Development of an ultraviolet constant angle Mie scattering detector toward the determination of aerosol growth kinetics in the transition and free molecular regime

Abstract

The measurement of aerosol growth kinetics at ever smaller sizes toward the transition and free molecular regime is of interest to provide for validation of theoretical predictions. Such measurements remain challenging to accomplish, particularly those occurring in the kinetic regime. Toward this goal, an instrument based on the ultraviolet constant angle Mie scattering (UV-CAMS) method was developed. The instrument utilizes adiabatic expansion to cause supersaturation and drive aerosol growth. Aerosol particles growing by water condensation are illuminated with a pulsed UV laser at 337?nm wavelength and a reference laser with red light (wavelength of 632?nm). The scattered light fluxes at 30° are measured simultaneously and are then compared with size resolved Mie scattering calculations providing aerosol growth measurements. The growth curves obtained from UV match those from the red laser. These measurements allow us to see the first Mie peak for UV scattering for particles in the 500?nm range. This is an almost two-fold resolution increase compared to the smallest particles that can be seen via red laser scattering in similar conditions (first Mie peak above 900?nm).     

Characterization and demonstration of a black carbon aerosol mimic for instrument evaluation

This study describes the characterization of a H₂O-dispersible, highly-absorbing carbonaceous nanomaterial that mimics the morphological and spectroscopic properties of aged black carbon aerosol (BC). When atomized from aqueous suspension, the material forms particles with a collapsed morphology resembling aged soot or BC. The material is >90 percent elemental carbon and has a mass absorption coefficient (MAC) and spectral dependence consistent with BC values published in the literature. The MAC at a wavelength of 532?nm decreased monotonically from 8.5 to 5.8?m² g^?1 for aerosol with mobility diameters between 150?nm to 500?nm. The single scatter albedo (SSA) at wavelengths of 405?nm and 660?nm was a function of both wavelength and mobility diameter and increased from 0.1 to 0.4 with mobility diameters between 150?nm to 400?nm. The Ångström absorption exponent (AAE) between λ?=?405?nm and 780?nm decreased monotonically from 1.4 to 0.6 for aerosol with mobility diameters between 150?nm to 400?nm. We demonstrate that this material can be used for fast, efficient calibration of aerosol photoacoustic spectrometers and for evaluation of spectroscopic-based measurements of aerosol mass concentration using in-situ photoacoustic spectroscopy (PAS) and filter-based light attenuation measurements for up 50?µg m^?3, enabling inter-method and inter-laboratory instrument comparison.

Copyright © 2019 American Association for Aerosol Research     

An inter-comparison of black-carbon-related instruments in a laboratory study of biomass burning aerosol

Black carbon (BC)-containing particles are the most strongly light absorbing aerosols in the atmosphere. Measurements of BC are challenging because of its semi-empirical definition based on physical properties and not chemical structure, the complex and continuously changing morphology of the corresponding particles, and the effects of other particulate components on its absorption. In this study, we compare six available commercial continuous instruments measuring BC using biomass burning aerosol. The comparison involves a Soot Particle Aerosol Mass Spectrometer (SP-AMS), a Single-Particle Soot Photometer (SP2), an aethalometer, a Multiangle Absorption Photometer (MAAP), and a blue and a green photoacoustic extinctiometer (PAX). An SP-AMS collection efficiency equal to 0.35 was measured for this aerosol system. The corrected SP-AMS BC mass measurements agreed within 6% with the SP2 refractory BC mass values. Two regimes of behavior were identified for the optical instruments corresponding to high and low organic/BC ratio. The mass absorption cross-sections (MAC) measured varied from 26% to two times the instrument default values depending on the instrument and the regime. The presence of high organic aerosol concentration in this system can lead to overestimation of the BC mass by the optical instruments by as much as a factor of 2.7. In general, the discrepancy among the BC measurements increased as the organic carbon content of the BC-containing particles increased.

