Clouds and “throat hit”: Effects of liquid composition on nicotine emissions and physical characteristics of electronic cigarette aerosols

Electronic cigarettes (ECIGs) heat and vaporize a liquid mixture to produce an inhalable aerosol that can deliver nicotine to the user. The liquid mixture is typically composed of propylene glycol (PG) and vegetable glycerin (VG), in which are dissolved trace quantities of flavorants and, usually, nicotine. Due to their different chemical and thermodynamic properties, the proportions of PG and VG in the liquid solution may affect nicotine delivery and user sensory experience. In social media and popular culture, greater PG fraction is associated with greater “throat-hit,” a sensation that has been attributed in cigarette smokers to increased presence of vapor-phase nicotine. VG, on the other hand, is associated with thicker and larger exhaled “clouds.” In this study, we aim to investigate how PG/VG ratio influences variables that relate to nicotine delivery and plume visibility. Aerosols from varying PG/VG liquids were generated using a digitally controlled vaping instrument and a commercially available ECIG, and analyzed for nicotine content by GC-MS. Particle mass and number distribution were determined using a six-stage cascade impactor and a fast particle spectrometer (TSI EEPS), with tightly controlled dilution and sampling biases. A Mie theory model was used to compute the aerosol scattering coefficients in the visible spectrum. Decreasing the PG/VG ratio resulted in a decrease in total particulate matter (TPM) and nicotine yield (R² > 0.9, p < .0001). Measured particle count median diameter ranged between 44 and 97nm, and was significantly smaller for PG liquids. Although the particle mass concentration was lower, aerosols produced using liquids that contained VG had an order of magnitude greater light scattering coefficients. These findings indicate that PG/VG ratio is a strong determinant of both nicotine delivery and user sensory experience.

Copyright © 2017 American Association for Aerosol Research     

Impact of power level and refill liquid composition on the aerosol output and particle size distribution generated by a new-generation e-cigarette device

The new high-power Electronic Nicotine Delivery System (ENDS) can generate aerosols with higher nicotine concentrations than older ENDS. Aerosol particle sizes affect deposition patterns and then plasma nicotine levels in vapers. Consequently, understanding the factors influencing particle size distribution of high-power ENDS is relevant to assess their performance in terms of nicotine delivery. The particle size distribution and the aerosol output (aerosol mass) were measured using cascade impactors. The effects of the refill liquid composition (80% PG/20% VG vs. 80% VG/20% PG; PG refers to propylene glycol and VG to vegetable glycerin) and the power level of the battery (from 7 W to 22 W) were investigated. The aerosol output increases significantly with the power level following a logarithmic law. The PG/VG ratio also has an impact on the aerosol output. The higher the VG content in the refill liquid, the higher is the aerosol output. Besides, particle size distribution is positively related to the power level, following linear correlations between the mass median aerodynamic diameter (MMAD) and the power level in the range of 7–22 W. A moderate impact of the PG/VG ratio on size distribution is equally observed. Changes in the power level allow the transition between a dominant mode with MMAD from 613 nm to 949 nm. We demonstrated that the power level can strongly change the aerodynamic properties of high-power ENDS, especially at high voltage. Associated with the aerosol nicotine level assessment, MMAD could be determined as a means for comparing ENDS devices and nicotine delivery.

