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

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

Copyright © 2019 American Association for Aerosol Research     

Investigating the dependence of light-absorption properties of combustion carbonaceous aerosols on combustion conditions

We performed controlled combustion experiments to investigate the dependence of the mass absorption cross-section (MAC) and absorption Ångström exponent (AAE) of combustion carbonaceous aerosol emissions on combustion conditions. Using benzene and toluene as fuels, we obtained a wide range of combustion conditions by varying the combustion temperature and equivalence ratio. We also used nitrogen as a passive diluent to tune the combustion conditions. We calculated MAC and AAE from multi-wavelength light-absorption measurements using a photoacoustic spectrophotometer and aerosol mass loadings estimated from thermal-optical analysis. Starting with relatively low-temperature and fuel-rich combustion conditions and progressively increasing the temperature and/or decreasing the equivalence ratio, we produced emissions with progressive change from weakly absorbing brown carbon (BrC) (MAC at 532?nm (MAC₅₃₂) = 0.24?m²/g and AAE = 8.6) to strongly absorbing BrC (MAC₅₃₂ = 2.1?m²/g and AAE = 3.1) to mixtures of black carbon (BC) and strongly absorbing BrC (MAC₅₃₂ = 7.7?m²/g and AAE = 1.5). These findings indicate that combustion conditions are important in dictating the light-absorption properties of the emitted aerosols. Furthermore, regardless of fuel type and combustion conditions, the emitted aerosols exhibit a unified continuum of light-absorption properties that can be characterized by MAC₅₃₂ and AAE pairs. The MAC₅₃₂ and AAE pairs are well-correlated with the elemental carbon-to-organic carbon ratio (EC/OC), which is a proxy of combustion conditions, confirming previous findings that EC/OC is a practical basis for parameterizing the light-absorption properties of combustion carbonaceous aerosols.

Copyright © 2019 American Association for Aerosol Research     

Electrostatic capillary collector for in-situ spectroscopic analysis of aerosols

Identification of particulate matter is important in assessing an individual’s exposure to potentially harmful particles, such as aeroallergens, toxins, and emissions from combustion sources, which can contribute to cardio-pulmonary diseases. Efficient collection of aerosols is essential for aerosol exposure studies such as analysis of chemical and biological components. We present the design and evaluation of a capillary collector that collects PM_2.5 onto the outer surface of a capillary for in-situ spectroscopic analysis. The capillary collector uses a needle-to-ring corona generator to charge particles; the electric field between a cylinder and a wire inserted into the bore of a capillary is used to collect the charged particles. Corona and repelling voltages are optimized for maximum collection of ambient PM_2.5 particles and fluorescent polystyrene latex microspheres in the PM_2.5 size range, on the capillary. The capillary collection efficiency of ambient PM_2.5 at 3 slpm operating flow rate and optimal operating voltages is 63%. Fluorescence spectroscopy is used to quantify the collection of polystyrene latex microspheres. The fluorescence-based capillary collection efficiency is in close agreement with the capillary collection efficiency of ambient PM_2.5. The collection and analysis methodology can be used to develop a compact, low-cost sensor for in-situ spectroscopic analysis of aerosols to determine their chemical composition for source apportionment.

Copyright © 2019 American Association for Aerosol Research     

Seasonal Variability of Particle-Associated Organic Compounds Near a Heavily Traveled Secondary Road

Size-resolved aerosol samples were collected both upwind and downwind of a large secondary road in the winter and spring of 2007 to assess contributions of on-road emissions to ambient aerosols. The aerosol samples were extracted and analyzed for a wide variety of organic compounds including polycyclic aromatic hydrocarbons (PAHs), alkanes, sugars, and organic acids. The results showed a strong seasonal pattern where the concentrations of most compounds were higher in winter than in spring. Some of the biogenic sugars were the exception, which might be the result of a “spring blooming season.” The surprising result was that the upwind site located in a residential neighborhood had very similar concentrations of most organic compounds compared to the near-roadway site. Possible reasons for the lack of differences in organic chemical concentrations between the near-road and control sites include: a large urban background concentration of aerosols superimposed on any local source; shifting wind directions that make the “downwind” site upwind during the night; and additional local sources in the residential neighborhood such as wood burning in winter.

