Secondary Organic Aerosol Coating Formation and Evaporation: Chamber Studies Using Black Carbon Seed Aerosol and the Single-Particle Soot Photometer

We report a protocol for using black carbon (BC) aerosol as the seed for secondary organic aerosol (SOA) formation in an environmental chamber. We employ a single-particle soot photometer (SP2) to probe single-particle SOA coating growth dynamics and find that SOA growth on nonspherical BC aerosol is diffusion-limited. Aerosol composition measurements with an Aerodyne high resolution time-of-flight aerosol mass spectrometer (AMS) confirm that the presence of BC seed does not alter the composition of SOA as compared to self-nucleated SOA or condensed SOA on ammonium sulfate seed. We employ a 3-wavelength photoacoustic soot spectrometer (PASS-3) to measure optical properties of the systems studied, including fullerene soot as the surrogate BC seed, nucleated naphthalene SOA from high-NO_x photooxidation, and nucleated α-pinene SOA from low-NO_x photooxidation. A core-and-shell Mie scattering model of the light absorption enhancement is in good agreement with measured enhancements for both the low- and high-NO_x α-pinene photooxidation systems, reinforcing the assumption of a core-shell morphology for coated BC particles. A discrepancy between measured and modeled absorption enhancement factors in the naphthalene photooxidation system is attributed to the wavelength-dependence of refractive index of the naphthalene SOA. The coating of high-NO_x α-pinene SOA decreases after reaching a peak thickness during irradiation, reflecting a volatility change in the aerosol, as confirmed by the relative magnitudes of f₄₃ and f₄₄ in the AMS spectra. The protocol described here provides a framework by which future studies of SOA optical properties and single-particle growth dynamics may be explored in environmental chambers.

Copyright 2013 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     

Calibration and Intercomparison of Filter-Based Measurements of Visible Light Absorption by Aerosols

Data on light absorption by atmospheric particles are scarce relative to the need for global characterization. Most of the existing data come from methods that measure the change in light transmission through a filter on which particles are collected. We present a calibration of a recently developed filter-based instrument for continuous measurement of light absorption (model PSAP, Radiance Research, Seattle, WA) that has been incorporated in several measurement programs. This calibration uses a reference absorption determined as the difference between light extinction and light scattering by unaltered (suspended) particles. In addition, we perform the same calibration for two other common filter-based methods: an Integrating Plate and the Hybrid Integrating Plate System. For each method, we assess the responses to both particulate light scattering and particulate light absorption. We find that each of the instruments exhibits a significant response to nonabsorbing aerosols and overestimates absorption at 550 nm by suspended particles by about 20-30%. We also present correction factors for the use of the PSAP.     

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 portable,four-wavelength,single-cell photoacoustic spectrometer for ambient aerosol absorption

Aerosols directly affect Earth's climate by scattering and absorbing solar radiation. Although they are ubiquitous in Earth's atmosphere, direct, in situ, wavelength-resolved measurements of aerosol optical properties remain challenging. As a result, the so-called aerosol direct effects are one of the largest uncertainties in predictions of Earth's future climate, and new instrumentation is needed to provide measurements of the absorption of sunlight by atmospheric particles. We have developed a portable, four-wavelength, single-cell photoacoustic spectrometer for simultaneous measurement of aerosol absorption at 406, 532, 662, and 785 nm, with an additional extinction measurement at 662 nm via a built-in cavity ringdown spectrometer. The instrument, dubbed MultiPAS-IV, is compact, robust, has low power requirements, and utilizes a multipass optical arrangement to achieve typical detection limits of 0.6–0.7 Mm^?1 for absorption (2σ, 2-min average). Tests with nigrosin aerosols show agreement with Mie theory calculations to within 2%, and comparison with a 7-wavelength aethalometer shows good correlation for ambient (Athens, GA, USA) aerosols. We demonstrate the utility of the broad spectral coverage and sensitivity of the MultiPAS-IV for calculating the absorption Ångström exponent of black carbon (AAE_BC, median value of 0.70) in ambient aerosols and use this value to derive the brown carbon contributions to absorption at 406 nm (43%) and 532 nm (13%) and its wavelength dependence (AAE_BrC = 6.3).

