Journal of Aerosol Science最新文献

Atmospheric soot (or black carbon, BC) affects climate through solar light absorption and scattering, which depend strongly on the particle morphology and composition. Initially, soot particles are fractal aggregates of spherules made of elemental carbon (EC), but condensation of atmospheric trace vapors adds non-EC materials and often results in particle compaction. The optical properties of such processed soot differ from those of fractal soot, and the changes are caused both by particle volume increase from coating addition and by restructuring of the EC backbone. In laboratory studies of soot optics, surrogates such as carbon black (CB) and nigrosin are often used in place of flame-generated soot. Our goal was to investigate if compositional and morphological differences between these surrogates and soot may produce different processing rates and optical responses. In our experiments, we generated fractal soot, compact CB, agglomerated CB (via coagulation of compact CB), and spherical nigrosin aerosol particles, subjected them to supersaturated vapor of dioctyl sebacate (DOS) to form a coating layer, and investigated the morphological response of these four particle types to coating addition and removal. Using coated and coated-denuded aerosol particles with known composition and morphology, we quantified the contributions of volume increase and restructuring to light scattering and absorption enhancements. By comparing experimental measurements against different particle optics models we show that it is crucial to account for larger, multiply charged particles present in the mobility-classified aerosol. Producing a disproportionately high contribution to absolute values of optical cross sections, such larger particles also result in lesser optical enhancements due to slower growth by vapor condensation. Scattering increases for all particle types due to the addition of a coating layer, and also due to restructuring for fractal soot (strongly) and agglomerated CB (weakly). Absorption increases only due to coating addition caused by the coating layer for all particle types. We find that simple optical models, such as Mie, are often sufficient to provide reasonable closure with experimental results for bare and coated aerosols, but only after accounting for the contributions from multiply charged particles, both in terms of their stronger optical cross sections and slower condensational growth. We conclude that CB is an appropriate surrogate for soot in aerosol aging studies where the effects of restructuring do not need to be considered and that nigrosin can be used as a general model for light-absorbing aerosols but is not representative of optical properties of soot.

The present study investigates the effect of the internal swirling atomizing air on the injector near-field spray characteristics and spray stability of a novel twin-fluid injector named swirl burst (SB) injector by incorporating an internal swirl. It involves primary atomization by internal bubbling and bubble bursting, and external secondary atomization by shear layer instabilities. A previous design integrated an external swirl and successfully enhanced the secondary atomization. It generated fine droplets immediately, rather than a typical jet core/film of conventional airblast or pressure swirl atomizers. It thus resulted in compact and ultra-clean lean-premixed combustion of distinct fuels, potentially enabling small-core fuel-flexible combustors. The current work aims to further enhance the primary atomization. The near-field flow patten and droplet size distribution and dynamics are investigated using high-speed laser-driven shadowgraph imaging accompanied by the internal bubble visualization. Results reveal that the internal swirl leads to more uniform, smaller and faster-moving bubbles that concentrate at the internal liquid tube tip regardless of the increased flow rates, generating ultra-stable and finer sprays with a wider working range, compared to the injector without the internal swirl. The frequency spectrum analysis of droplet sizes consistently substantiates the significantly improved spray steadiness, enhancing clean spray combustion stability.

Organic aerosols are ubiquitous, playing important roles in various atmospheric physicochemical processes such as the formation of cloud droplets and haze. Condensation of organic vapors, as a net effect of association with particles and dissociation from the condensed phase, is a fundamental process that drives the formation of organic aerosols. Kinetic models are often used to simulate the condensation fluxes of low-volatility organic vapors and aerosol growth. However, the widely used kinetic growth models usually calculate the evaporation of a certain species based on previous particulate compositions, without including the co-condensation of other species. Here we present a new kinetic partitioning method for calculating the condensation fluxes of organic vapors in a wide volatility range with low computational cost. In this method, the organic vapors are assumed to be in a quasi-steady state, but never reach real association-dissociation equilibrium during the simultaneous condensation of multiple species. We show a good consistency between the kinetic partitioning method and kinetic models in simulating particle mass fractions and condensation fluxes. Under relevant atmospheric conditions, we reveal that the kinetic partitioning method also reproduce the trend that low-volatility species are almost non-volatile while volatile organic compounds almost reach association-dissociation equilibrium, while there is a transition regime between them. This transition regime varies with atmospheric conditions, such as temperature and vapor concentrations. Compared with previous studies combining kinetic growth methods with equilibrium partitioning theories to simplify the condensation flux calculation, this method helps to improve accuracy without a significant expense of computation cost, and it can be applied in a wider range of atmospheric conditions such as in extremely cold atmospheres and polluted exhaust plumes.