Copyright 2014 American Association for Aerosol Research     

Factors affecting the formation of alpha and beta polymorphs in glutaric acid aerosols

Dicarboxylic acids are prevalent in the atmosphere and frequently investigated by aerosol and atmospheric scientists. Glutaric acid, being a water-soluble dicarboxylic acid, is commonly used to model water activity in dicarboxylic acid aerosols. Different values associated with glutaric acid aerosols, specifically DRH and hygroscopic data, have been reported by several groups. We hypothesize that this variability is caused by its polymorphism. Glutaric acid exhibits dimorphism, meaning the molecule can form into two different types of crystal structures, an alpha and a beta polymorph. Glutaric acid naturally forms the beta polymorph, which is more stable. The alpha polymorph is the metastable phase. These polymorphs manifest themselves as a bimodal aerosol size distribution when analyzed by Scanning Mobility Particle Sizers or other aerosol mobility sizing methods. In this article, we discuss the formation of the nanoscale alpha glutaric acid polymorph generated from an aqueous glutaric acid solution. The formation of both polymorphs from an aqueous solution has not been documented elsewhere. We have found that the alpha polymorph forms at faster crystallization speeds, with a lower aqueous solution concentration and a lower solution flow rate into the atomizer. This identification of the presence of polymorphism in glutaric acid aerosols under common laboratory generation conditions could explain the variability present in studies associated with glutaric acid aerosols. Understanding the conditions at which the alpha polymorph is present will allow researchers to avoid it when using glutaric acid as a dicarboxylic acid standard and explain previously noted discrepancies.

Copyright © 2019 American Association for Aerosol Research     

Development of polydisperse aerosol generation and measurement procedures for wind tunnel evaluation of size-selective aerosol samplers

Accurate development and evaluation of inlets for representatively collecting ambient particulate matter typically involves the use of monodisperse particles in aerosol wind tunnels. However, the resource requirements of using monodisperse aerosols for inlet evaluation creates the need for more rapid and less-expensive techniques to enable determination of size-selective performance in aerosol wind tunnels. The goal of recent wind tunnel research at the U.S. EPA was to develop and validate the use of polydisperse aerosols, which provide more rapid, less resource-intensive test results, which still meet data quality requirements necessary for developing and evaluating ambient aerosol inlets. This goal was successfully achieved through comprehensive efforts regarding polydisperse aerosol generation, dispersion, collection, extraction, and analysis over a wide range of aerodynamic particle sizes. Using proper experimental techniques, a sampler’s complete size-selective efficiency curve can be estimated with polydisperse aerosols in a single test, as opposed to the use of monodisperse aerosols, which require conducting multiple tests using several different particle sizes. While this polydisperse aerosol technique is not proposed as a regulatory substitute for use of monodisperse aerosols, the use of polydisperse aerosols is advantageous during an inlet’s development where variables of sampling flow rate and inlet geometry are often iteratively evaluated before a final inlet design can be successfully achieved. Complete Standard Operating Procedures for the generation, collection, and analysis of polydisperse calibration aerosols are available from EPA as downloadable files. The described experimental methods will be of value to other researchers during the development of ambient sampling inlets and size-selective evaluation of the inlets in aerosol wind tunnels.

© 2018 American Association for Aerosol Research     

Numerical,wind-tunnel,and atmospheric evaluation of a turbulent ground-based inlet sampling system

The use of inlets for transferring aerosols from the environment to instrumentation can introduce uncertainty in the measurement of aerosol properties. Aerosol loss during this process is a non-negligible issue that may bias the subsequent measurements. These loss mechanisms include aspiration at the inlet head and deposition/evaporation/condensation during transport through the sampling lines. Coarse-mode aerosol is significantly impacted by the aspiration and inertial loss mechanisms within an inlet system. This work uses wind tunnel experiments to investigate aerosol losses through the Storm Peak Laboratory’s (SPL) new aerosol inlet system. The inlet is used extensively for both intensive field campaigns and long-term aerosol monitoring. The results of numerical simulations of the SPL aerosol inlet sampling efficiency are provided at several wind speeds, and experimental results demonstrate the system has a 50% cut off for the coarse-mode at an aerodynamic diameter of approximately 13?μm and wind speed of 0.5?m s^?1. This investigation will lead to improved accuracy of in situ aerosol measurements at SPL and this system can be replicated at other atmospheric stations.

