Size Calibration Corrections for the Active Scattering Aerosol Spectrometer Probe (ASASP-100X)

The response of the active scattering aerosol spectrometer probe (ASASP-100X) is affected by the optical properties of measured particles. Response functions of the ASASP-100X probe were calculated for different complex refractive indices corresponding to different types of atmospheric aerosol particles under various relative humidity conditions. Based on these response functions, corrected calibration bin diameters were determined for 15 size channels at six relative humidity values (0%, 50%, 70%, 80%, 90%, and 99%) and three typical aerosol types (rural, urban, and maritime). Sample calculations with these corrected calibration data show that a significant underestimation of the aerosol volume distribution can result if uncorrected manufacturer's size calibration data are used.     

Response of Particle Measuring Systems Airborne ASASP and PCASP to NaCl and Latex Particles

The response of a Particle Measuring Systems, Inc. (PMS) airborne Passive Cavity Aerosol Spectrometer Probe (PCASP)-100X and a PMS airborne Active Scattering Aerosol Spectrometer Probe (ASASP)-100X to nearly monodisperse aerosols of NaCl and polystyrene latex spheres in the size range 0.074–1.07 μm diameter (± 5%) are examined. Particles < 0.34 μm are size classified by electrical mobility using a Thermo System Inc. Electrostatic Classifier. The particles are also sized with the aid of a scanning electron microscope. Three equivalent diameters for the near cubic NaCl particles are considered: average of length and breadth, and cross section equivalent and volume equivalent diameters. For the linear diameter, the probes' response to latex and NaCl particles is quite similar. However, the use of cross section and volume equivalent diameter leads NaCl to be sized significantly less than the latex spheres consistent with reported differences between spherical (i.e., latex) and cubical geometry (i.e., NaCl). Using the linear diameter the lower detection limit of the ASASP-100X is found to be 0.175 μm ±5%, not 0.120 μm as given by the manufacturer. The lower detection limit of the PCASP-100X is found to be 0.125 μm ±5%, compared with the value of 0.10 μm suggested by PMS. In spite of this discrepancy the PCASP still represents a significant advancement for the airborne measurement of aerosol size distributions. Size distributions measured simultaneously with the two probes from an aircraft agreed more favorably with the new calibrations than with those of the manufacturer.     

Size Measurements of Latex Particles by Laser Aerosol Spectrometer

Size measurements of PSL (polystyrene latex) particles in a size range from 0.109 to 0.330 μm were made by laser aerosol spectrometer (PMS, LAS-X). The results were compared with those by electron microscopy. For example, the geometric standard deviation, σ_g, of nominally 0.176-μm PSL particles was measured as 1.05, assuming that their sizes distribute log-normally. The value of 1.05 was very close to 1.02 measured by electron microscopy. It was found that the spectrometer had very high size resolution, although the size resolution of the light scattering type spectrometer has been said to be poor. For some samples of PSL particles, however, there were large differences between particle sizes measured by LAS-X and those by electron microscopy. It was also found that LAS-X had a problem in calibration of size response curve.     

Counting efficiencies of six commercial particle counters

Counting efficiencies of a condensation nuclei counter (TSI 3020), a white-light optical particle counter (Climet CI-8060) and four laser instruments (PMS LAS-X, LAS-250X, LPC 525 and HP-LAS) were determined relative to the LAS-X.

Measurements were made in the geometric diameter range of 0.1–4 μm using latex spheres as well as monodisperse organic and inorganic particles produced by a vibrating orifice generator.

The high-pressure in-line counter (HP-LAS) shows a nonlinear response to gas velocity which can be taken into account by a calibration. The lower detection limits (50% points) of the conventional laser counters (LAS-X and LAS-250X) agree within 0.05 μm with their nominal specifications; for the white-light counter (CI-8060) the actual lower limit is at about 0.4 μm. For all counters, the degree of inlet losses for larger particles varies greatly with inlet design and flow velocity of each counter.     

