Dependence of Real Refractive Indices on O:C,H:C and Mass Fragments of Secondary Organic Aerosol Generated from Ozonolysis and Photooxidation of Limonene and α-Pinene

The refractive index is a fundamental property controlling aerosol optical properties. Secondary organic aerosols have variable refractive indices, presumably reflecting variations in their chemical composition. Here, we investigate the real refractive indices (m_r) and chemical composition of secondary organic aerosols (SOA) generated from the oxidation of α-pinene and limonene with ozone and NO_x/sunlight at different HC/NO_x ratios. Refractive indices were retrieved from polar nephelometer measurements using parallel and perpendicular polarized 532-nm light. Particle chemical composition was monitored with a high-resolution time-of-flight aerosol mass spectrometer (HR-Tof-AMS). For photochemically generated SOA, the values of refractive indices are consistent with prior results, and ranged from about 1.34 to 1.55 for limonene and from 1.44 to 1.47 for α-pinene, generally increasing as the particles grew. While AMS fragments are strongly correlated to the refractive index for each type of SOA, the relationships are in most cases quite different for different SOA types. Consistent with its wide range of refractive index, limonene SOA shows larger variations compared to α-pinene SOA for most parameters measured with the AMS, including H:C, O:C, f₄₃ (m/z 43/organic), f_C4H7 ⁺, and others. Refractive indices for α-pinene ozonolysis SOA also fell in narrow ranges; 1.43–1.45 and 1.46–1.53 for particles generated at 19–22 and 23–29°C, respectively, with corresponding small changes of f₄₃ and H:C ratio and other parameters. Overall, H:C ratio, m/z 43 and 55 (C₂H₃O⁺, C₄H₇ ⁺) were the best correlated with refractive index for all aerosol types investigated. The relationships between m_r and most fragments support the notion that increasing condensation of less oxygenated semivolatile species (with a possible role for a concomitant decrease in low refractive index water) is responsible for the increasing m_rs observed as the experiments progress. However, the possibility that oligomerization reactions play a role cannot be ruled out.     

An Intensive Study of the Size and Composition of Submicron Atmospheric Aerosols at a Rural Site in Ontario,Canada

Atmospheric sampling was conducted at a rural site near Egbert, about 70 km north of Toronto, Ontario, Canada from March 27 to May 8, 2003 to characterize the physical and chemical properties of the ambient aerosol in near real-time. The instrumentation included a tapered element oscillating microbalance (TEOM), an ultrafine condensation particle counter (UCPC), a scanning mobility particle sizer (SMPS), an aerodynamic particle sizer (APS), an aerosol mass spectrometer (AMS), and a particulate nitrate monitor (R&P 8400N) for aerosol measurements. Gas-phase non-methane hydrocarbon compounds (NMHCs) were measured by gas chromatograph-flame ionization detection (GC-FID). Filter samples were also collected for analysis of inorganic ions by ion chromatography (IC). Aerosol properties varied considerably depending upon meteorological conditions and airmass histories. For example, urban and industrial emissions advected from the south strongly influenced the site occasionally, resulting in higher particulate mass with the higher fractions of nitrate and organics. Cleaner northwesterly winds carried aerosols with relatively higher fractions of organics and sulfate. The AMS derived mass size distributions showed that the inorganic species in the particles with vacuum aerodynamic diameters between about 60 nm and 600 nm had mass modal vacuum aerodynamic diameters around 400–500 nm. The particulate organics often exhibited two modes at about 100 nm and 425 nm, more noticeable during fresh pollution events. The small organic mode was well correlated with gas-phase nonmethane hydrocarbons such as ethylbenzene, toluene, and propene, suggesting that the likely sources of small organic particles were combustion related emissions. The particulate nitrate exhibited a diurnal variation with higher concentrations during dark hours and minima in the afternoon. Particulate sulfate and organics showed evidence of photochemical processing with higher levels of sulfate and oxygenated organics in the afternoon. Reasonable agreement among all of the co-located measurements is found, provided the upper size limit of the AMS is considered.     