Copyright © 2018 American Association for Aerosol Research     

Size-dependent correction factors for absorption measurements using filter-based photometers: PSAP and COSMOS

Filter-based absorption photometers have been widely used to measure mass concentrations of black carbon (BC) by measurement of the absorption coefficient of BC. In these techniques, correction for the effect of multiple scattering by the filter medium is necessary, even if only BC particles are extracted by evaporating co-existing volatile compounds using a heated inlet. The correction depends on particle size, because it varies with the aerosol penetration depth into the filter. The size dependence has not, however, been taken into account in previous studies. For the first time, we quantify the particle size dependence of the sensitivities of two filter-based photometers, PSAP and COSMOS, using mono-disperse nigrosin particles, which were generated by the combination of a differential mobility analyzer and an aerosol particle mass analyzer. At diameters smaller than 200 nm, the absorption coefficients measured by PSAP and COSMOS were much larger than those calculated by Mie theory. The size-dependent correction factors for PSAP and COSMOS are determined by comparing the observed absorption coefficients at a flow rate of 0.7 standard liter per minute with those calculated by Mie theory. The correction factors to the mass absorption cross-section are also estimated for typical size distributions of ambient black carbon particles. The new factors reduce the mass absorption cross-sections measured by PSAP and COSMOS by 28–36% for typical ambient black carbon particles observed with an inlet heated to 400 °C.     

A Newly Designed and Constructed Instrument for Coupled Infrared Extinction and Size Distribution Measurements of Aerosols

Although atmospheric particles are often non-spherical, Mie theory is commonly used to acquire aerosol optical depth, composition, and transport information from satellite retrievals. In the infrared (IR) region, the radiative effects of aerosols, usually modeled with Mie theory, are subtracted from satellite spectral data to determine key atmospheric and oceanic properties. To gain a better understanding of the infrared radiative effects of aerosols and the methods used to model them, an instrument has been designed to simultaneously measure infrared extinction spectra and particle size distributions obtained from a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS). Infrared extinction spectra are simulated with Mie theory using the measured particle size distributions and available literature optical constants. As a result, the errors associated with using Mie theory to model the infrared extinction of mineral dust aerosol can be quantitatively examined. Initial results for this instrument are presented here. For ammonium sulfate, the Mie theory simulation is in good agreement with our measured extinction spectrum. This is in accordance with the nearly spherical shape of ammonium sulfate particles. However, for illite, an abundant clay mineral, there is poor agreement between the experimental spectrum and the Mie simulation. This result is attributed to particle shape effects.     

An intercomparison of aerosol absorption measurements conducted during the SEAC4RS campaign

During the SEAC⁴RS campaign in 2013, inflight measurements of light-absorption by aerosol in biomass burning and agriculture fire plumes were collected along with concomitant measurements of aerosol extinction, scattering, and black carbon mass concentration. Here, we compare three measurements of aerosol absorption coefficients: from a photoacoustic spectrometer (PAS), a particle soot absorption photometer (PSAP), and a continuous light absorption photometer (CLAP). Each of these absorption measurements was collected in three visible spectral regions: red, green, and blue (although the precise wavelength and bandwidth vary with each instrument). The absorption measurements were compared during the plumes, in the boundary layer, and in the free troposphere. The slopes from the comparison ranged from 0.6 to 1.24. For biomass burning plumes, the uncertainty in the absorption measurements translates into a range in single scattering albedos of 0.93–0.94 at a wavelength of 660?nm, 0.94–0.95 at 532?nm and 0.92–0.95 at 405?nm. Overall, the aerosol absorption instruments agreed within their stated accuracies. Comparisons with simultaneous measurements of refractive black carbon mass concentration (collected by a single particle soot photometer), were used to derive the mass absorption coefficients (MAC). For all wavelengths, the MAC was high by greater than a factor of three compared to the expected MAC for black carbon.