Calibration Correction of an Active Scattering Spectrometer Probe to Account for Refractive Index of Stratospheric Aerosols: Comparison of Results with Inertial Impaction

Real-time particle size spectra are being acquired on our research aircraft with relative ease and speed by techniques that make use of the real-time interaction of laser light with aerosols and cloud droplets. The results are, however, sometimes ambiguous, because the optical “signatures” of the particles depend on their refractive indices in addition to physical dimensions. The calibration supplied by the manufacturer is based on instrument response to a specific test aerosol, e.g., latex spheres (refractive index = 1.59). Such a calibration is strictly valid only for sample aerosols of refractive index and shape similar to the test aerosol. Whenever the sample aerosol differs from the test aerosol, a calibration correction is in order. Of concern here is the use of an active scattering spectrometer probe (ASAS-X), to measure sulfuric acid aerosols on high-flying U-2 and ER-2 research aircraft. Correcting the calibration of the ASAS-X for dilute sulfuric acid droplets (refractive index = 1.44) that predominate the stratospheric aerosol changes the inferred sizes by up to 32% per size interval from that determined from the nominal calibration. This results in an average increase in particle surface area and volume of 42 ± 10% and 71 ± 19%, respectively. The calibration correction of the optical spectrometer probe for stratospheric aerosol is validated by independent and simultaneous sampling of the particles with impactors. Sizing and counting of particles on microphotographs of scanning electron microscope images give results on total particle surface areas and volumes. After the calibration correction, the optical spectrometer data (averaged over four size distributions) agree with the impactor results (similarly averaged) to within a few percent. We conclude that the optical properties, or chemical makeup, of the sample aerosol must be known for accurate size analysis by optical aerosol spectrometers.     

Optimizing Thermal-Optical Methods for Measuring Atmospheric Elemental (Black) Carbon: A Response Surface Study

The chemical, physical, and morphological complexity of atmospheric aerosol elemental carbon (EC) presents major problems in assuring measurement accuracy. Since EC and black carbon are often considered equivalent, methods based on thermal-optical analysis (TOA) are widely used for EC in ambient air samples because no prior knowledge of the aerosol's absorption coefficient is required. Nevertheless, different TOA thermal desorption protocols result in wide EC-to-total-carbon (TC) variation. We created three response surfaces with the following response variables: EC/TC, maximum laser attenuation in the He phase ( L max ), and laser attenuation at the end of the He phase ( L He4 ). A two-level central-composite factorial design comprised of four factors considered the temperatures and durations of all desorption steps in TOA's inert (He) phase and the initial step in TOA's oxidizing (O 2 -He) phase. L max was used to assess the positive bias caused by nonvolatile unpyrolized organic carbon (OC char) being measured as native EC. A negative bias that the attenuated laser response does not detect is caused by the loss of native EC in the He phase. L He4 was used as a surrogate indicator for the loss of native EC in the He phase. The intersection between the L max and L He4 surfaces revealed TOA conditions where both the production of OC char in the He phase was maximized and the loss of native EC in the He phase was minimized, therefore leading to an optimized thermal desorption protocol. Based on the sample types used in this study, the following are generalized optimal conditions when TOA is operated in the fixed-step-durations, laser-transmission mode (i.e., TOT): step 1 in He, 190°C for 60 s; step 2 in He, 365°C for 60 s; step 3 in He, 610°C for 60 s; step 4 in He, 835°C for 72 s. For steps 1-4 in O 2 -He, the conditions are 550°C for 180 s, 700°C for 60 s, 850°C for 60 s, and 900°C for 90 s to 120 s, respectively.     

Characterization of an Automated,Water-Based Expansion Condensation Nucleus Counter for Ultrafine Particles

The design and experimental characterization of a condensation nucleus counter (CNC) is presented. The counter produces supersaturation by means of fast volume-controlled adiabatic expansion. The aerosol number concentration is derived from observing scattered laser light in the forward direction under a solid angle between 1.1° and 4.4° over the full annular sector. The number concentration is derived by application of Mie theory from the characteristic pattern in the temporal evolution of the detected signal during the droplet growth process. The equation for calculation of the aerosol number density by this method is presented. Theoretical considerations for the smallest aerosol particles that can be activated indicate a lower size cut-off between 2.5 and 3.0 nm. Model calculations of the expected Mie scatter signal during expansion agree very well with the experimental observations. The Expansion-CNC can be operated fully automated under computer (PC) control in 10-second sample cycles. For characterization it is compared with a TSI 3025A Ultrafine-CPC (TSI UCPC) for measurements of monodisperse sodium chloride and sulfuric acid aerosol particles, indicating good agreement between the two counters down to particle sizes as low as 3.5 nm under laboratory conditions. In addition, ambient aerosol measurements in urban air show excellent agreement with simultaneous TSI UCPC measurements for particle number concentrations ranging from roughly 50 cm^{? 3} to 130000 cm^{? 3}.     