Characterization of Organic Particles from Incense Burning Using an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer

Incense burning is a common ritual in Asian communities both indoors in residential homes and outdoors in temple premises. Organic particles from burning of incense sticks, incense coils, and mosquito coils after extensive dilution (>1000×) were characterized by the Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The obtained mass spectra in general resemble those reported for biomass burning aerosols. Ion peaks with m/z values higher than 100 accounted for 15%–25% of the organic signals in the unit-mass-resolution (UMR) mass spectra. In the high-resolution (HR) mass spectra, the ion peaks at m/z 60 and 73 are found to be related to the sugar anhydrides as in particles from other biomass burning processes. In addition, the ion peaks at m/z 107, 121, 137, 151, 167, and 181, some of which (e.g., m/z 137 and 167) have been observed in particles from biomass burning but not yet assigned, were assigned to lignin-related components. Elemental analysis from the HR data reveals that a large portion of particulate organics from incense burning are oxygenated (O/C between 0.3 and 0.5) and unsaturated (and/or cyclic) in nature. Results from this study also highlight that mass spectra from HR-ToF-AMS measurements concerning primary emissions such as incense burning contain very useful information in the high m/z (>100) region about the chemical characteristics of those primary organic particles.

Copyright 2012 American Association for Aerosol Research     

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     

Using Raman Microspectroscopy to Determine Chemical Composition and Mixing State of Airborne Marine Aerosols over the Pacific Ocean

Chemical composition and mixing state of aerosols collected over an 11,000 km latitudinal cruise in the Pacific Ocean are reported here as determined by a new application of Raman spectroscopy. The Raman microspectroscopy technique employs a Raman spectrometer coupled to an optical microscope to identify the chemical composition and internal mixing state of single particles. By analyzing multiple particles in a collected ensemble, the degree of external mixing of particles was also determined. To lend context to the Pacific aerosol population sampled, atmospheric aerosol concentration, and the critical supersaturation required for the aerosols to activate as cloud condensation nuclei, and chlorophyll a concentration in the underlying water (a metric for phytoplankton biomass in the ocean) were also obtained. Our results indicate that long chain organic molecules were prevalent in the marine aerosol samples throughout the cruise, including during coastal and open ocean locations, in both hemispheres, and in the seasons of autumn and spring. Long chain organic compounds tended to be present in internal mixtures with other organic and inorganic components. Although variations in the fraction of aerosols activated as CCN were observed, no simple correlation between organics and CCN activation was found. According to our measurements, marine aerosol in the Pacific Ocean may be generally characterized as multicomponent aerosol containing and often dominated by a high organic fraction. Our results suggest that the prevalence of organics and the high degree of internal mixing of aerosol must be accounted for in accurate modeling of the role of marine aerosols in cloud formation and climate.

Copyright 2014 American Association for Aerosol Research     

Contribution of Selected Dicarboxylic and ω-Oxocarboxylic Acids in Ambient Aerosol to the m/z 44 Signal of an Aerodyne Aerosol Mass Spectrometer

The Aerodyne aerosol mass spectrometer (AMS) employs flash vaporization (600°C) followed by 70-eV electron impact ionization (EI) to detect organic and inorganic aerosols. The signal at mass-to-charge ratio (m/z) 44 (mainly CO ₂ ⁺ ) is considered the most reliable marker of oxygenated organic aerosol. This study is the first to evaluate the contribution of selected low molecular weight dicarboxylic acids (diacids) and ω-oxocarboxylic acids (ω-oxoacids) to the particle-phase m/z 44 signal of the AMS mass spectrum. Ambient measurements were conducted at a surface site in Tokyo (35°39 ′ N, 139°40 ′ E) during August 3–8, 2003. Diacids and ω-oxoacids were measured using a filter sampling followed by extraction, derivation, and gas chromatograph-flame ionization detector (GC-FID) analysis. The mass concentrations of diacids and ω-oxoacids show tight correlation with the m/z 44 signal (r ² = 0.85–0.94) during the measurement period. Laboratory experiments were also performed to determine the fragment patterns of selected diacids (C₂–C₆ diacids and phthalic acids) and ω-oxoacid (glyoxylic acid) in ambient aerosols. Here, we report for the first time that the selected organic acids could account for 14 ± 5% of the observed m/z 44 signal on average during the measurement period. Oxalic acid (C₂) is the largest contributor, accounting for 10 ± 4% of the observed m/z 44 signal. These results would be useful for interpreting the m/z 44 signals obtained from ambient measurements in various locations.     

