73.8 ± 0.5% r.h. regardless of initial compositions. For sulfate aerosols containing 0.75 to 0.95 mole fraction (NH₄)₂SO₄ (the balance being H₂SO₄), the onset of growth occurs at 69.0 ± 0.5% r.h.. In the composition range of 0.5 to 0.75, a deliquescence humidity of 39.0 ± 0.5% is noted. Below 0.5 mole fraction, however, the mixed-sulfate aerosols are expected to exhibit hygroscopic properties on the basis of thermodynamic considerations.     

Linear and Nonlinear Drying Behavior in Tuberous Crop Slices

Understanding drying physics is a complex task because interactions between phases and variations in thermal properties change over time. In this investigation we used two models to simulate the drying of potatoes slices. Drying kinetics were modeled by both the drying characteristic curve (DCC) method and by a mechanistic approach implemented in COMSOL Multiphysics. The DCC was developed on the basis of experimental data and a referential drying rate, which for potatoes is the maximum evaporation rate during the process. The surface thermal evolution was considered to estimate the drying rate curve and the drying stages. The phenomenological model considers both the transport of free water and water vapor by applying a mechanistic approach. In order to simulate free water transport we took into account the capillary diffusivity term, and to simulate water vapor evacuation we considered the desorption isotherm. Two drying conditions were analyzed, 1.0 and 2.2 m/s of air flow with 60°C and 30% relative humidity (RH). The mechanistic model solves the primary unknown's moisture content, temperature, and dry air density. Both models were compared against experimental data. The simulation correctly describes the drying kinetics for the trial at 2.2 m/s and fails to simulate the phenomena at 1.0 m/s. Two different drying behaviors influenced by air flow speed were identified by following the evolution of surface temperature and mass flux. The dependence of mass flux on the difference in temperature (T_air?T_surface) shows that the area of exchange is a very important parameter to be considered in simulations, because both linear and nonlinear behaviors are manifested.     

Determining Aerosol Volatility Parameters Using a “Dual Thermodenuder” System: Application to Laboratory-Generated Organic Aerosols

Thermodenuders (TD) are a tool widely used for measuring aerosol volatility in the laboratory and field. Extracting the parameters that dictate organic aerosol volatility from TD data is challenging because gas-particle partitioning rarely reaches equilibrium inside a TD operating under atmospheric conditions, thus a wide variety of parameter sets can explain observed evaporation. Component volatilities (as represented by saturation vapor pressure, C_sat), cannot be directly extracted due to uncertainties in potential limitations to mass transfer (represented by mass accommodation coefficient, α) and components’ enthalpies of evaporation (ΔH_vap). To address these limitations, we have developed a “dual TD” experimental approach in which one line uses a temperature-stepping TD (TS-TD) with a relatively long residence time (RT) and the other operates isothermally at variable residence time (VRT-TD). Data from this approach are used in tandem with an optimizing evaporation kinetics model to extract the values of parameters dictating volatility (C_sat, and associated values of ΔH_vap and α). The system was evaluated using laboratory generated dicarboxylic acid aerosols (adipic acid and succinic acid). Excellent agreement with previously published evaporation data collected with other TD systems was observed. Parameter values reported in the literature for the tested acids vary widely, but our results are generally consistent with those from studies that allow for nonunity values of α. For example, our results suggest that α for these aerosols are of order 0.1, in agreement with results determined by Saleh et al. (2009, 2012). Modeling results suggest that the addition of VRT-TD data provides tighter constraint on feasible ΔH_vap and α values. The dual TD approach presented here does not rely on equilibration in the TD and thus can be directly applied to extract volatility parameters for more complex laboratory and ambient organic aerosol systems.

Copyright 2015 American Association for Aerosol Research     

Development and Characterization of a Fast-Stepping/Scanning Thermodenuder for Chemically-Resolved Aerosol Volatility Measurements

Numerous sources of liquid aerosols are to be found in industrial environments. Such aerosols may, for instance, be cutting fluids, pesticides, etc., that are harmful or even toxic to humans. To control and reduce worker exposure to potentially toxic aerosols, these latter are usually filtered through fibrous filters. When non-saturated air traverses a clogged filter, however, the drops deposited on the fibers may evaporate. Consequently, workers are exposed to greater amounts of more concentrated vapors than the initial state of the filtered aerosol. Furthermore, exposure readings are distorted by an artifact that may be significant. This study offers an experimental approach to long-term monitoring of the evaporation of a semi-volatile n-hexadecane liquid aerosol deposited on filters of varying efficiency. Results were modeled using two semi-empirical models for identifying the basic parameters of liquid aerosol evaporation on fibers. For the first time ever it has been demonstrated that the Fick's first law, as previously suggested by models proposed in the literature, does not control evaporation kinetic.     