Copyright © 2019 American Association for Aerosol Research     

Simulated sensitivity of secondary organic aerosol in the South Coast Air Basin of California to nitrogen oxides and other chemical parameters

Sensitivity of secondary organic aerosol (SOA) concentrations in the South Coast Air Basin (SoCAB) of California to nitrogen oxide (NO_x) emission is simulated using gas-phase chemistry and gas-particle partitioning modules. These modules are implemented into a three-dimensional air quality model applied for high-pollution summer meteorology and 2008 emissions. To test sensitivity, NO_x emissions in all locations and at all times are scaled by factors ranging from 0.1 to 10.0 in separate model runs. The basin-wide average SOA concentration exhibits a ‘turnover’ NO_x emission multiplicative factor, above and below which the average SOA concentration decreases. For the entire SoCAB, this critical NO_x emission factor is ～0.3; while the magnitude of SOA concentrations changes with time, this peak value (～0.2–0.3) appears to be relatively independent of the hour of the simulated day. When considering individual locations within the SoCAB, this peak factor shows a slightly broader range. Projected emissions for 2023 indicate a decrease in basin-average SOA concentration; the response at individual locations, however, can be either positive or negative, indicating the need for location-specific considerations. Ensembles of module simulations based on parameter values selected using efficient sampling techniques (Latin Hypercube method) are used to identify parameters to which SOA predictions are significantly sensitive. Total SOA predictions are most sensitive (in no particular order) to concentrations of O₃, unsaturated species formed from the gas-phase oxidation of monoaromatic compounds, and substituted products from long-chain alkane oxidation. Secondary inorganic aerosol species, likely through influencing aerosol liquid water, control at least partially the formation of SOA upwind. In addition, the rate at which unsaturated bicyclic oxidation products of monoaromatic compounds are oxidized by hydroxyl radical impacts significantly SOA prediction. These findings emphasize the need for consideration of long-chain alkanes and monoaromatic species when designing emission control strategies.

© 2018 American Association for Aerosol Research     

In-situ characterization of e-cigarette aerosols by 90°-light scattering of polarized light

The polarization ratio method is used for fast in-situ characterization of unimodal condensed aerosols of e-cigarettes. The method is based on 90°-light scattering of polarized 680 nm laser light by the droplet ensemble inside an optically defined measuring volume. Mass median droplet diameter (MMD) is derived from the ratio of scattered light from horizontally and vertically polarized incident light beams assuming a fixed value of the geometric standard deviation of the aerosol mass distribution. MMD is used to correct for the size dependence of the mass-based scattering signal of vertically polarized light to obtain the mass concentration if the sensor is calibrated once with an aerosol with a fixed MMD. The sensor uses commercially available aerosol photometers, and its application to e-cigarette aerosols was validated with an impactor for MMD and with a filter measurement for mass concentration. Good correlation (r² > 0.97) for both parameters was observed. Application ranges are mass concentration range 0.5–50 mg/L, MMD 0.2–1.2 µm, 100 ms time resolution, and 0.2–3 L/min flow rate. The usefulness of this simple sensor for e-cigarette aerosol characterization is demonstrated by developing a scaling law between MMD and operating parameters of an e-cigarette, i.e., puff flow rate and mass concentration.