Copyright © 2018 American Association for Aerosol Research     

Fate of pyrazines in the flavored liquids of e-cigarettes

Popularity of electronic cigarettes (ECIGs) has increased tremendously among young people, in part due to flavoring additives in ECIG liquids. Pyrazines are an important class of these additives, and their presence in tobacco cigarettes has been correlated with increased acceptability of smoking among smokers and bystanders. Pyrazine use by the tobacco industry is therefore thought to encourage smoking. However, the extent of transfer of pyrazines present in the liquid to aerosols upon vaping remains unclear. We present a simple analytical method to quantify six pyrazine derivatives in liquids and aerosols of ECIGs that allows the isolation of pyrazines from interfering compounds, like nicotine. Standard pyrazine solutions and commercial ECIG samples of different brands and flavors were tested for their pyrazine content in the liquids and in the generated aerosols from these solutions. Testing on ECIG commercial liquids revealed a heterogeneous distribution in the levels and types of pyrazines, with acetyl and alkyl pyrazines present in more than 70% of the samples. This method confirmed that pyrazine additives are common in ECIG and that labels do not usually reflect the type and quantity of pyrazines in the liquid. Pyrazines were not correlated with the nicotine content or the brand of the liquid. The aerosols showed similar pyrazine profiles to their corresponding liquids. The efficiency of transfer of pyrazines into the particle phase was approximately 46%. Therefore, addition of pyrazines to ECIGs should be regulated, because they act synergistically with nicotine to increase product appeal, ease smoking initiation, and discourage cessation.

Copyright © 2018 American Association for Aerosol Research     

Computational analysis of deposition and translocation of inhaled nicotine and acrolein in the human body with e-cigarette puffing topographies

Recently, toxicants such as formaldehyde and acrolein were detected in electronic cigarette (EC) aerosols. It is imperative to conduct research and provide sufficient quantitative evidence to address the associated potential health risks. However, it is still a lack of informative data, i.e., high-resolution local dosimetry of inhaled aerosols in lung airways and other systemic regions, due to the limited imaging resolutions, restricted operational flexibilities, and invasive nature of experimental and clinical studies. In this study, an experimentally validated multiscale numerical model, i.e., Computational Fluid-Particle Dynamics (CFPD) model combined with a Physiologically Based Toxicokinetic (PBTK) model is developed to predict the systemic translocation of nicotine and acrolein in the human body after the deposition in the respiratory system. In-silico parametric analysis is performed for puff topography influence on the deposition and translocation of nicotine and acrolein in human respiratory systems and the systemic region. Results indicate that the puff volume and holding time can contribute to the variations of the nicotine and acrolein plasma concentration due to enhanced aerosol deposition in the lung. The change in the holding time has resulted in significant difference in the chemical translocation which was neglected in a large group of experimental studies. The capability of simulating multiple puffs of the new CFPD-PBTK model paves the way to a valuable computational simulation tool for assessing the chronic health effects of inhaled EC toxicants.

Copyright © 2018 American Association for Aerosol Research     

Prototype e-cigarette and the capillary aerosol generator (CAG) comparison and qualification for use in subchronic inhalation exposure testing

Our objective was to evaluate the suitability of using a capillary aerosol generator (CAG) instead of using e-cigarette devices in 90-day or longer inhalation studies. Aerosol characteristics for both the CAG (which uses heat to produce a condensation aerosol) and e-cigarette generators have been previously reported, but a side-by-side comparison with the identical formulation has not been reported. Aerosols from both devices were analyzed immediately after generation for chemicals in the formulation (propylene glycol [PG], glycerin, water, and nicotine), selected carbonyls (acetaldehyde, acrolein, and formaldehyde) by ultra-performance liquid chromatography with ultraviolet detection (UPLC-UV), and a chemical fingerprint analysis using gas chromatography-mass spectroscopy (GC-MS). Aerosol capture methods for chemical analysis included Cambridge filter pads or two impingers in series each containing solution to trap and stabilize selected carbonyl compounds. Particle size distribution (cascade impactor) and exposure port uniformity (gravimetric) was measured in four rodent inhalation exposure chambers under inhalation study conditions. The aerosol of both generators contained the same known and unknown chemicals. Similar levels of compounds in the formula except for PG were detected in the aerosol of both generators. CAG produced more consistent particulate aerosol than e-cigarette generator and had lower levels of carbonyls primarily due to lower levels of formaldehyde. Exposure port concentrations were consistent and closer to target values with the CAG compared to the e-cigarette aerosol generator. CAG was easier to operate on a daily basis although more difficult to maintain because it required daily cleaning compared to single-use e-cigarettes. CAG was determined to be suitable for use in 90-day or longer inhalation studies.