盐湖老卤蒸发气候条件研究与蒸发模型建立

通过开展环境气候条件实验研究,揭示环境气候主要参数大气湿度和老卤蒸发速度之间的关系,建立了老卤蒸发模型,并利用模型推算出老卤自然蒸发的大气相对湿度极限值为33.2%。实验证明蒸发环境大气相对湿度小于该值时老卤将会自然蒸发,大于该值时不但不蒸发,还将倒吸空气中的水分。利用本研究蒸发数学模型,可以预测盐湖气候季节变化过程中老卤蒸发情况,为合理设计老卤蒸发方案提供科学依据。     

Water vapor absorption and permeability of films based on chitosan and sodium caseinate

The objective of this work was to characterize the moisture sorption and water vapor permeation behavior of edible films made from sodium caseinate and chitosan for future applications as protective layers on foods. Glycerol was used as a plasticizer, and the films were obtained by a casting/solvent‐evaporation method. The moisture sorption kinetics and water vapor permeability (WVP) were investigated. The effect of the addition of glycerol on the WVP characteristics of the films was determined at 25°C with a relative humidity (RH) gradient of 0–64.5% (internal to external). Experimental data were fitted with an exponential function with two fitting parameters. WVP increased with increasing glycerol content in both films, chitosan samples being much more permeable than caseinate ones at any glycerol content. WVPs of sodium caseinate, chitosan, and chitosan/caseinate films with 28 wt % glycerol were also determined for two RH gradients, 0 to 64.5% and 100 to 64.5%, higher WVPs being measured at higher RHs. The moisture sorption kinetics of caseinate films prepared with various glycerol contents were determined by the placement of the films in environments conditioned at 20°C and 75% RH. Peleg's equation and Fick's second law were used to predict the moisture sorption behavior over the entire time period. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Size Distribution Measurements of Ultrafine Aerosols,d p > 1.8 nm,Formed by Radiolysis in a Diameter Measurement Analyzer Aerosol Charger

A University of Vienna differential mobility analyzer-electrometer arrangement was used to measure the size distribution of charged ultrafine aerosols (d _p > 1.8 nm) generated from a N₂-SO₂-H₂O mixture by radiolysis in a bipolar diffusion charger loaded with a 2.5 mCi Am-241 α-source. Despite the short residence time in the charger, t_r < 1s, nucleation mode diameters near 2 nm were found for dry gas mixtures with water vapor concentrations < 32 ppm. At 20% relative humidity number concentrations increased and modal diameters shifted to larger sizes with increasing SO₂ concentration. The addition of trace concentrations of nitric oxide or ethanol vapor to the gas mixtures resulted in a near complete suppression of particle formation.     

Effect of moisture on the oxidation behavior of ZrB2

Oxidation studies of ZrB₂ were performed under wet air and dry air conditions at 1200°C, 1400°C, and 1500°C for 1, 4, and 10 h. Compared to dry air, the presence of water vapor was found to enhance the oxidation kinetics by a factor of 7 to 30, depending on the temperature. Thermodynamic calculations suggested that water vapor promoted the formation of additional volatile species such as boric acid (HBO₂), in addition to boria (B₂O₃) produced in dry air, which increased the evaporation rate of B₂O₃. Compared to dry air, the presence of water vapor leads to more rapid evaporation of boria and the transition from parabolic oxidation kinetic behavior (ie, rate controlled by diffusion through boria) to linear (ie, underlying ZrB₂ is directly exposed to the oxidizing environment) at shorter times and lower temperatures.     