Investigating the chemical species in submicron particles emitted by city buses

Detailed chemical characterization of exhaust particles from 23 individual city buses was performed in Helsinki, Finland. Investigated buses represented different technologies in terms of engines, exhaust after-treatment systems (e.g., diesel particulate filter, selective catalytic reduction, and three-way catalyst) and fuels (diesel, diesel-electric (hybrid), ethanol, and compressed natural gas). Regarding emission standards, the buses operated at EURO III, EURO IV, and EEV (enhanced environmentally friendly vehicle) emission levels. The chemical composition of exhaust particles was determined by using a soot particle aerosol mass spectrometer (SP-AMS). Based on the SP-AMS results, the bus emission particles were dominated by organics and refractory black carbon (rBC). The mass spectra of organics consisted mostly of hydrocarbon fragments (54–86% of total organics), the pattern of hydrocarbon fragments being rather similar regardless of the bus type. Regarding oxygenated organic fragments, ethanol-fueled buses had unique mass-to-charge ratios (m/z) of 45, 73, 87, and 89 (mass fragments of C₂H₅O⁺, C₃H₅O₂⁺, C₄H₇O₂⁺, and C₄H₉O₂⁺, respectively) that were not detected for the other bus types at the same level. For rBC, there was a small difference in the ratio of C₄⁺ and C₅⁺ to C₃⁺ for different bus types but also for the individual buses of the same type. In addition to organics and rBC, the presence of trace metals in the bus emission particles was investigated.

Copyright © 2017 American Association for Aerosol Research     

Chemical classes of atmospheric aerosol particles at a rural site in Central Europe during winter

Ambient atmospheric particles were studied at an ecosystem research site in the Fichtelgebirge mountains in Central Europe by single-particle analysis and bulk impactor measurements. Fuzzy clustering analysis of mass spectra of individual aerosol particles allowed chemical classification of the atmospheric aerosol. During the campaign, inorganic salts, mineral particles, and carbonaceous particles, with varying degrees of secondary components, were identified. These chemical classes exhibited preferential size ranges leading to a characteristic pattern of relative abundances with respect to particle size. A more detailed analysis revealed that 65–80% of all particles were assigned almost exclusively to one chemical class. These particle populations are assumed to be externally mixed with respect to the identified chemical classes. The temporal variations of the ratio of nitrate to ammonium (ranging between 0.37 and 0.81) determined by both impactor measurements and single-particle analyses were in good agreement. Through Monte-Carlo-type simulations, confidence intervals of the mean NO₃⁻/NH₄⁺ ratio were calculated for sub-samples of the total particle population.     

Determination of Particle Effective Density in Urban Environments with a Differential Mobility Analyzer and Aerosol Particle Mass Analyzer

Effective densities of atmospheric aerosols in various locations of the Los Angeles Basin were determined by a DMA-APM technique. Effective density was calculated by comparing voltage distributions of sampled atmospheric aerosols with PSL particles of known density. The five sites chosen for field experiments were: (1) Interstate-710 Freeway, impacted by heavy-duty diesel vehicles; (2) State Route CA-110, open only to gasoline vehicles; (3) Riverside, a receptor site known for secondary particle formation; (4) University of Southern California, a typical urban and industrial environment; and (5) Coast for marine aerosol. The size range selected for this study was from 50 nm to 414 nm. While 50 nm particles exhibited a single effective density multiple effective densities were measured for each of the other particle sizes as significant fractions of these particles are transported from background sources. Regardless of location, 322–414 nm particle effective densities were considerably lower than unity. The lowest effective densities (～ 0.1 g cm ^{? 3} ) were reported for I-710, confirming that diesel combustion aerosols are rich in chain agglomerates with large void spaces. Riverside exhibited high effective densities (～ 1.2–1.5 g cm ^{? 3} ) for 50–202 nm particles, which we hypothesize is due to transformations that occur during advection from Los Angeles. Measurements of diurnal variation of effective density at Riverside support this hypothesis. Overall, our results suggest that effective density declines as the particle mobility diameter increases irrespective of location. Fractal dimensions calculated from average effective densities were lowest for I-710 ( D _f = 2.41) and CA-110 (D _f = 2.54) aerosols, presumably due to the influence of vehicular combustion emission on these sites. By contrast, average fractal dimensions at USC, Riverside and Coast were found to be 2.79, 2.83, and 2.92, respectively. High fractal dimensions at these sites may be the effects of aging, moisture absorption and/or organic vapor condensation on the particles, which fills void space and makes particles more spherical.     