Copyright © 2016 American Association for Aerosol Research     

Dihydroxyacetone levels in electronic cigarettes: Wick temperature and toxin formation

Recently, we reported the presence of dihydroxyacetone (DHA), the active ingredient in sunless tanners, in the aerosol of an electronic cigarette. DHA has been shown to react with DNA in vitro. The FDA restricts the use of DHA to external application only. It states that it should not be inhaled, ingested, or come into contact with any areas containing mucous membranes, due to unknown risk. Herein, the quantification of DHA in the aerosols of three brands of e-cigarettes has been carried out. These included two devices with horizontal heating coil configurations as well as one with a sub-ohm resistance vertical heating coil. In order to understand and begin to address the origin of DHA and related aerosol products, the wicking properties of the three e-cigarettes were compared. DHA levels were analyzed by a combination of GS/MS and ¹H NMR. DHA was found in all three e-cigarettes, with substantially less in the sub-ohm, vertical coil device as compared to the horizontal coil devices (e.g., 0.088 µg/puff vs. 2.29 µg/puff, respectively). Correspondingly, the temperature of the wet layer of the wick for the vertical coil was relatively stable, compared to the wicks for the horizontal coils, upon increasing battery power output. This result is in agreement with prior studies of e-cigarette wicking efficiency and aerosol toxin formation. The temperature measurements reported are a simple means for comparing devices with different design properties during operation.

Copyright © 2018 American Association for Aerosol Research     

The First Combined Thermal Desorption Aerosol Gas Chromatograph—Aerosol Mass Spectrometer (TAG-AMS)

To address the critical need for improving the chemical characterization of the organic composition of ambient particulate matter, we introduce a combined thermal desorption aerosol gas chromatograph—aerosol mass spectrometer (TAG-AMS). The TAG system provides in-situ speciation of organic chemicals in ambient aerosol particles with hourly time resolution for marker compounds indicative of sources and transformation processes. However, by itself the TAG cannot separate by particle size and it typically speciates and quantifies only a fraction of the organic aerosol (OA) mass. The AMS is a real-time, in-situ instrument that provides quantitative size distributions and mass loadings for ambient fine OA and major inorganic fractions; however, by itself the AMS has limited ability for identification of individual organic compounds due to the electron impact ionization detection scheme used without prior molecular separation.

The combined TAG-AMS system provides real-time detection by AMS followed by semicontinuous analysis of the TAG sample that was acquired during AMS operation, achieving simultaneous and complementary measurements of quantitative organic mass loading and detailed organic speciation. We have employed a high-resolution time-of-flight mass spectrometer (HR-ToF-MS) to enable elemental-level determination of OA oxidation state as measured on the AMS, and to allow improved compound identification and separation of unresolved complex mixtures (UCM) measured on the TAG. The TAG-AMS interface has been developed as an upgrade for existing AMS systems. Such measurements will improve the identification of organic constituents of ambient aerosol and contribute to the ability of atmospheric chemistry models to predict ambient aerosol composition and loadings.

Copyright 2014 American Association for Aerosol Research     

A practical set of miniaturized instruments for vertical profiling of aerosol physical properties

In situ atmospheric aerosol measurements have been performed from a Manta unmanned aircraft system (UAS) using recently developed miniaturized aerosol instruments. Flights were conducted up to an altitude of 3000 m (AMSL) during spring 2015 in Ny-Ålesund, Svalbard, Norway. We use these flights to demonstrate a practical set of miniaturized instruments that can be deployed onboard small UASs and can provide valuable information on ambient aerosol. Measured properties include size-resolved particle number concentrations, aerosol absorption coefficient, relative humidity, and direct sun intensity. From these parameters, it is possible to derive a comprehensive set of aerosol optical properties: aerosol optical depth, single scattering albedo, and asymmetry parameter. The combination of instruments also allows us to determine the aerosol hygroscopicity.