© 2018 American Association for Aerosol Research     

Retrieval of Aerosol Complex Refractive Index by Combining Cavity Ring Down Aerosol Spectrometer Measurements with Full Size Distribution Information

Cavity ring down aerosol extinction measurements are combined with size distribution measurements to provide a multi-parameter basis for the retrieval of the aerosol complex refractive index. We show that two distinct size distributions of small particles (< 300 nm) suffice to obtain robust convergence of the Mie theory fit of the extinction function. Experiments are performed both for purely scattering and for absorbing aerosol. Thus this method provides a perspective to use cavity ring down aerosol spectroscopy in field and laboratory measurements that often suffer from low particle concentrations and a lack of large particles.     

Estimate of scattering truncation in the cavity attenuated phase shift PMSSA monitor using radiative transfer theory

The recently developed cavity attenuated phase shift particulate matter single scattering albedo (CAPS PM_SSA) monitor has been shown to be fairly accurate and robust for real-time aerosol optical properties measurements. The scattering component of the measurement undergoes a truncation error due to the loss of scattered light from the sample tube in both the forward and backward directions. Previous studies estimated the loss of scattered light typically using the Mie theory for spherical particles, assuming particles are present only on the sampling tube centerline, and without accounting for the effects of sampling tube surface reflection. This study overcomes these limitations by solving the radiative transfer equation in an axisymmetric absorbing and scattering medium using the discrete-ordinates method to estimate the scattering truncation error. The effects of absorption coefficient, scattering coefficient, asymmetry parameter of the scattering phase function, and the reflection coefficient at the sampling tube inner surface were investigated. Under typical conditions of CAPS PM_SSA operation of low extinction coefficients below about 5000 Mm^?1, the scattering loss remains independent of the absorption and scattering coefficients but is dependent on the scattering phase function and the reflection coefficient of the sampling glass tube inner surface. The proposed method was used to investigate the effects of asymmetry parameter and surface reflection coefficient on truncation for absorbing aerosol particles whose scattering phase function can be well represented by the Henyey-Greenstein approximation. The scattering loss increases with increasing the asymmetry parameter and the surface reflection coefficient.

Copyright © 2018 National Research Council Canada     

Ultraviolet Extinction and Visible Transparency by Ivy Nanoparticles

Though much research has been conducted for nanoparticles, naturally occurring nanoparticles have not yet been well explored for their diverse properties and potential applications. This paper reports the optical absorption and scattering properties of nanoparticles secreted by English ivy. Both experimental and theoretical studies have been conducted. Strong ultraviolet extinction and excellent visible transparency are observed, compared to the inorganic TiO₂ and ZnO nanoparticles at similar concentrations. The contributions of absorption and scattering to the total extinction are quantified by simulation of the Mie scattering theory.     

烟气膜吸收法脱除SO2的超声波强化处理

本文提出了以柠檬酸盐溶液为吸收剂、中空纤维膜组件为脱除SO₂的基本设备,并配有超声波强化吸收的技术模式,首次将超声波和膜技术应用于烟气脱硫过程。重点探讨了超声空化作用及其促进传质过程的机理,利用自主设计的超声膜吸收器进行了烟气脱硫研究,同时分析了超声对吸收液和中空纤维稳定性的影响。研究结果表明：在超声频率为20kHz,柠檬酸盐溶液浓度为0.5mol&#8226;L^-1, pH值为4.5的条件下,超声波对膜吸收SO₂有明显的促进作用,对吸收液温度有加热的副作用,同时超声波的加入对吸收液和膜材料的稳定性没有影响。     

Optical properties of black carbon in cookstove emissions coated with secondary organic aerosols: Measurements and modeling

Cookstoves are a major source of black carbon (BC) particles and associated organic compounds, which influence the atmospheric radiative balance. We present results from experiments that characterize BC emissions from a rocket stove coated with secondary organic aerosol. Optical properties, namely, BC mass absorption cross-section (MAC_BC) and mass scattering cross-section (MSC), as a function of the organic-to-black carbon ratio (OA:BC) of fresh and aged cookstove emissions were compared with Mie and Rayleigh–Debye–Gans (RDG) calculations. Mie theory reproduced the measured MAC_BC across the entire OA:BC range. However, Mie theory failed to capture the MSC at low OA:BC, where the data agreed better with RDG, consistent with a fractal morphology of fresh BC aggregates. As the OA:BC increased, the MSC approached Mie predictions indicating that BC-containing particles approach a core–shell structure as BC cores become heavily coated. To gain insight into the implications of our findings, we calculated the spectral simple forcing efficiency (dSFE) using measured and modeled optical properties as inputs. Good agreement between dSFE estimates calculated from measurements and Mie-modeled dSFE across the entire OA:BC range suggests that Mie theory can be used to simulate the optical properties of aged cookstove emissions.