A VUV Photoionization Aerosol Time-of-Flight Mass Spectrometer with a RF-Powered VUV Lamp for Laboratory-Based Organic Aerosol Measurements

A vacuum ultraviolet (VUV) photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS) has been developed for real-time, quantitative chemical analysis of organic particles in laboratory environments. A nozzle of ～ 0.12 mm orifice combined with an aerodynamic lens assembly and a three stage differential pumping system is used to sample particles at atmospheric pressure. The particles are vaporized on a thermal heater, and then the nascent vapor is photoionized by light generated with a RF-powered VUV lamp. A 0.41 V/cm electric field is used to drive the ions from the ionization region into the ion extraction region where a positive electric pulse repels the ions into a reflectron mass spectrometer. The mass resolution of the spectrometer is ～ 350 and the detection limit is ～ 400 μ m ³ . The signal intensities observed are linear with the mass concentration of aerosols. Oleic acid particles are well quantified with an uncertainty of 15% in mass concentrations ranging from 3.9 mg/m ³ to 392 mg/m ³ . The VUV-ATOFMS has substantial potential for the use in laboratory investigations on organic aerosol chemistry.     

Relative Humidity-Dependent HTDMA Measurements of Ambient Aerosols at the HKUST Supersite in Hong Kong,China

The hygroscopic tandem differential mobility analyzer (HTDMA) has been frequently used to measure the hygroscopic properties of atmospheric aerosols at a fixed high relative humidity (RH) of about 90%. To evaluate if such measurements could be used to determine the hygroscopicity of aerosols at lower RH, simultaneous hygroscopic growth factor (GF) and size-resolved composition measurements were made with an HTDMA and a high-resolution aerosol mass spectrometer (HR-AMS), respectively, at a coastal site in Hong Kong from January to June and in August 2012. A total of 58 cycles of dehydration (decreasing RH) and hydration (increasing RH) of 100 nm and 200 nm particles with organic-to-inorganic mass ratio ranging from 0.19 to 1.97 were measured at RH = 10–93%. The Kappa (κ) equation developed by Petters and Kreidenweis in the year 2007 was used to determine (i) κ at individual RHs (κ_RH) and (ii) best-fit κ covering the range of RHs measured (κ_f) for the more-hygroscopic (MH) mode, which describes more than 80% of the particles in each cycle, during dehydration. Overall, κ at 90% RH or above (κ_>90) fell between 0.18 and 0.48, and was within 15% of κ_f in 83% of the datasets. Regression analysis between κ_>90 or κ_f and AMS mass fractions showed that κ was positively correlated with sulfate but negatively correlated with organic and nitrate. In most cases, κ_RH increased as RH decreased and the average increase in κ was 45% from 90% RH to 40% RH, but these differences yielded insignificant changes in the GF-RH curves. The Zdanovskii-Stokes-Robinson (ZSR) estimated κ were mostly within 20% of κ_>90 and κ_f. GF predictions using the empirical correlation of κ with AMS mass fractions or the ZSR estimated κ were within 10% of additional measurements and hence κ_>90 is useful for predicting GF at lower RHs.