Copyright © 2017 American Association for Aerosol Research     

In-situ characterization of e-cigarette aerosols by 90°-light scattering of polarized light

The polarization ratio method is used for fast in-situ characterization of unimodal condensed aerosols of e-cigarettes. The method is based on 90°-light scattering of polarized 680 nm laser light by the droplet ensemble inside an optically defined measuring volume. Mass median droplet diameter (MMD) is derived from the ratio of scattered light from horizontally and vertically polarized incident light beams assuming a fixed value of the geometric standard deviation of the aerosol mass distribution. MMD is used to correct for the size dependence of the mass-based scattering signal of vertically polarized light to obtain the mass concentration if the sensor is calibrated once with an aerosol with a fixed MMD. The sensor uses commercially available aerosol photometers, and its application to e-cigarette aerosols was validated with an impactor for MMD and with a filter measurement for mass concentration. Good correlation (r² > 0.97) for both parameters was observed. Application ranges are mass concentration range 0.5–50 mg/L, MMD 0.2–1.2 µm, 100 ms time resolution, and 0.2–3 L/min flow rate. The usefulness of this simple sensor for e-cigarette aerosol characterization is demonstrated by developing a scaling law between MMD and operating parameters of an e-cigarette, i.e., puff flow rate and mass concentration.

Copyright © 2018 American Association for Aerosol Research     

Simulation of cigarette smoke dynamics in denuder tubes considering particle phase chemistry

The aerosol dynamics model ADiC was extended to include chemical reactions. It is used to computationally replicate denuder tube experiments where freshly generated cigarette smoke is drawn through a vertically arranged, acid covered tube to capture alkaline substances. The calculated deposition rates and total deposition are compared to experimental findings from several studies that investigated respective quantities for nicotine (and ammonia). Further, the form of deposition, vaporous and condensed phase, is considered. The model does not apply any parameters changing physico-chemical properties to fit simulation and experimental findings.

The only variable parameter used in all simulations is the choice of the amount of acid initially in the system to establish a certain pH value. An initial pH of 5.9 to 6.25 (i.e. the baseline scenarios) allows for replicating the nicotine deposition rate and total deposition in the lower tube sections. For the same simulation, ammonia deposition rate and total deposition are of the order of the experimental data. For the simulation featuring the initially lower pH value, the deposition of ammonia is lower than the experimental data – in the other case it is higher. Increasing the molality of alkaline substances initially in the system by roughly 20% drastically reduces the differences between simulated and measured nicotine deposition rate.

The present model describes some aspects of the dynamics of the complex cigarette smoke in a simplified manner; however, since it is independent of experiment specific parameters it may be applied to other environments such as deposition in the respiratory tract.     

Generation of monodisperse aerosols by combining aerodynamic flow-focusing and mechanical perturbation

A novel instrument has been developed for generating highly monodisperse aerosol particles with a geometrical standard deviation of 1.05 or less. This aerosol generator applies a periodic mechanical excitation to a micro-liquid jet obtained by aerodynamic flow-focusing. The jet diameter and its fastest growth wavelength have been optimized as a function of the flow-focusing pressure drop and the liquid flow rate. The monodisperse aerosol generated by this instrument is also charge neutralized with bipolar ions produced by a non-radioactive, corona discharge device. Monodisperse droplet generation in the 15- to 72-μm diameter range from a single 100-micron nozzle has been demonstrated. Both liquid and solid monodisperse particles can be generated from 0.7- to 15-μm diameter by varying solution concentration, liquid flow rate, and excitation frequency. The calculated monodisperse particle diameter agrees well with independent measurements. The operation of this new monodisperse aerosol generator is stable and reliable without nozzle clogging, typical of other aerosol generators at the lower end of the operating particle size ranges.

Copyright © 2016 American Association for Aerosol Research     

Economics of Treating Waste Gases from an Air Stripping Tower Using Photochemically Generated Ozone

Air stripping towers have been recommended for the removal of volatile organic compounds (VOCs) in drinking water supply and industrial waste treatment systems. This technique removes VOCs economically in the liquid phase. It can, however, create adverse secondary environmental impacts by removing VOCs from the water and discharging them to the air.

A commonly proposed method for controlling .VOC emissions is filtration of the off-gas through adsorption of the stripped organics in the off-gas by granular activated carbon. The high incremental cost of this alternative has produced an interest in alternative control technologies.