Mechanism of the effects of microwave irradiation on the relative volatility of binary mixtures

The use of microwave irradiation to enhance distillation processes has been reported recently. However, there is an ongoing debate in the scientific community on whether the observed enhancement is mainly a consequence of the shift of the “equilibrium” of vapor–liquid mass transfer. In this article, a developed instrument was used to determine the relative volatility of various binary mixtures under microwave irradiation. By comparing the relative volatility in the presence/absence of microwave irradiation, the shift of the “equilibrium” of vapor–liquid mass transfer was observed for certain binary mixtures under microwave irradiation. The effects of microwave irradiation on the relative volatility of binary mixtures (in addition to the mechanisms involved therein) were analyzed using the non‐equilibrium thermodynamic principle. The results demonstrate that differences in the dielectric properties, microwave field intensity, intermolecular forces, and boiling point play dominant roles in determining the effects of microwaves on the relative volatility. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1328–1337, 2017     

浓海水鼓泡滩晒过程初探

实验搭建了一块120 m²的浅滩鼓泡浓海水晒盐池,盐池中含9120个直径为1 mm的鼓气孔,采用额定流量为420 m³·h^-1的鼓风机鼓泡。测定了鼓泡池及1 m²对照盐池液位、温度、波美度、环境温湿度的变化,分析了气象条件和鼓气量对蒸发量的影响。研究表明,从上午12点至下午3点鼓泡晒盐效果最佳,在最佳条件下比对照池每小时可多蒸发120 kg水蒸气,但通过鼓气直接带走的水蒸气量不足8.7 kg,漂浮的气泡使盐池表面积增加50%~60%,多蒸发约50%的水蒸气,其余的水蒸气通过其他方式蒸发。与传统晒盐方法相比,浅滩鼓泡晒盐具有更快的蒸发速率,若能显著降低鼓泡管路成本和能耗,将具有较好的应用前景。     

Rapid,Size-Resolved Aerosol Hygroscopic Growth Measurements: Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe (DASH-SP)

We report on a new instrument developed to perform rapid, size-resolved aerosol hygroscopicity measurements. The differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP) employs differential mobility analysis in-concert with multiple humidification and optical sizing steps to determine dry optical size and hygroscopic growth factors for size-selected aerosols simultaneously at three elevated relative humidities. The DASH-SP has been designed especially for aircraft-based measurements, with time resolution as short as a few seconds. The minimum particle diameter detected with 50% efficiency in the optical particle counters (OPCs) is 135 ± 8 nm, while the maximum detectable particle diameter is in excess of 1 μm. An iterative data processing algorithm quantifies growth factors and “effective” refractive indices for humidified particles using an empirically derived three-dimensional surface (OPC pulse height–refractive index–particle size), based on a calculated value of the “effective” dry particle refractive index. Excellent agreement is obtained between DASH-SP laboratory data and thermodynamic model predictions for growth factor dependence on relative humidity for various inorganic salts. Growth factor data are also presented for several organic acids. Oxalic, malonic, glutaric, and glyoxylic acids grow gradually with increasing relative humidity up to 94%, while succinic and adipic acids show no growth. Airborne measurements of hygroscopic growth factors of ship exhaust aerosol during the 2007 Marine Stratus/Stratocumulus Experiment (MASE II) field campaign off the central coast of California are presented as the first report of the aircraft integration of the DASH-SP.     

A Two-Dimensional Laminar Flow Model for Thermodenuders Applied to Vapor Pressure Measurements

Thermodenuders (TD) have been used to quantify the volatility of aerosol species, frequently with the aid of modeling. Here we present a two-dimensional model of flow, heat transfer, and aerosol dynamics that is fast, yet includes spatial resolution of the complete aerosol size distribution. We first demonstrate the utility of the model by interpreting nonequilibrium TD measurement data previously reported in the literature. It is shown that the thermogram (temperature vs. mass fraction remaining) curve is remarkably insensitive to radial variations in temperature and vapor concentration under typical conditions. Therefore, the discrepancies among vapor pressure estimates determined in TD studies are unlikely to be due to oversimplified flow models, but are instead likely due to faulty assumptions concerning evaporation kinetics. We then show that the best-fit range for the parameters that dictate equilibrium partitioning (saturation vapor pressure at a reference temperature and enthalpy of vaporization) can also be obtained by fitting nonequilibrium TD data using a three-parameter model that accounts for mass transfer limitations (by also fitting the evaporation coefficient). The degree of agreement between experiments and model simulations are examined for two dicarboxylic acids using the model developed in this study. The best-fit parameters were within the uncertainty range previously found using an “equilibrated” TD approach for butanedioic acid, whereas significantly better model-experiment agreement was obtained for a much lower value of enthalpy of vaporization than previously reported for hexanedioic acid.