© 2018 American Association for Aerosol Research     

Mass,charge, and radius of droplets in a linear quadrupole electrodynamic balance

Single particle levitation is a key tool in the analysis of the physicochemical properties of aerosol particles. Central to these techniques is the ability to determine the size of the confined particle or droplet, usually achieved via optical methods. While some of these methods are extremely accurate, they are not suitable for all applications and sample types, such as solid or optically absorbing particles. In this work, measurements of the radius, mass, and charge of droplets in a linear quadrupole electrodynamic balance (LQ-EDB) are reported. Using the elastic light scattering pattern produced by laser illumination, a method to determine the radius is described, with an accuracy of as good as ±60?nm and a sensitivity to changes on the order of 10?nm. The effect of refractive index on these measurements is explored by application of the technique to simulated data using Mie theory. In addition to radius, the relative and absolute mass and charge of droplets in the trap is measured from the voltage required to stabilize their vertical position. These measurements are facilitated by stacking multiple droplets in the LQ-EDB and solving the force balance equations to yield both parameters. These approaches are demonstrated through measurements of the evaporation of pure ethylene glycol and pure water droplets, the change in density of an aqueous glycerol solution as water evaporates, and the mass and charge of pure glycerol droplets.

Copyright © 2019 American Association for Aerosol Research     

Identifying time-dependent changes in the morphology of an individual aerosol particle from its light scattering pattern

The physical, chemical, and biological properties of an aerosol droplet/particle are dependent on the morphology of the droplet/particle itself; for example, a liquid droplet will be processed by oxidants in the gas phase in a fundamentally different way than a solid particle. Additionally, given their small size, aerosol droplets may change phase over timescales in the order of milliseconds (e.g., deliquescence or crystallisation). Thus, ability to rapidly and easily estimate the morphology of a droplet/particle is critical, especially in the interpretation of complex aerosol processes such as spray drying and dissolution. To be reported here is a novel method that uses the forward scattered light (～32° < θ < ～58°) passed through a droplet to determine the droplet/particles morphology. The algorithm was developed through the qualitative analysis of over one million individual phase functions of various particle morphologies. The algorithm can differentiate between four different morphologies: homogeneous, core/shell, with inclusions, and non-spherical/inhomogeneous. The algorithm is applicable to droplets between ～5 to ～30 microns in radius. The rate of phase analysis is dependent on the rate in which the light scatter can be collected, in the data presented here a particle’s morphology is reported every 10 milliseconds. The accuracy of the phase identification with the algorithm proposed in this work is very high (>90%); its utility is strengthened by the high frequency of the collection of scattered light, which allows an individual droplet to be probed upwards of over 100 times per second. Although not absolute on every phase function analysis, when coupled with repetition and high throughput, the algorithm presented here can be a valuable tool to easily and readily determine particle morphology in dynamic aerosol systems.

Copyright © 2019 American Association for Aerosol Research     

Dual-cavity spectrometer for monitoring broadband light extinction by atmospheric aerosols

Abstract

Atmospheric Aerosols affect Earth’s climate directly by scattering and absorbing solar radiation. In order to study the optical properties of aerosols, we developed a broadband cavity-enhanced spectrometer that uses a supercontinuum laser source and a compact spectrometer, to measure simultaneously the extinction coefficient of aerosols over a broad wavelength region from 420 to 540?nm. The system employs a dual cavity approach with a reference and a sample cavity, accounting for changes in gases background and for laser spectral and intensity fluctuations. We tested the system with aerosolized salt particles and polystyrene latex spheres. We performed calculations using Mie theory and found good agreement with the measured extinction. We also found that the extinction coefficient of non-absorbing aerosol favorably compares with the scattering coefficient measured by a nephelometer. Finally, we generated soot particles and found an extinction Ångström exponent in good agreement with values reported in the literature. Wavelength dependent detection limits (1σ) for the instrument at 5?nm wavelength resolution and for an integration time of ～10?min were found to be in the range ～5?Mm^?1 to 13?Mm^?1. The broadband dual-cavity extinction spectrometer is simple and robust and might be particularly useful for laboratory measurements of the extinction coefficient of brown carbon aerosol. The laboratory tests suggest that the prototype is promising for future developments of a field-deployable instrument.