One alternative currently available is based on short wavelength ultraviolet (UV) radiation. This technique combines the effects of ozone generation, free radical formation and photolysis of the contaminants to effectively control the VOC emissions. This technique is known as Advanced Photo Oxidation (APO)^R.

The cost for APO is $0.27/m³ for a 3.8 m³/hr contaminated water system. A system of this size is adequate for a groundwater decontamination project where a moderate length of time is available for restoration of the site. The cost of a conventional air stripping tower with Granular Activated Carbon (GAC) adsorption emissions control in this size range would be $0.40 to $0.45/m³ (J.M. Montgomery, 1986).

Additional testing will be required to fully develop design guidelines for different contaminants and larger systems. Another area for additional technical documentation is the application of this technique to the liquid phase oxidation of VOCs.     

Opto-aerodynamic focusing of aerosol particles

We describe a new method for focusing and concentrating a stream of moving micron-sized aerosol particles in air. The focusing and concentrating process is carried out by the combined drag force and optical force that is generated by a double-layer co-axial nozzle and a focused doughnut-shaped hollow laser beam, respectively. This method should supply a new tool for aerosol science and related research.

Copyright © 2018 American Association for Aerosol Research     

Ionization efficiency of evolved gas molecules from aerosol particles in a thermal desorption aerosol mass spectrometer: Numerical simulations

Thermal desorption aerosol mass spectrometers (TDAMSs) with electron ionization are widely used to quantitatively measure aerosol chemical compositions. The physical and chemical mechanisms affecting the ionization efficiency of evolved gas molecules are not fully understood. We have developed a numerical model for simulating the dynamics of gas molecules evolved from aerosol particles. The simulation model is composed of two main sections. The first section simulates the elastic collisions of the evolved gas molecules in a small region near the vaporization source (collision domain), where the mean free paths of the molecules are much shorter than those in the surrounding high vacuum environment. The second section simulates the free-molecular dynamics from the boundary of the first section to the ionizer. The ionization efficiencies of ammonia and hydrogen iodide molecules that evolved from ammonium iodide particles were evaluated. Our results suggest that the molecular collisions during the early stage of plume expansion and possible changes in the molecular velocities induced by these collisions could be an important mechanism affecting the observed variability in the ionization efficiency. However, the physical and chemical processes of the vaporization and ionization of aerosol particles in TDAMSs may be too complex to be quantitatively reproduced using simplified numerical models.

Copyright © 2019 American Association for Aerosol Research     

A coupled CFD-Monte Carlo method for simulating complex aerosol dynamics in turbulent flows

A coupled computational fluid dynamics (CFD)-Monte Carlo method is presented to simulate complex aerosol dynamics in turbulent flows. A Lagrangian particle method-based probability density function (PDF) transport equation is formulated to solve the population balance equation (PBE) of aerosol particles. The formulated CFD-Monte Carlo method allows investigating the interaction between turbulence and aerosol dynamics and incorporating individual aerosol dynamic kernels as well as obtaining full particle size distribution (PSD). Several typical cases of aerosol dynamic processes including turbulent coagulation, nucleation and growth are studied and compared to the sectional method with excellent agreement. Coagulation in both laminar and turbulent flows is simulated and compared to demonstrate the effect of turbulence on aerosol dynamics. The effect of jet Reynolds (Re_j) number on aerosol dynamics in turbulent flows is fully investigated for each of the studied cases. The results demonstrate that Re_j number has significant impact on a single aerosol dynamic process (e.g., coagulation) and the simultaneous competitive aerosol dynamic processes in turbulent flows. This newly modified CFD-Monte Carlo/PDF method renders an efficient method for simulating complex aerosol dynamics in turbulent flows and provides a better insight into the interactions between turbulence and the full PSD of aerosol particles.