Copyright © 2016 American Association for Aerosol Research     

Large-area uniform hydrogen-free diamond-like carbon films prepared by unbalanced magnetron sputtering for infrared anti-reflection coatings

The hydrogen-free diamond-like carbon (DLC) films are potential materials to be used as infrared anti-reflection protective coatings if their optical absorption can be reduced to get relatively thick films needed. In this study, hydrogen-free DLC films were deposited by the physical vapor deposition (PVD) method in an unbalanced magnetron sputtering (UBMS) system with a rectangle graphite target of 440 × 80 mm in the argon atmosphere. The UBMS system was described in detail and the magnetron field distribution of the target was denoted in this work. The film thickness uniformity was investigated and the results showed that this system is capable of depositing uniform films larger than 150 mm in diameter. The infrared transmission spectra of DLC films were analyzed by a FTIR spectrometer, the results indicating that transparent films were obtained in the infrared region for the single side DLC coated on the silicon and germanium substrates, and about 68.83% and 63.05% transmittance were achieved respectively at the wave number of 2983 /cm, close to theoretical value for non-absorption carbon material. No obvious absorption peaks were found between 5000 and 800 /cm. The refractive index and extinction coefficient of the DLC films deposited under optimized conditions were about 2.08 and 0.067 respectively at the wavelength of 1600 nm. These important optical characteristics showed that the hydrogen-free DLC films prepared in the UBMS system were suitable for infrared transmission enhancement applications.     

Measurement of aerosol size distribution function using Mie scattering—Mathematical considerations

This paper analyzes the applications of Mie scattering to measure the size distribution function (SDF) of aerosols. Measurement of SDF by Mie scattering usually involves solving the Fredholm integral equations of the first kind based on discrete inputs with uncertainties (e.g., extinction measurements at multiple wavelengths or at multiple angles). A set of inputs which are not mutually independent within the measurement error implies that redundancy exists in the measurements and not all the measurements provide useful information for solving the integral equations. To avoid such redundancy, this paper develops a method to analyze the dependency among the kernel functions associated with Mie scattering. Applications of this method are demonstrated and the results provide valuable insights into the optimization of SDF measurement based on Mie scattering, in terms of minimizing the number of measurements needed and revealing the optimal wavelengths and angles to perform the measurements.     

Absorption and Scattering Properties of Dust Clouds at 10.5-μm

Mass-density-normalized absorption and extinction coefficients for arid region soil-based dust were measured at a wavelength of 10.5 μm using photoacoustical techniques, short-path transmissometry, and aerosol dosimetry. An environmental chamber incorporating strong circulation, as well as the various aerosol sampling systems, was specifically designed for aerosol size distributions with particles as large as 40 μm in radius.

The mass extinction coefficient was found to be 0.22 m²/g, while the single scattering albedo, determined from the absorption and extinction coefficients, was 0.5. Calculations of these properties were based on two approaches: analyses of size distributions from photomicrographs of filter samples and analyses of the results obtained using a mixed-medium settling theory. In both cases, Mie theory was applied despite the clearly irregular particle forms. Agreement was close to the measured value for both approaches. The expected overestimation of the optical properties for the former model did not occur. Larger particles in this range are included because of their relevance to arid region dust clouds.     