Copyright © 2020 American Association for Aerosol Research     

Accuracy of black carbon measurements by a filter-based absorption photometer with a heated inlet

Long-term measurements of black carbon (BC) aerosols by filter-based absorption photometers with a heated inlet (COSMOS) in different regions have been useful in elucidating spatial variations and radiative impacts of BC. Evaluations of mass concentrations of BC (M_BC) measured by the COSMOS have been made by our previous studies through comparisons with other measurement techniques. However, how variations in the microphysical properties of BC and the co-existing light scattering aerosols affect the COSMOS measurements should be evaluated in more detail. In this study, we assessed these potential effects under various field environments in the Arctic and in the East Asia. From the slopes of the correlation plots between the M_BC values measured by the COSMOS and a single-particle soot photometer, the average accuracy of the COSMOS was estimated as ～10% in the M_BC range 1–3000?ng m^?3. On an hourly basis, the estimated sensitivity of the COSMOS M_BC values to the changes in the BC size distributions was less than 10%, within the typical variabilities of BC size at individual observation sites. The COSMOS measurements depended little on the mixing states of BC and the concentrations of co-existing light scattering aerosols, except in the maritime air masses of East Asia, where the relative abundance of sea salt to BC was very high. The M_BC measured by COSMOS also well agreed with elemental carbon measurements. Our results demonstrate the high reliability of COSMOS measurements under various environments.

Copyright © 2019 American Association for Aerosol Research     

Complex refractive index,single scattering albedo,and mass absorption coefficient of secondary organic aerosols generated from oxidation of biogenic and anthropogenic precursors

Refractive index and optical properties of biogenic and anthropogenic secondary organic aerosol (SOA) particles were investigated. Aerosol precursors, namely longifolene, α-pinene, 1-methylnaphthalene, phenol, and toluene were oxidized in a Teflon chamber to produce SOA particles under different initial hydrocarbon concentrations and hydroxyl radical sources, reflecting exposures to different levels of nitrogen oxides (NO_x). The real and imaginary components (n and k, respectively) of the refractive index at 375?nm and 632?nm were determined by Mie theory calculations through an iterative process, using the χ² function to evaluate the fitness of the predicted optical parameters with the measured scattering, absorption, and extinction coefficients from a Photoacoustic Extinctiometer and Cavity Attenuated Phase Shift Spectrometer. Single scattering albedo (SSA) and bulk mass absorption coefficient (MAC) at 375?nm were calculated. SSA values of SOA particles from biogenic precursors (longifolene and α-pinene) were ～0.98–0.99 (～6.3% uncertainty), reflecting purely scattering aerosols regardless of the NO_x regime. However, SOA particles from aromatic precursors were more absorbing and displayed NO_x-dependent SSA values. For 1-methylnaphthalene SOA particles, SSA values of 0.92–0.95 and ～0.75–0.90 (～6.1% uncertainty) were observed under intermediate- and high-NO_x conditions, respectively, reflecting the absorbing effects of SOA particles and NO_x chemistry for this aromatic system. In mixtures of longifolene and phenol or longifolene and toluene SOA under intermediate- and high-NO_x conditions, k values of the aromatic-related component of the SOA mixture were higher than that of 1-methylnaphthalene SOA particles. With the increase in OH exposure, k_phenol decreased from 0.10 to 0.02 and 0.22 to 0.05 for intermediate- and high-NO_x conditions, respectively. A simple relative radiative forcing calculation for urban environments at λ?=?375?nm suggests the influence of absorbing SOA particles on relative radiative forcing at this wavelength is most significant for aerosol sizes greater than 0.4?µm.

Copyright © 2019 American Association for Aerosol Research     

Simultaneous transmission and absorption photometry of carbon-black absorption from drop-cast particle-laden filters

Simultaneous transmissivity and absorptivity measurements were carried out in the visible at a laser wavelength of 532?nm on drop-cast, carbon-black-laden filters under ambient (laboratory) conditions. The focus of this investigation was to establish the feasibility of this approach to estimate the mass absorption coefficient of the isolated particles and compare results to earlier work with the same carbon-black source. Transmissivity measurements were carried out with a laser probe beam positioned normal to the particle-laden filter surface. Absorptivity measurements were carried out using a laser-heating approach to record in time the sample temperature rise to steady-state and decay back to the ambient temperature. The sample temperature was recorded using a fine-wire thermocouple that was integrated into the transmission arrangement by placing the thermocouple flush with the filter back surface. The advantage of this approach is that the sample absorptivity can be determined directly (using laser heating) instead of resolving the difference between reflectivity (filter surface scattering) and transmissivity. The current approach also provides the filter optical characteristics, as well as an estimate of filter effects on the absorption coefficient due to particle absorption enhancement or shadowing. The approach may also be incorporated into other filter-based techniques, like the particle/soot absorption photometer, with the simple addition of a thermocouple to the commercial instrument. For this investigation, measurements were carried out with several blank uncoated quartz filters. A range of solution concentrations was prepared with a well-characterized carbon black in deionized water (i.e., a water-soluble carbonaceous material referred to as a surrogate black carbon or ‘carbon black’). The solution was then drop cast using a calibrated syringe onto blank filters to vary particle loading. After evaporation of the water, the measurements were repeated with the coated filters. The measurement repeatability (95% confidence level) was better than 0.3?K for temperature and 3?×?10^?5 mW for laser power. From the measurements with both the blank and coated filters, the absorption coefficient was determined for the isolated particles. The results were then compared with an earlier investigation by You et al. and Zangmeister and Radney, who used the same carbon-black material. The measurements were also compared with Lorenz–Mie computations for a polydispersion of spherical particles dispersed throughout a volume representative of the actual particles. The mass absorption coefficient for the polydispersion of carbon-black particles was estimated to be about 7.7?±?1.4?m² g^?1, which was consistent with the results expected for these carbon black particles.     