Copyright © 2018 American Association for Aerosol Research     

The Use of Nonlinear Acoustics as an Energy-Efficient Technique for Aerosol Removal

This article is a feasibility study on using nonlinear acoustic effects, acoustic streaming and acoustic radiation pressure, for aerosol removal in an air duct. Unlike previous research, which used acoustics solely to cause aerosol agglomeration prior to aerosol removal in traditional duct collection systems, this article considers the acoustic streaming effect, which is significant but was previously neglected. Monodispersed polystyrene spheres with diameters ranging from 0.3 to 6 μm were tested. The proposed system removed 12–20% of the submicron aerosols and 25–32% of the micron aerosols when the airflow rate was approximately 90 L/min. Acoustic streaming introduces stagnation points on the surface of the air duct and removes the aerosols by deposition. Acoustic radiation pressure causes aerosols to form agglomerates. This enhances inertial impaction and/or gravitational sedimentation, which further enhances the removal efficiency of micron aerosols. The particle-removal efficiency is proportional to the duration that the aerosols are exposed to the acoustic field. The pressure drop due to the nonlinear acoustic effects is negligible; thus, power consumption is minimal. This system has the potential to be developed into an energy-efficient technique for aerosol removal.

Copyright 2014 American Association for Aerosol Research     

Numerical modeling of the performance of high flow DMAs to classify sub-2 nm particles

While there are several computational studies on differential mobility analyzers (DMA), there is none for high flow DMA to classify nanoparticles less than 3?nm. A specific design of a high flow DMA, a half mini DMA, is investigated to predict its performance through numerical modeling in the incompressible flow regime. The governing equations for flow field, electric field and aerosol transport are solved using COMSOL 5.3. The transfer function of the half mini DMA is compared with that of a nano DMA (TSI 3085). The results show that both the height of the transfer function and resolution (R) of the half mini DMA are much better than those of nano DMA in sub-2?nm particle size range. Finally, the transfer function of half mini DMA is evaluated for different values of aerosol flow rate to the sheath flow rate (q/Q). Comparison of the simulated transfer function with existing models from Knutson–Whitby and Stolzenburg is also elucidated. It is found that the former model overestimates the resolution; whereas the latter is close to the simulation results for q/Q above 0.067. This work provides a useful method to study the flow regimes and transfer function of a high flow DMA.

Copyright © 2018 American Association for Aerosol Research     

Numerical analysis of the dynamics of aerosol inertial collection and aggregation on raindrops

A three-dimensional stochastic model is developed for predicting atmospheric aerosol collection and aggregation on the surface of a falling raindrop at its terminal velocity. Potential flow and viscous flow are assumed as the flow fields in the vicinity of the large and the small raindrops, respectively. The results show that hydrophobic coarse mode aerosols collected by either small raindrops (d_c < 100 μm) or large drops (d_c > 100 μm) form aggregations on the surfaces of drops, and accumulation mode aerosols tend to be captured by the aggregations or hydrophobic coarse particles which have been collected by the drops, and this may significantly enhance the capability of the raindrop for fine aerosol collection. When the aggregation effect is considered in the calculation, fine aerosol efficiency can be promoted by one to two orders of magnitude. Therefore, fine particle collision efficiency by raindrops is underestimated by employing the classical dynamic theory which neglects the particle aggregation effect. However, the collection efficiency of coarse particles remains almost constant with the increase in the amount of particles collected by large drops, while there is only a slight increase in efficiency by small raindrops upon increasing in particle concentration. This implies that the traditional limiting trajectory method can still be used for the calculation of coarse particle collection efficiencies by either small or large raindrops.

Copyright © 2018 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     

Emission measurements with gravimetric impactors and electrical devices: An aerosol instrument comparison

Particulate matter in the atmosphere is known to affect Earth’s climate and to be harmful to human health. Accurately measuring particles from emission sources is important, as the results are used to inform policies and climate models. This study compares the results of two ELPI?+?devices, two PM10 cascade impactors and an eFilter, in combustion emission measurements. The comparison of the instruments in a realistic setting shows what types of challenges arise from measuring an emission aerosol with unknown particle morphologies and densities, different particle concentrations and high temperature. Our results show that the PM10 cascade impactors have very good intercorrelation when the collected mass is greater than 150?µg, but below that, the uncertainty of the results increases with decreasing mass. The raw signals of two ELPI?+?devices were nearly identical in most samples, as well as the particle number concentrations and size distributions calculated from raw signals; however, transforming the current distributions into mass distributions showed variation in the mass concentration of particles larger than 1?µm. The real-time time signal measured by eFilter was similar to the total current measured by ELPI+. The eFilter and PM10 cascade impactors showed similar particle mass concentrations, whereas ELPI?+?showed clearly higher ones in most cases. We concluded that the difference is at least partially due to volatile components being measured by ELPI+, but not by the mass collection measurements.