Here, a heterogeneous ice nucleation parameterization associated with aerosol acting as ice nucleating particle (INP) was implanted into a two-dimensional numerical cumulus model. To explore the impact of INP on microphysical and electrical processes, a comparison was conducted with the original approach, which employs an empirical formula. Simulation results indicate that INP greatly impacted microphysical evolution in the heterogeneous ice nucleation process, reducing total liquid precipitation amounts and causing a slight precipitation delay, as well as increasing the diameter and mixing ratio of ice crystals and the expansion of the vertical distribution of ice crystals. This led to a notable change in electrification in thunderstorms. The increase of ice crystal diameter was the dominant contributor to the enhancement of electrification using the new parameterization. Additionally, unlike the structure of thunderclouds in a mature stage, which always retains a normal dipole structure adopting the empirical formula, a tripole structure developed a lower positive charge, and the polarity inversion with upper negative and lower positive charges occurred when the new parameterization was adopted. This predominately was the result of abundant ice crystals present below the reversal temperature. The electrification characteristics of thunderstorms may have a close connection with lightning activity. It has been found that charge structure changed significantly in the two cases, with the tripolar charge structure facilitating the production of inverted intra-cloud (IC) flashes and negative Cloud-to-ground (CG) flashes simulated by the new parameterization; additionally, clouds simulated using the empirical formula may be able to develop normal IC lightning and positive CG flashes. Therefore, it will be very meaningful to obtain greater insight into the characteristics of thunderstorms electrification in cumulus model with aerosols.

While significant progress has been made in developing models for the formation and transport of aerosol aggregates, there is still a need for a simple, versatile tool capable of estimating intrinsic properties of aggregated particles. Scalar friction factor is an important parameter used extensively in the field of aerosol science. The scalar friction factor for non-spherical particles can be computed with the information on two geometric parameters, hydrodynamic radius (R_h) and orientationally averaged projected area (PA), depending on the momentum transfer regime. Although the existing methods for the estimation of these descriptors are efficient, many applications involve frequent estimation of these geometric descriptors, which can be time-consuming. We propose a Machine Learning (ML) based tool that can predict these descriptors using Fractal Dimension, pre-exponential factor, number of monomers and anisotropy factors as the input. An extensive database comprising fractal parameters, anisotropy factors, R_h, and PA is developed for testing and training the ML models. Five ML methods were assessed, with random forest (RF) identified as the most effective. The RF model demonstrated high accuracy in the testing phase, with R-squared value of 0.9875 for R_h and 0.9979 for PA, and average errors of 3.17% and 1.21% for R_h and PA, respectively. The predicted R_h and PA values were then used to estimate other relevant 3-dimensional properties such as mobility diameter, shape factor, and aerodynamic diameter, with the results indicating high accuracy of the prediction tool. Python-based tool offers ease of use, and can be easily integrated with other numerical codes.

The deposition pattern or “linewidth” of an inertial particle deposit from acceleration of an aerosol through a high aspect ratio slit nozzle-substrate system is influenced by particle size-dependent aerodynamic focusing. In addition to this, high aspect ratio jets will eventually undergo downstream “jet-axis switching”, wherein the jet profile shrinks along its major axis and elongates in the direction of the minor axis. Jet axis switching in an aerosol may also lead to “rotated” particle deposits. In this study, we use a converging–diverging slit nozzle system with a major axis length of 8 mm and a throat width (minor axis) of 200 $\mu$m to examine the deposition patterns of monodisperse particles in the 100 nm–5 $\mu$m diameter range, in air with an upstream pressure of 252 Torr and variable downstream pressure in the 3–50 Torr range. The nozzle-to-substrate distance $L^{\ast }$ is varied from 90 to 248 times longer than the throat width. We find deposition patterns that are strongly dependent on particle size, downstream pressure, and $L^{\ast }$. Depending on particle diameter and operating conditions, we obtain deposits resembling the nozzle dimensions and orientation, deposits which are completely switched (perpendicular to the nozzle), or deposits which are relatively symmetric and focused at a center point. The latter appear to be the result of the combined effects of jet axis switching and aerodynamic focusing. In general, the influence of jet axis switching is more pronounced on smaller particles at higher downstream pressures, and with larger distances to the substrate. The extent of switching and area of the deposit are also both inversely related to the particle Stokes number.

Airborne Staphylococcus aureus is detected in various locations and linked to human infection. Reliable quantification of viable S. aureus bioaerosols by filter-based samplers helps characterize personal exposure, requiring efficient sampling and post-sampling processing. However, the efficiencies of filtration sampling and cell extraction are undetermined for viable S. aureus. In coupled with quantitative PCR and propidium monoazide, the performance of three widely-used samplers (IOM, Button, and Cassette) loaded with polycarbonate (PC) or gelatin filters was evaluated over 30–270 min of sampling and compared to that of BioSampler containing deionized water (DW). Effects of sampler type, filter type, and sampling time on cell recovery efficiency of sampling methods were assessed. Methods to extract cells from filters were also studied. The 1.5-min vortexing in peptone-Tween mixture and 10-min heating in DW were respectively granted optimal for cell extraction from PC and gelatin filters with extraction efficiencies averaged 1.0–1.76 (n = 4). Both Button and IOM with 3-μm gelatin filter performed best to capture S. aureus, significantly greater than Button with 0.8-μm PC by a factor of 9–11 (P < 0.05) and Cassette or IOM with 0.2-μm PC by a factor of 15–79 (P < 0.05). Cassette and IOM with 0.2-μm PC also showed less efficiencies than BioSampler/DW by a factor of 4–16 (P < 0.05). Cell recovery efficiency was not affected by sampling time except for the Button with 0.8-μm PC. Overall, filter type is the most critical factor governing cell recovery efficiency. Button and IOM with gelatin filter and 10-min heating in DW are considered the most efficient filtration sampling and extraction methods for viable S. aureus.