A refractive-index and position-independent single-particle detector for large,nonabsorbing, spherical particles

We show that for spherical particles with real refractive index and diameters greater than ca. 10 microns, the differential scattering cross-section is only independent of the refractive index at angles near 37?±?5°. We built a device with a modified Gaussian incident beam profile so that the beam transit time of a particle passing through the beam can determine the true incident intensity for the scattering of the particle. By combining the modified Gaussian incident beam profile with detection of scattered light near 37?±?5°, we demonstrate a refractive-index independent measurement of single spherical particles as they pass through the beam.

Copyright © 2018 American Association for Aerosol Research     

Multiparametric optical characterization of airborne dust with single particle extinction and scattering

Abstract

We describe a robust, portable, deployable instrument for multiparametric optical characterization of single airborne particles. It is based on the Single Particle Extinction and Scattering method with additional sensors at 45° and 90° angles. Four independent optical parameters are associated to each particle. Basically, it provides a rigorous measurement of the extinction cross section and the complex amplitude of the forward scattered field. Moreover, thanks to the multiparametric single particle approach, it is possible to roughly classify the particles within a size range from a few hundreds of nanometers to some micrometers. By assigning a reasonable single scattering albedo for each population, our data are enough to fit the phase function with acceptable uncertainties. We report here the results of tests performed with water droplets, generating well controlled data without any free parameters. Data analysis is described in detail. We also report measurements performed on urban aerosol collected in the city of Milan by recovering the optical properties and feeding radiative transfer models. The findings reported here support the importance of an accurate measurement of the phase function, as already established by the Community.

Copyright © 2020 American Association for Aerosol Research     

Measuring ultrafine aerosols by direct photoionization and charge capture in continuous flow

Direct ultraviolet (UV) photoionization enables electrical charging of aerosol nanoparticles without relying on the collision of particles and ions. In this work, a low-strength electric field is applied during particle photoionization to capture charge as it is photoemitted from the particles in continuous flow, yielding a novel electrical current measurement. As in conventional photocharging-based measurement devices, a distinct electrical current from the remaining photocharged particles is also measured downstream. The two distinct measured currents are proportional to the total photoelectrically active area of the particles. A three-dimensional numerical model for particle and ion (dis)charging and transport is evaluated by comparing simulations of integrated electric currents with those from charged soot particles and ions in an experimental photoionization chamber. The model and experiment show good quantitative agreement for a single empirical constant, K_cI, over a range of particle sizes and concentrations providing confidence in the theoretical equations and numerical method used.

Copyright © 2018 American Association for Aerosol Research     

Particle aggregation and capture by walls in a particulate aerosol channel flow

A soft-sphere discrete-element method is used to examine particle aggregate formation and particle capture by walls in a laminar channel flow. Adhesive particulates have been identified as a leading cause of failure in many different microfluidic devices, including those currently being developed by different research groups for rapid biological and chemical contaminant sensing, fluid drag reduction, etc. As these microfluidic devices enter into the marketplace and become more extensively used in field conditions, the importance of particle adhesion and clogging will increasingly limit the reliability of such systems. At a larger scale, clogging of vehicle radiators by small adhesive particles is currently a major problem for construction vehicles operating in various environmental conditions and soil types. Cooling system fouling leads to the need for frequent maintenance and machine down time. Dust fouling of equipment is also of concern for potential human occupation on dusty planets, such as Mars. The paper provides a detailed investigation of the fundamental mechanics leading to adhesion of particle aggregates to channel walls, which involves a combination of aggregate capture, aggregate deformation by particle rolling, and shearing of aggregates from the wall. Cases with different adhesion potential, particle sizes, and flow Reynolds number are examined, with both single-size particles and a binary particle mixture.     