Copyright © 2019 American Association for Aerosol Research     

Observations of a Correlation Between Primary Particle and Aggregate Size for Soot Particles

For decades, soot has been modeled as fractal-like aggregates of nearly equiaxed spherules. Cluster–cluster aggregation simulations, starting from a population of primary particles, give rise to structures that closely match real aerosols of solid particles produced in flames. In such simulations, primary particle size is uncorrelated with aggregate size, as all aggregates contain primary particles drawn from the same population. Aerosol measurements have been interpreted with this geometric model. Examination of transmission electron micrographs of soot samples from various sources shows that primary particle sizes are not well mixed within an aerosol population. Larger aggregates tend to contain larger primary particles and the variation in size is much larger between aggregates than within aggregates. The soot sources considered here are all substantially not well-mixed (aircraft jet engine, inverted diffusion flame, gasoline direct injection engine, heavy-duty compression ignition engine). The observed variations in primary particle size can be explained if soot aggregates are formed and grew by coagulation in small zones of the combustion chamber, prior to dilution and transport (with minimal coagulation) to the sampling system.

Copyright 2014 American Association for Aerosol Research     

Development and qualification of a VH-TDMA for the study of pure aerosols

We create and qualify a Volatility and Hygroscopicity Tandem Differential Mobility Analyzer (VH-TDMA) for the study of aerosols. This VH-TDMA measures size distributions, volatility, and hygroscopicity and includes an auxiliary conditioner that allows quick connection to other external aerosol conditioners. The differential mobility analyzers are not temperature controlled, allowing the surrounding environment to influence the measurement conditions, and this is fully accounted for when measuring aerosol volatility and hygroscopicity. For the volatility conditioner, the VH-TDMA uses a 15?m coil of tubing in an oven to evaporate aerosol samples at elevated temperatures. We measured several single component model aerosols to qualify the differential mobility particle sizer (DMPS) channel and each of the conditioners: hygroscopicity and volatility. Due to insufficient power supply calibration in this study, the TDMA channel is limited to particle sizes greater than 70?nm. The DMPS channel was able to reproduce ammonium sulfate size distributions when compared to common scanning mobility particle sizers. For hygroscopicity, the standard deviation in the measured ammonium sulfate growth factors was 0.03 over a 4-h experiment. From this data, the TDMA has an observed relative humidity error of ±0.6% with manufacturer reported error of ±1.2% relative humidity. The volatility channel reproduced the previously published saw tooth pattern of room temperature saturation vapor pressures from atomized C3-C9 diacids. The maximum percent difference in room temperature saturation vapor pressure was approximately 80%. The enthalpy of sublimation derived from the diacids increased monotonically (except for suberic acid) and resembled measurements from mass effusion techniques.

Copyright © 2019 American Association for Aerosol Research     

Cloud condensation nuclei activity and droplet formation of primary and secondary organic aerosol mixtures