Copyright 2013 American Association for Aerosol Research     

The Effect of Water Vapor on the Particle Structure and Size of Silica Nanoparticles During Sintering

In this article, the influence of the water vapor concentration on structural changes of SiO₂ aerosol nanoparticle agglomerates during tempering was studied. The presence of water vapor in the carrier gas was shown to strongly accelerate the kinetics of sintering. While dry sintering at temperatures between 1100°C and 1500°C generated aggregates only, the addition of water to the process yields individual, completely coalesced nanoparticles at a temperature of 1300°C. Furthermore, depending on the water vapor concentration and temperature of the process, evaporation and condensation processes could be observed, leading to bimodal size distributions. The results prove the significant role of the water concentration in high temperature synthesis of silica and may be used to improve the control over morphology and specific surface area in these processes.     

Undersizing of droplets from a vented nebulizer caused by aerosol heating during transit through an anderson impactor

The use of Anderson cascade impaction to measure droplet sizes from a vented nebulizer connected directly to the impactor was examined by performing two sets of impactor measurements. In the first set, the impactor was operated in room temperature air. In the second set, the impactor was immersed in a cooled water bath at the same temperature as the aerosol exiting the nebulizer (10°C). Normal saline was nebulized in five Pari LC Jet+nebulizers driven by a single Pulmo-Aide compressor, operating under ambient conditions of 36% RH and 22°C. Impaction was done once steady temperatures were reached. When the impactor was operated in room temperature air, the air travelling through the impactor warmed from 10°C at the entrance to the impactor to room temperature at the exit of the impactor. This heating resulted in significant droplet shrinkage due to humidification by evaporation from the droplets, since the impactor immersed in the cooled water bath gave an MMAD that was on average 70% larger than was obtained when the impactor was operated in room air (3.4 μm vs 2.1 μm). These results emphasize the need for caution when using impactors to measure nebulized hygroscopic aerosols, since even if these aerosols enter the impactor in vapor pressure equilibrium with their surrounding air, significant size changes can occur during transit through the impactor if the temperature of the aerosol differs significantly from that surrounding the impactor.     

Bioinspired Poly(vinyl alcohol) Film Actuator Powered by Water Evaporation under Ambient Conditions

Soft humidity‐responsive materials are highly desirable for applications such as actuators, sensors, generators, and soft robots. However, it remains a huge challenge to develop a durable, cost‐effective, fast responsive version of such a smart material powered by water evaporation at ambient conditions. Herein, this challenge is addressed to demonstrate sustained response to humidity gradient from ambient water evaporation by using common poly(vinyl alcohol) (PVA) film as an actuator. The resultant PVA film displays strong mechanical properties in both dry and wet conditions, which cause rapid adsorption and desorption of water vapor to drive the film undergoing swift locomotion with flipping frequency of up to 65 r min^‐1. Based on these features, a mimosa inspired humidity‐responsive actuator is developed which is far superior in response speed and durability than real mimosa. Furthermore, it is demonstrated that the film actuator can convert water evaporation energy into electricity when attached to a piezoelectric element.     

The Relative Influence of Atomization and Evaporation on Metered Dose Inhaler Drug Delivery Efficiency

Factors influencing the delivery efficiency from HFA-134a MDIs were examined theoretically and experimentally. The time required for evaporation of HFA-134a and ethanol droplets were theoretically calculated. HFA-134a droplets were shown to evaporate approximately seven times faster than ethanol droplets of the same size, even though HFA-134a droplets cool to approximately 78 degrees below ambient temperature during evaporation. MDI delivery efficiency was experimentally shown to decrease with increasing ethanol concentration, however, the corresponding decrease in vapor pressure was not the primary variable responsible for the decreased efficiency. Rather, this was shown to be primarily due to the increased time required for the droplets to evaporate as ethanol concentration increased. Droplets that evaporate slowly remain for a longer period in the size range that is more likely to deposit via turbulent deposition in the actuator mouthpiece or USP Inlet during cascade impaction tests. Tests with experimental MDIs using alternative cosolvents confirmed that MDI delivery efficiency is much more sensitive to the time required for evaporation of the droplets than on the formulation vapor pressure or the size of the atomized droplets. This indicates that factors affecting the evaporation of an MDI aerosol play a larger role in determining MDI delivery efficiency than do atomization effects.