Evaluation of the regional intranasal delivery of locally acting drugs in children is challenging. Anatomical nasal airway replicas potentially can provide a robust pre-clinical tool to test the performance of devices and formulations. However, there is often a challenge in identifying the nasal geometries that can reasonably be indicative of in vivo regional mass distribution of administered drug. This in vitro study was designed to investigate the regional intranasal drug delivery in 20 children, 2–11 years old (50% 2–6 years old and 50% female), using two commercially available suspension nasal spray products with different nozzle designs, plume characteristics, and active pharmaceutical ingredients. High-resolution computed tomography scans of the sinonasal region of pediatric human subjects with healthy nasal airways, reviewed and scored by a head and neck surgeon, were used to develop 20 three-dimensional (3D) replicas of the nasal airways. The 3D replicas were segmented into the two regions: anterior and posterior to the internal nasal valve (INV). They were then rapid prototyped in high clarity rigid plastic (Accura ClearVue). Each side of the septum of the 20 subjects was examined separately, resulting in 40 singular nasal cavities. A nozzle-specific spray tip holder was designed for each case to ensure consistent administration (insertion length, sagittal angle, and coronal angle) in all replicates. The wide range of posterior drug delivery observed in the forty geometries indicated significant intersubject variability in pediatric intranasal drug delivery. Three nasal geometries representing low, medium, and high levels of drug delivery to the target region, posterior to the INV, were chosen from the 40 nasal cavities. Our vision is that these three nasal geometries can potentially be beneficial in determining whether performance differences between test and reference nasal spray products are present that may affect their bioequivalence in children. They also may be useful when applied in parallel with similar adult nasal geometries, previously developed following a similar procedure, to provide additional insights into pediatric nasal drug delivery with innovator products in children in lieu of extending clinical studies to include pediatric subjects.

An analysis is presented of the attachment of electrospray nanodroplets by larger particles in a grounded metallic tube where both are carried by a laminar air flow. Droplets and particles that get charged by attaching droplets migrate toward the tube wall under the action of the electric field induced by their charges, but they do so at different velocities owing to the disparity of their electrical mobilities. The electric currents they cause in the tube wall originate therefore in different regions of the tube, and can be separately measured by splitting the tube into two insulated segments. It is shown that, in two limiting conditions, the cross-section for droplet–particle collisions can be extracted from these measurements.

Determining the soot volume fraction (f_v) in combustion environments requires detailed knowledge of the optical properties of the soot particles, and in particular of their absorption function E(m). This study addresses a fundamental lack of information on the optical properties of 2–4 nm soot particles. Recent works based on the modeling of the photoelectron emission yields and UV-vis-NIR-absorption measurements found a sharp decrease of E(m) with the particle size in the vis-NIR spectral region, which is inconsistent with the in situ detection of 2–4 nm particles in the near-infrared region by laser-induced incandescence (LII) or sensitive absorption methods like cavity ring-down extinction (CRDE). The objective of this study is twofold: first, an original method for the determination of E(m) of soot particles, including 2–4 nm particles is proposed. Then, the dynamic of two widespread in situ diagnostics, LII and CRDE, are compared over three orders of magnitude of f_v in atmospheric premixed ethylene/air flames with different flow rates and C/O. The determination of the absolute value of $E\left(m\right)$ and of its variation in the flames is derived from an original analysis, which does not require complex LII modeling. This analysis is based on the comparison between the experimental and calculated LII/LII_max signals in the low fluence regime, LII_max being the plateau value of the fluence curve, which is reached at fluence larger than 1 J/cm² for the smallest C/O. E(m) is found to vary between 0.15 at low C/O up to 0.36 for the richest flames. Concerning the comparison of the dynamics of LII and CRDE, an excellent agreement is found above a threshold (C/O)_limit, while LII exhibits a stronger decrease with C/O below (C/O)_limit. This discrepancy is attributed to the spectral dependence of E(m) which is negligible above (C/O)_limit, but increases when C/O decreases below (C/O)_limit. The particle size distribution function (PSD), measured by scanning mobility particle sizing, reveals monomodal or bimodal PSDs with soot having mobility diameter in the range 2.3–7.5 nm depending on the flame conditions. It is suggested that the particles contained in the first PSD mode, which is dominant in the low C/O range, could be affected by a significant spectral dependence of E(m) in comparison with the second PSD mode.