Copyright © 2017 American Association for Aerosol Research     

Identification and quantification of oxidized organic aerosol compounds using derivatization,liquid chromatography,and chemical ionization mass spectrometry

A systematic approach for identifying and quantifying molecular components of complex organic aerosol mixtures is presented. The approach combines methods developed previously for derivatizing carbonyl, hydroxyl, carboxyl, and ester functional groups, which are commonly present in oxidized organic aerosol, with liquid chromatography, UV detection, and chemical ionization-ion trap mass spectrometry. The original derivatization-spectrophotometric methods were modified for compatibility with liquid chromatography and then evaluated by analyzing a variety of standard compounds that contain one or more functional groups. Detection limits for carbonyl, hydroxyl, carboxyl, and ester analysis are approximately 0.003, 0.02, 0.01, and 1 nmole, respectively. Mass spectral analysis of derivatives using isobutane and ammonia as reagent gases for chemical ionization can be used to determine compound molecular weight, and characteristic fragmentation patterns provide structural information for use in compound identification. The methods will be useful for analyzing the chemical composition of secondary organic aerosol (SOA) formed in laboratory studies to obtain information needed to develop quantitative reaction mechanisms that can be incorporated into atmospheric models to better predict the formation, composition, and fate of SOA.

Copyright © 2017 American Association for Aerosol Research     

Sensitivity of Aerosol Refractive Index Retrievals Using Optical Spectroscopy

Accurate refractive index values are required to determine the effects of aerosol particles on direct radiative forcing. Theoretical retrievals using extinction data alone or extinction plus absorption data have been simulated to determine the sensitivity of each retrieval. A range of aerosol types with a range of different refractive indices were considered. The simulations showed that the extinction-only retrieval was not able to accurately or precisely retrieve refractive index values, even for purely scattering compounds, but the addition of a simulated absorption measurement greatly improved the retrieval.

Copyright 2014 American Association for Aerosol Research     

Eliminating the main source of instability and turbulence in TSI's 3071 DMA

The transfer function of TSI's widely used 3071 Differential Mobility Analyzer (DMA) widens drastically when the flow rate Q of sheath gas exceeds 30–40 lit/min, limiting its ability to resolve very small particles. This flow instability is unexpected at the prevailing relatively small Reynolds number (Re < 400). Here, we note that the rings holding the laminarization screens penetrate into the flow channel, generating unsteady vortices. A screen step exists not only on the outer screen region, but also on the inner screen region. Using a new step-free screen, no critical transition is observed up to the highest flow rate achieved of Q = 103 lit/min. The original DMA widens the flow cross-section in the mixing region where the aerosol joins the sheath gas. The flow deceleration then arising at small aerosol input flow rates introduces another source of transfer function broadening, which, however, has negative resolution effects only at Q > 60 lit/min. This feature is suppressed here by modifying a single inlet piece. Although the two flow improvements implemented increase the resolving power in the analysis of very small particles, a substantial non-ideality of unclear origin remains: the best resolving power R found with electrosprayed ions of the protein Immunoglobulin is R = 13.9 for the trimer (IgG)₃, and 12 for the monomer, even at a sheath/aerosol flow ratio of 100.

© 2017 American Association for Aerosol Research     

Aerosol Measurement by Induced Currents

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     

Effects of temperature on the formation of secondary organic aerosol from amine precursors

Aerosol formation is directly influenced by meteorological properties such as temperature and relative humidity. This study examines the influence of temperature on the physical properties and chemical composition of the aerosol produced from radical oxidation of aliphatic amines. Aerosol formation for temperatures ranging from 10 to 40°C was investigated in dual 90 m³ indoor atmospheric chambers. Further, chemical and physical responses of aerosol formed at one temperature and then raised/cooled to another were investigated in detail. Around two to three times more aerosol formation occurred at 10°C than at 40°C. This has important implications for locations influenced by amine emissions during the winter months. Significant aerosol formation occurred with the oxidation of amines with nitrate radical (100–600 μg/m³) and consisted largely of amine nitrate salts. These reactions are important contributors to aerosol formation during the nighttime hours, when nitrate radical is the dominant oxidant and temperatures tend to be cooler. Solid/gas partitioning of amine nitrate salt aerosol was consistent with literature results. A novel, temperature dependent, mechanism describing peroxy and hydroperoxy radical reactions was observed in the trimethylamine with hydroxyl radical oxidation experiments.

Copyright © 2016 American Association for Aerosol Research