Copyright 2015 American Association for Aerosol Research     

Determining Aerosol Volatility Parameters Using a “Dual Thermodenuder” System: Application to Laboratory-Generated Organic Aerosols

Thermodenuders (TD) are a tool widely used for measuring aerosol volatility in the laboratory and field. Extracting the parameters that dictate organic aerosol volatility from TD data is challenging because gas-particle partitioning rarely reaches equilibrium inside a TD operating under atmospheric conditions, thus a wide variety of parameter sets can explain observed evaporation. Component volatilities (as represented by saturation vapor pressure, C_sat), cannot be directly extracted due to uncertainties in potential limitations to mass transfer (represented by mass accommodation coefficient, α) and components’ enthalpies of evaporation (ΔH_vap). To address these limitations, we have developed a “dual TD” experimental approach in which one line uses a temperature-stepping TD (TS-TD) with a relatively long residence time (RT) and the other operates isothermally at variable residence time (VRT-TD). Data from this approach are used in tandem with an optimizing evaporation kinetics model to extract the values of parameters dictating volatility (C_sat, and associated values of ΔH_vap and α). The system was evaluated using laboratory generated dicarboxylic acid aerosols (adipic acid and succinic acid). Excellent agreement with previously published evaporation data collected with other TD systems was observed. Parameter values reported in the literature for the tested acids vary widely, but our results are generally consistent with those from studies that allow for nonunity values of α. For example, our results suggest that α for these aerosols are of order 0.1, in agreement with results determined by Saleh et al. (2009, 2012). Modeling results suggest that the addition of VRT-TD data provides tighter constraint on feasible ΔH_vap and α values. The dual TD approach presented here does not rely on equilibration in the TD and thus can be directly applied to extract volatility parameters for more complex laboratory and ambient organic aerosol systems.

Copyright 2015 American Association for Aerosol Research     

Development of an Aerosol Mass Spectrometer for Size and Composition Analysis of Submicron Particles

The importance of atmospheric aerosols in regulating the Earth's climate and their potential detrimental impact on air quality and human health has stimulated the need for instrumentation which can provide real-time analysis of size resolved aerosol, mass, and chemical composition. We describe here an aerosol mass spectrometer (AMS) which has been developed in response to these aerosol sampling needs and present results which demonstrate quantitative mea surement capability for a laboratory-generated pure component NH₄ NO₃ aerosol. The instrument combines standard vacuum and mass spectrometric technologies with recently developed aerosol sampling techniques. A unique aerodynamic aerosol inlet (developed at the University of Minnesota) focuses particles into a narrow beam and efficiently transports them into vacuum where aerodynamic particle size is determined via a particle time-of-flight (TOF) measurement. Time-resolved particle mass detection is performed mass spectrometrically following particle flash vaporization on a resistively heated surface. Calibration data are presented for aerodynamic particle velocity and particle collection efficiency measurements. The capability to measure aerosol size and mass distributions is compared to simultaneous measurements using a differential mobility analyzer (DMA) and condensation particle counter (CPC). Quantitative size classification is demonstrated for pure component NH₄ NO₃ aerosols having mass concentrations 0.25_mu g m -3. Results of fluid dynamics calculations illustrating the performance of the aerodynamic lens are also presented and compared to the measured performance. The utility of this AMS as both a laboratory and field portable instrument is discussed.     

Monodisperse fine aerosol generation using fluidized bed

Monodisperse, fine aerosols are needed in many applications: filter testing, experiments for testing models, and aerosol instrument calibration, among others. Usually, monodisperse fine aerosols are generated in very low concentrations, or mass flow rates, in the laboratory scale. In this work, we needed to generate aerosols with higher mass flow rate than typically available by the laboratory-scale methods, such as atomizers, nebulizers, ultrasonic generators, vibrating orifice generators, and condensation generators. Therefore, we constructed a fluidized bed aerosol generator to achieve particle mass flow rates in the range of 15-100 g/h. Monodisperse, spherical SiO₂ particles of two sizes with geometrical diameters of 1.0 and 2.6 µm were used in the aerosol generator. The aerosol generator was used at both atmospheric pressure, and at high pressures up to 5 bar (abs).The particle size, mass concentration and the net average particle charge were measured after mixing the aerosol with nitrogen. The particle size distributions with both particle sizes were monodisperse, and no particle agglomerates were entrained from the fluidized bed. The behavior of the fluidized bed generator was found to be markedly different with the two particle sizes in regard to particle concentration, presumably due to different particle charging inside the generator. After determining the net average charge of the particles, an ion source Kr-85 was used to reduce the charge of the particles. This was found to be effective in neutralizing the particles.     