Towards a determination of the active surface area of polycrystalline and nanoparticle electrodes by Cu upd and CO oxidation

Methods for the determination of the surface area for Pt, Ru and Se modified Pt and Ru are compared, in view of their possible application for technical and nanoparticle electrodes. The hydrogen adsorption charge can hardly be used as a reliable measure for the surface area for Ru because it is paralleled by anion adsorption. The charge necessary for the oxidation of adsorbed CO also contains a large contribution due to anion or oxygen adsorption, which amounts to approx. 45% of the charge in the case of Ru. The mass spectrometrically determined amount of CO₂ formed gives a more reliable measure for the surface area, provided that the maximum coverages are constant and independent of the particular surface. Values obtained in this way agree to within 20% with surface area values obtained from measuring the charge needed for the desorption of a complete monolayer of Cu upd on Pt(111) and polycrystalline Pt, polycrystalline Ru, submonolayers of Ru on polycrystalline Pt and on Pt(111) and for nanoparticle, carbon supported electrodes. Se modified Ru has recently found attention as a methanol tolerant cathode material for oxygen reduction. CO does not adsorb on Pt or Ru saturated by Se. For surfaces partially covered by Se, a comparison of the charge measured by cyclic voltammetry in the hydrogen region and of the mass spectrometrically determined amount of CO₂ suggests that the latter can be used for a determination of the area not covered by Se. Cu upd, on the other hand, also takes place on surfaces completely covered by Se; the Cu desorption charge is independent of the Se coverage on Pt and Ru modified Pt as long as it does not exceed 70% of full coverage. In the presence of multilayers of Se, Cu _x Se is formed. On Se modified bulk Ru the amount of Cu upd decreases with increasing Se coverage, approaching only 105 μC m⁻² for full Se coverage.     

直接滴定法测定甜菜碱产品中活性物和游离叔胺的含量

研究了用盐酸-异丙醇直接滴定法和两相滴定法分别测定烷基羧基甜菜碱样品中游离叔胺含量和活性物含量。结果表明,用盐酸-异丙醇直接滴定法测定游离叔胺,对双十二烷基甲基羧基甜菜碱样品滴定终点为浅黄色,与用标准盐酸溶液滴定单纯的叔胺产品相同,但对十六烷基二甲基羧基甜菜碱样品,滴定终点为蓝绿色而非黄色,以黄色为终点将高估叔胺含量。与滴定单纯的叔胺产品相比,大量甜菜碱的存在使滴定过程中pH的突跃消失,体系pH随滴定体积的增加缓慢下降,但滴定单位质量叔胺所消耗的盐酸-异丙醇的体积不变。由于指示剂颜色亦呈现缓慢的变化,仍可以根据指示剂颜色变化来确定终点。终点颜色不随叔胺含量而变化,但可能因甜菜碱的分子结构而异。在酸性条件下,叔胺与甜菜碱都显示阳离子性质,可以用两相滴定法测出两者总量。于是在用盐酸-异丙醇直接滴定法独立确定样品中游离叔胺的情况下,通过两相滴定即可测出样品中甜菜碱活性物含量。     

Lipase purification by various techniques and its thermostabilization in presence of surface active additives

Crude porcine pancreatic lipase was purified by removing water-insoluble impurities (residual lipid material) associated with it by a conventional method, using hydrotropic additives and by liquid coacervate extraction. Maximum yield with good recovery of activity was obtained when hydrotropes were used to separate the associated lipids from lipase. The thermostability of the enzyme was also checked in the solutions of additives such as sodium butyl monoglycol sulfate (Na-BMGS), proline and Triton X-114. In Na-BMGS solutions above a concentration of 0·2 mol dm⁻³ the lipase activity decreased beyond 50°C whereas in 1 mol dm⁻³ proline solution it was retained even at 80°C, showing a good thermostabilizing effect. However, in the presence of Triton X-114 the enzyme was completely inactivated with increase in temperature. © 1998 SCI.