Optical and Optoelectronic Properties Enhancement of Porous Silicon Treated with Indium Oxide

This paper presents the results of the effect of Indium oxide (In2O3) on the structural, optical, and optoelectronic properties of porous silicon (PS). The results show an important improvement of optoelectronic property of PS coated with In2O3 as a antireflective thin film. The treated PS with In2O3 thin film was thermally annealed at various temperatures to improve the efficiency of the photovoltaic cells. The deposition of In2O3 onto the PS sample was performed by simple immersion method. Surface morphology and chemical composition modification of the samples In2O3/PS were analyzed by SEM, EDX, and FTIR spectroscopy. Total reflectivity of In2O3/PS layers decreases significantly compared to as-prepared PS owing to an improvement in the light absorption. The PS treated with In2O3 shows a significant enhancement in the minority carrier lifetime (τeff) indicating an improved surface quality in comparison with the untreated PS. Photoluminescence (PL) measurements of PS layer treated with In2O3 has revealed an increase in PL-intensity and a bleu shift in emission PL band as a function of annealing temperature. The PL and τeff enhancement are due to the surface passivation improvement and the decrease of recombination process.     

Bias in Filter-Based Aerosol Light Absorption Measurements Due to Organic Aerosol Loading: Evidence from Laboratory Measurements

Light absorption by soot or nigrosin dye aerosol particles were measured in the laboratory using a particle soot absorption photometer (PSAP) and a photo-acoustic spectrometer (PAS) to assess the influence of non-absorbing organic aerosol (OA) on the PSAP measurements. For the PSAP, particle light absorption is measured after collection on a filter, whereas for the PAS light absorption is measured while the particles remain suspended in the gas phase. OA was generated from the reaction of α -pinene with ozone. It was observed that the presence of this OA in an external mixture of absorbing aerosol and OA can cause an increase in the light absorption measured by the PSAP, relative to that measured by the PAS, by more than a factor of two. This enhancement in the PSAP absorption was found to increase as the amount of OA increased relative to the absorbing compound. Additionally, experiments where absorbing aerosol was deposited on a PSAP filter prior to addition of OA demonstrated that the non-absorbing OA can actually appear as if it were absorbing, with measured single scattering albedo values as low as 0.92. These results indicate that filter-based measurement techniques may significantly overestimate light absorption by aerosols in the atmosphere under conditions where the organic loading is large, with consequent implications for understanding and calculating the Earth's radiation budget. These laboratory experiments aid in the interpretation of results from a recent field study, discussed in a companion article (Lack et al. 2008 Lack, D. A., Cappa, C. D., Baynard, T., Massoli, P., Covert, D. S., Sierau, B., Bates, T. S., Quinn, P. K., Lovejoy, E. R. and Ravishankara, A. R. 2008. Bias in Filter-Based Aerosol Absorption Measurements Due to Organic Aerosol Loading: Evidence from Ambient Measurements. Aerosol Sci. and Technol., 42: 1033–1041. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar]).     

The Influence of Water Coating on the Optical Scattering Properties of Fractal Soot Aggregates

The effect of water coating of constituent monomers on the optical single-scattering properties of fractal soot aggregates is investigated numerically using core-mantle theory and approximations involving two effective medium theories. A cluster–cluster aggregation algorithm is used to numerically generate fractal aggregates, and the core-mantle Generalized Multi-particle Mie (GMM) method is used to compute the exact single-scattering properties of soot aggregates with water-coated monomers. Comparisons are then made with results obtained using approximations that combine either the GMM method or the Rayleigh–Debye–Gans (RDG) method with either the Maxwell-Garnett or the Bruggeman effective medium approximation (a total of four approximation methods). The optical properties calculated are the extinction and absorption cross sections, the single-scattering albedo, and the phase matrix of water-coated fractal aggregates; these calculations are done for two wavelengths, 0.628 μm and 1.1 μm. Water coating of the fractal aggregates is shown to increase the extinction and absorption cross sections, the single-scattering albedo, and forward scattering, but decrease backward scattering. The combination GMM + Maxwell-Garnett gives approximations that are quite good over a range of coating thicknesses and aggregate size. The combination GMM + Bruggeman performs less well, overestimating the extinction and absorption cross sections and underestimating the single-scattering albedo. In the case of RDG, the better combination is with the Bruggeman approximation, but the errors involved are greater than with the GMM + Maxwell-Garnett combination. Results from simple idealized calculations indicate that the differences between results with the Maxwell-Garnett and Bruggeman approximations should be even larger in cases of aerosol cores that are less absorptive than soot.

Copyright 2012 American Association for Aerosol Research