A Novel Optical Instrument for Estimating Size Segregated Aerosol Mass Concentration in Real Time

A novel optical instrument has been developed that estimates size segregated aerosol mass concentration (i.e., PM ₁₀ , PM ₄ , PM _2.5 , and PM ₁ ) over a wide concentration range (0.001–150 mg/m ³ ) in real time. This instrument combines photometric measurement of the particle cloud and optical sizing of single particles in a single optical system. The photometric signal is calibrated to approximate the PM _2.5 fraction of the particulate mass, the size range over which the photometric signal is most sensitive. The electrical pulse heights generated by light scattering from particles larger than 1 micron are calibrated to approximate the aerodynamic diameter of an aerosol of given physical properties, from which the aerosol mass distribution can be inferred. By combining the photometric and optical pulse measurements, this instrument can estimate aerosol mass concentrations higher than typical single particle counting instruments while providing size information and more accurate mass concentration information than traditional photometers. Experiments have shown that this instrument can be calibrated to measure aerosols with very different properties and yet achieve reasonable accuracy.     

Vapor wall loss of semi-volatile organic compounds in a Teflon chamber

We have investigated the vapor wall loss of semi-volatile organic compounds (SVOCs) in a Teflon smog chamber. We studied the vapor wall loss of seven SVOCs with known saturation concentrations, including alkanes (hexacosane, pentacosane, docosane, eicosane, and d₆₂-squalane), an organic acid (oleic acid), and a polyol (levoglucosan) in single-component and binary-component (organic) systems, using ammonium sulfate (AS) seeds to constrain the particle wall loss. We coated inorganic particles with SVOCs and measured the loss of organics from those particles to constrain the wall losses, observing loss rates proportional to the saturation concentrations of the SVOCs. The loss rate of oleic acid mixed with d₆₂-squalane was proportional to its mole fraction in the mixture. Our results show that the vapor wall-loss rates of SVOCs are significant, quasi-irreversible, and proportional to the SVOC vapor concentrations. The vapor wall-loss rate constant of the SVOCs that we studied in the CMU chamber is 3.8 ± 0.3 h^?1; this is comparable to values in other chambers with similar surface area to volume ratios. Our results are also consistent with a relatively high mass accommodation coefficient for SVOCs, α_org > 0.1.

© 2016 American Association for Aerosol Research     

An Aerosol Chemical Reactor for Coating Metal Oxide Particles with (NH4)SO4-H2SO4-H2O. 1. New Particle Formation

In situ atmospheric measurements (notably single-particle mass spectrometry) show that tropospheric aerosols are internally mixed, including both water-soluble and insoluble components. This fact notwithstanding, most process study laboratory work has concentrated on water-soluble electrolytes because the generation of particles composed of both soluble and insoluble components is difficult to achieve in the laboratory. Even so, such an aerosol is essential for accurate process studies of atmospheric aerosols and for the quantitative calibration of single-particle mass spectrometers. In the completed work, particles composed of a (NH₄)₂SO₄-H₂SO₄-H₂O coating on a TiO₂, Al₂O₃, or ZrO₂ core are prepared in a novel chemical reactor, which is a tube furnace with a linear-temperature gradient along its longitudinal axis. Reactor controls on the number size distribution are reported, including the linear flow velocity, the SO₃ vapor pressure, the NH₃ vapor pressure, the reactor temperature gradient, and the presence or the absence of insoluble seed nuclei (viz. TiO₂, Al₂O₃, and ZrO₂).     