The Detection Efficiency of the Single Particle Soot Photometer

A single particle soot photometer (SP2) uses an intense laser to heat individual aerosol particles of refractory black carbon (rBC) to vaporization, causing them to emit detectable amounts of thermal radiation that are used to quantify rBC mass. This approach is well suited for the detection of the majority of rBC mass loading in the ambient atmosphere, which occurs primarily in the accumulation mode (～ 1–300 fg-rBC/particle). In addition to operator choices about instrument parameters, SP2 detection of rBC number and/or mass can be limited by the physical process inherent in the SP2 detection technique — namely at small rBC mass or low laser intensities, particles fail to heat to vaporization, a requirement for proper detection. In this study, the SP2's ability to correctly detect and count individual flame-generated soot particles was measured at different laser intensities for different rBC particle masses. The flame-generated soot aerosol used for testing was optionally prepared with coatings of organic and non-organic material and/or thermally denuded. These data are used to identify a minimum laser intensity for accurate detection at sea level of total rBC mass in the accumulation mode (300 nW/(220-nm PSL)), a minimum rBC mass (～ 0.7-fg rBC-mass corresponding to 90 nm volume-equivalent diameter) for near-unity number detection efficiency with a typical operating laser intensity (450 nW/(220-nm PSL)), and a methodology using observed color temperature to recognize laser intensity insufficient for accurate rBC mass detection. Additionally, methods for measurement of laser intensity using either laboratory or ambient aerosol are presented.     

Sizing Small Sulfuric Acid Particles with an Ultrafine Particle Condensation Nucleus Counter

The sizing capability of an ultrafine particle condensation nucleus counter (which uses butanol as the condensing fluid) equipped with pulse height analysis was evaluated in terms of particle composition for sulfuric acid aerosol and sulfuric acid aerosol to which gas-phase ammonia had been added. The response of the counter depended on composition for a range of particle sizes when the water partial pressure was low. For water partial pressures < 5 Torr and for particles > 4 nm in diameter, the response (pulse heights) of the instrument to particles of a given size was substantially different for sulfuric acid particles and those that were neutralized with ammonia. For water partial pressures > 5 Torr, however, neutralizing the particles with ammonia had little effect on pulse height distributions. For particles smaller than 4 nm diameter the pulse heights were insensitive to exposure to ammonia.     

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.     

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     

Acid-base method for the demineralization of pyrolytic carbon black

The carbon black material used as reinforcing filler in tires was recovered by vacuum pyrolysis at a temperature of 500°C and a total pressure of 20 kPa. The pyrolytic carbon black obtained (CBp) was contamined by various additives of the original tire. Contaminants were also produced by chemical reactions occurring in the pyrolysis reactor. The contamination is reflected by the high content of ash and gritty materials (coke) present in the CBp. A characterization of the recovered carbon black was performed and a possible reduction of the ash content by sulfuric acid and sodium hydroxide treatment was investigated. The variables which were studied included the ratio of reactant to carbon black, the reactant concentration, the treatment temperature and the reaction time. Properties of the commercial carbon black filler grade N539 were compared to those of the CBp recovered before and after the demineralization treatment.     

Density measurement of size selected multiwalled carbon nanotubes by mobility-mass characterization

We employ a combination of gas phase particle mobility and mass methods to make the first absolute density measurement of gas phase grown carbon nanotubes (CNTs). The approach combines a tandem differential mobility analyzer and aerosol particle mass analyzer in series to achieve two steps of electrical mobility classifications of the CNTs and one of mass classification. In the first mobility classification step a stream of monodisperse catalytic particles was produced by pulsed laser ablation. These mobility-classified catalysts seeded the aerosol growth of CNTs, where were directly passed to a second electrical mobility classification step which allows classification of the diameter-controlled CNTs in length. These diameter- and length-classified CNTs were finally introduced into the aerosol particle mass analyzer to measure their mass distribution. We found that the condensed phase density of CNTs was 1.74 ± 0.16 g/cm³ for two different groups of CNTs with diameters of ∼15 and ∼22 nm. This value is lower (about 3 sigma) than for graphite, and about 1 sigma lower than the average value for density measurements for carbon black.     

Electrical and Mechanical Properties of Some Composites with Polymeric Matrix

Abstract

The present study aims to investigate the dielectric properties as well as the mechanical properties of polymethyl methacrylate-carbon black and polyester-carbon black composites using semi-reinforcing furnace (SRF) and high abrasion furnace (HAF) carbon blacks.

The permittivity ε' was found to increase by increasing carbon black content and showed anomalous dispersion. The absorption spectra (ε'' vs. frequency) from 100 Hz to 10 MHz were analyzed using a computer program based on Fröhlich terms. Four absorption regions were obtained for either the polymer matrix or loaded samples.

The first region in the lower frequency range could be attributed to Maxwell-Wagner effect resulting from the differences in the conductivities of the ingredients of the composite. The second and third regions in the higher frequency range could be attributed to the large and small aggregates caused by movements of the main chain. The fourth region may occur due to segmental rotation (ester group or carbonyl group) or local twisting motion of the main chain. At higher carbon black concentration, the relaxation time for different regions except that for Maxwell-Wagner effect became higher and carbon black seems to interact with the polymer matrix giving rise to large aggregates resulting high relaxation times.

The mechanical properties for the investigated samples were also examined, the results showed that the tensile strength is higher in the case of smaller particle size carbon black HAF. This could be attributed to a good incorporation of carbon black of small particle size into the polymer matrix. Also, the morphology study indicates the presence of aggregates and some arrangements on adding carbon black.     

Investigation of Refractory Black Carbon-Containing Particle Morphologies Using the Single-Particle Soot Photometer (SP2)

An important source of uncertainty in radiative forcing by absorbing aerosol particles is the uncertainty in their morphologies (i.e., the location of the absorbing substance on/in the particles). To examine the effects of particle morphology on the response of an individual black carbon-containing particle in a Single-Particle Soot Photometer (SP2), a series of experiments was conducted to investigate black carbon-containing particles of known morphology using Regal black (RB), a proxy for collapsed soot, as the light-absorbing substance. Particles were formed by coagulation of RB with either a solid substance (sodium chloride or ammonium sulfate) or a liquid substance (dioctyl sebacate), and by condensation with dioctyl sebacate, the latter experiment forming particles in a core-shell configuration. Each particle type experienced fragmentation (observed as negative lagtimes), and each yielded similar lagtime responses in some instances, confounding attempts to differentiate particle morphology using current SP2 lagtime analysis. SP2 operating conditions, specifically laser power and sample flow rate, which in turn affect the particle heating and dissipation rates, play an important role in the behavior of particles in the SP2, including probability of fragmentation. This behavior also depended on the morphology of the particles and on the thermochemical properties of the non-RB substance. Although these influences cannot currently be unambiguously separated, the SP2 analysis may still provide useful information on particle mixing states and black carbon particle sources.

Copyright 2015 American Association for Aerosol Research     

Comparative performance of a thermal denuder and a catalytic stripper in sampling laboratory and marine exhaust aerosols

The performance of a thermal denuder (thermodenuder—TD) and a fresh catalytic stripper (CS) was assessed by sampling laboratory aerosol, produced by different combinations of sulfuric acid, octacosane, and soot particles, and marine exhaust aerosol produced by a medium-speed marine engine using high sulfur fuels. The intention was to study the efficiency in separating non-volatile particles. No particles could be detected downstream of either device when challenged with neat octacosane particles at high concentration. Both laboratory and marine exhaust aerosol measurements showed that sub-23 nm semi-volatile particles are formed downstream of the thermodenuder when upstream sulfuric acid approached 100 ppbv. Charge measurements revealed that these are formed by re-nucleation rather than incomplete evaporation of upstream aerosol. Sufficient dilution to control upstream sulfates concentration and moderate TD operation temperature (250°C) are both required to eliminate their formation. Use of the CS following an evaporation tube seemed to eliminate the risk for particle re-nucleation, even at a ten-fold higher concentration of semi-volatiles than in case of the TD. Particles detected downstream of the CS due to incomplete evaporation of sulfuric acid and octacosane aerosol, did not exceed 0.01% of upstream concentration. Despite the superior performance of CS in separating non-volatile particles, the TD may still be useful in cases where increased sensitivity over the traditional evaporation tube method is needed and where high sulfur exhaust concentration may fast deplete the catalytic stripper adsorption capacity.