The Structure of Agglomerates Consisting of Polydisperse Particles

Agglomeration is encountered in many natural or industrial processes, like growth of aerosol particles in the atmosphere and during material synthesis or even flocculation of suspensions, granulation, crystallization, and with colloidal particle processing. These particles collide by different mechanisms and stick together forming irregular or fractal-like agglomerates. Typically, the structure of these agglomerates is characterized with the fractal dimension, D_f , and pre-exponential factor, k_n , of simulated agglomerates of monodisperse primary particles (PP) for ballistic or diffusion-limited particle–cluster and cluster–cluster collision mechanisms. Here, the effect of PP polydispersity on D_f and k_n is investigated with agglomerates consisting of 16-1024 PP with closely controlled size distribution (geometric standard deviation, σ_g = 1–3). These simulations are in excellent agreement with the classic structure (D_f and k_n ) of agglomerates consisting of monodisperse PPs made by four different collision mechanisms as well as with agglomerates of bi-, tridisperse, and normally distributed PPs. Broadening the PP size distribution of agglomerates decreases monotonically their D_f , and for sufficiently broad PP distributions (σ_g > 2.5) the D_f reaches about 1.5 and k_n about 1 regardless of the collision mechanism. Furthermore, with increasing PP polydispersity, the corresponding projected area exponent, D_α , and pre-exponential factor, k_a , decrease monotonically from their standard values for agglomerates with monodisperse PPs. So D_f as well as D_α and k_a can be an indication for PP polydispersity in mass mobility and light scattering measurements if the dominant agglomeration mechanism is known, like diffusion-limited and/or ballistic cluster–cluster coagulation in aerosols.

Copyright 2012 American Association for Aerosol Research     

Collection Efficiency of the Aerosol Mass Spectrometer for Chamber-Generated Secondary Organic Aerosols

The collection efficiency (CE) of the aerosol mass spectrometer (AMS) for chamber-generated secondary organic aerosol (SOA) at elevated mass concentrations (range: 19–207 μg m^?3; average: 64 μg m^?3) and under dry conditions was investigated by comparing AMS measurements to scanning mobility particle sizer (SMPS), Sunset semi-continuous carbon monitor (Sunset), and gravimetric filter measurements. While SMPS and Sunset measurements are consistent with gravimetric filter measurements throughout a series of reactions with varying parent hydrocarbon/oxidant combinations, AMS CE values were highly variable ranging from unity to <15%. The majority of mass discrepancy reflected by low CE values does not appear to be due to particle losses either in the aerodynamic lens system or in the vacuum chamber as the contributions of these mechanisms to CE are low and negligible, respectively. As a result, the largest contribution to CE in the case of chamber-generated SOA appears to be due to particle bounce at the vaporizer surface before volatilization, which is consistent with earlier studies that have investigated the CE of ambient and select laboratory-generated particles. CE values obtained throughout the series of reactions conducted here are also well correlated with the f ₄₄/f ₅₇ ratio, thereby indicating both that the composition of the organic fraction has an important impact on the CE of chamber-generated SOA and that this effect may be linked to the extent to which the organic fraction is oxidized.

Copyright 2013 American Association for Aerosol Research     

Evaluation of the new capture vaporizer for aerosol mass spectrometers: Characterization of organic aerosol mass spectra

The Aerosol Mass Spectrometer (AMS) and Aerosol Chemical Speciation Monitor (ACSM) are widely used for quantifying submicron aerosol mass concentration and composition, in particular for organic aerosols (OA). Using the standard vaporizer (SV) installed in almost all commercial instruments, a collection efficiency (CE) correction, varying with aerosol phase and chemical composition, is needed to account for particle bounce losses. Recently, a new “capture vaporizer” (CV) has been shown to achieve CE～1 for ambient aerosols, but its chemical detection properties show some differences from the SV due to the increased residence time of particles and vaporized molecules inside the CV. This study reports on the properties and changes of mass spectra of OA in CV-AMS using both AMS and ACSM for the first time. Compared with SV spectra, larger molecular-weight fragments tend to shift toward smaller ions in the CV due to additional thermal decomposition arising from increased residence time and hot surface collisions. Artifact CO⁺ ions (and to a lesser extent, H₂O⁺), when sampling long chain alkane/alkene-like OA (e.g., squalene) in the CV during the laboratory studies, are observed, probably caused by chemical reactions between sampled OA and molybdenum oxides on the vaporizer surfaces (with the carbon derived from the incident OA). No evidence for such CO⁺ enhancement is observed for ambient OA. Tracer ion marker fractions (f_m/z =, i.e., the ratio of the organic signal at a given m/z to the total OA signal), which are used to characterize the impact of different sources are still present and usable in the CV. A public, web-based spectral database for mass spectra from CV-AMS has been established.

Copyright © 2018 American Association for Aerosol Research