Copyright © 2018 American Association for Aerosol Research     

撞击流-沉淀法制取超细白炭黑中试研究

介绍了以工业级硫酸和工业级水玻璃为原料,利用撞击流反应-沉淀法制取超细白炭黑的中试试验。其反应过程在无旋立式循环撞击流反应器中进行。反应后物料经过陈化、过滤、洗涤,最后通过再浆进行喷雾干燥制得了超细白炭黑。用激光颗粒分析仪和比表面积-孔隙分析仪测定粒径及比表面积,结果表明:产品的平均粒径为2—3μm、比表面积高达822 m2/g,进一步证明了无旋立式循环撞击流反应器性能优越,操作稳定可靠,有利于制备超细粉体材料。     

Investigation of the specific attenuation cross-section of aerosols deposited on fiber filters with a polar photometer to determine black carbon

A polar photometer was used to investigate phase functions of glass fiber filters with varying aerosol particle loadings. Angle-resolved analysis revealed that the shape of the normalized phase function S_n(θ) in the transmitted hemisphere is independent of aerosol composition and determined exclusively by the fibrous filter matrix, while S_n(θ) in the reflected hemisphere depends on the scattering properties of the aerosol sample. Cross sensitivities of the specific attenuation cross section with respect to scattering components were examined with well-defined laboratory generated aerosol samples. The specific attenuation cross section at θ=165° (reflected hemisphere) was found considerably less dependent of the aerosol composition than any angular position in the transmitted hemisphere, and is therefore best suited for the determination of black carbon (BC). The angular position θ=165° was calibrated with an almost pure BC test aerosol. BC mass loadings of ambient aerosol samples (0.15–20 μg cm^-2) determined with this calibration function were in good agreement with values analyzed by coulometry.     

Generation of Radiolabeled "Soot-Like" Ultrafine Aerosols Suitable for Use in Human Inhalation Studies

We have developed a method for radiolabeling ultrafine carbon particle aggregates with technetium-99m. The carbon aggregate aerosol was chosen to mimic the physical properties of urban combustion or ''soot-like'' particulate. The radioisotope is a short lived (t_1/2 = 6.02 h) gamma emitter commonly used in human studies where scintigraphic methods are employed. Primary carbon parti cles, the aggregation of which is controlled by concentration and time, were produced by arcing between graphite electrodes under an argon atmosphere. Radiolabeling of particles was accomplished by applying a pertechnetate solution onto the tips of electrodes prior to arcing. The activity median diameter of experimental aerosols could be varied from 50 to 150 nm. The specific activity of aerosols increased with the amount of activity applied to the electrodes and decreased with time of generator operation. In-vitro leaching of the radioisotope from particles into solution was also measured. Leaching appeared to increase with the specific activity of the aerosol but was not affected by particle size.     

Investigation of Volatility Method for Measuring Aqueous Sulfuric Acid on Mixed Aerosols

The thermal discrimination or volatility technique is a widely used method exploiting differences in aerosol volatility to discriminate between particles of different chemical composition. In recent years numerous investigators applied this technique to determine the existence and the amount of sulfuric acid in the aerosol phase of aircraft contrails forming in the upper troposphere and lower stratosphere (UT/LS). Although the potential for systematic errors due to incomplete evaporation and recondensation of volatile material as well as internal wall losses was recognized by other investigators, we are not aware of any study on polydisperse aerosol (broad size distribution) incorporating these effects into the volatility technique. Here, a tandem differential mobility analyzer (TDMA) is employed to investigate the performance of a thermal discriminator designed at the University of Missouri-Rolla (UMR). Since sulfuric acid is of particular interest for atmospheric aerosol, this study focused on aqueous sulfuric acid (H 2 SO 4 /H 2 O) aerosol. For an operating temperature of 300°C and an aerosol residence time of more than 0.25 s, we found that complete evaporation of H 2 SO 4 /H 2 O aerosol occurred up to diameters of at least 1.9 w m, which is consistent with theoretical calculations. No evidence for recondensation was found for H 2 SO 4 /H 2 O particle surface area and mass concentrations typical for UT/LS background and aircraft plume conditions. Wall losses were measured and incorporated into a size-resolved version of the volatility method, allowing more accurate measurements of the volatile (H 2 SO 4 /H 2 O) volume fraction of polydisperse aerosol. The increased accuracy was demonstrated using well-characterized, mixed (partially volatile) H 2 SO 4 /H 2 O/NaCl aerosol.