Understanding the mixing behavior of anthropogenic primary and biogenic secondary organic aerosol (POA and SOA) is important for characterizing their interactions with water vapor. The following work expands upon previous studies and investigates cloud condensation nuclei (CCN) activity and droplet kinetics of α-pinene SOA formed in an environmental chamber and mixed with diesel or motor oil-diesel fuel POA. The changes in the aerosol mixing are similar to previously published work but this study provides new CCN activity and droplet information. The CCN activity of the unmixed aerosol systems are measured separately; κ = 0.15, 0.11, 0.022 for α-pinene SOA, diesel POA and motor oil-diesel fuel POA, respectively. In the α-pinene SOA + diesel POA mixture, the CCN activity, characterized by κ-hygroscopicity, decreases from κ = 0.15 to 0.06 after an initial injection of the POA but increases to κ = 0.12. The increase in CCN activity occurs after particle collision (coagulation and wall-loss) rates dominate aerosol processes in the chamber. The α-pinene SOA + motor oil-fuel POA does not readily mix and the CCN activity of the complex system increases with time (from κ = 0.022 to 0.10). An empirical equation using unit mass resolution (UMR) AMS data of two different ion fragments reasonably predicts CCN activity of the POA and SOA mixtures. CCN measurement may be a promising tool to gain additional insight into the complex mixtures of organic aerosol and subsequent interactions with water vapor.

Copyright © 2018 American Association for Aerosol Research     

The First Combined Thermal Desorption Aerosol Gas Chromatograph—Aerosol Mass Spectrometer (TAG-AMS)

To address the critical need for improving the chemical characterization of the organic composition of ambient particulate matter, we introduce a combined thermal desorption aerosol gas chromatograph—aerosol mass spectrometer (TAG-AMS). The TAG system provides in-situ speciation of organic chemicals in ambient aerosol particles with hourly time resolution for marker compounds indicative of sources and transformation processes. However, by itself the TAG cannot separate by particle size and it typically speciates and quantifies only a fraction of the organic aerosol (OA) mass. The AMS is a real-time, in-situ instrument that provides quantitative size distributions and mass loadings for ambient fine OA and major inorganic fractions; however, by itself the AMS has limited ability for identification of individual organic compounds due to the electron impact ionization detection scheme used without prior molecular separation.

The combined TAG-AMS system provides real-time detection by AMS followed by semicontinuous analysis of the TAG sample that was acquired during AMS operation, achieving simultaneous and complementary measurements of quantitative organic mass loading and detailed organic speciation. We have employed a high-resolution time-of-flight mass spectrometer (HR-ToF-MS) to enable elemental-level determination of OA oxidation state as measured on the AMS, and to allow improved compound identification and separation of unresolved complex mixtures (UCM) measured on the TAG. The TAG-AMS interface has been developed as an upgrade for existing AMS systems. Such measurements will improve the identification of organic constituents of ambient aerosol and contribute to the ability of atmospheric chemistry models to predict ambient aerosol composition and loadings.

Copyright 2014 American Association for Aerosol Research     

Droplet-based microfluidics detector for bioaerosol detection

Detection of bioaerosols is important in fields ranging from environmental health monitoring to biosurveillance, and current detector weaknesses have motivated the development of new technologies. In this work, a detector was built, which applies the principles of droplet microfluidics to bioaerosol detection. Droplet microfluidics is a subfield of microfluidics based on the creation of monodisperse microdroplets with compartmentalized reagents and supports enhanced assays and fluidic manipulations. The bioaerosol detector operates by aerodynamically focusing aerosols directly into these droplets to harness the benefits of the microreactor environment. A breadboard detector system, which consisted of an aerodynamic focusing lens, aerosol-focusing capillary, microfluidic droplet chip, and optical microscope, was constructed. Computational fluid dynamic simulations and Lagrangian particle tracking modeling were conducted to identify the optimal conditions for focusing. Preliminary experiments, where aerosols were deposited onto a solid substrate, demonstrated sub 200-µm spot diameters for aerodynamic diameters of 2–5 µm. Test aerosols were then generated, and collected into the microfluidic liquid interface on the chip as verified by microscopy. Recovery efficiency of the aerosols was dependent on aerosol size and ranged from about 27% to nearly 100%. Finally, to prove bioaerosol collection and detection, a droplet propidium iodide (PI) assay was performed: the system distinguished between E. coli and non-biological aerosols within 20 s. Overall, this work established the technique of direct collection of bioaerosols into a convenient droplet microfluidic platform for detection.

Copyright © 2017 The Johns Hopkins University Applied Physics Laboratory     

Highly sensitive nano-aerosol detection based on the whispering-gallery-mode in cylindrical optical fiber resonators

Highly sensitive detection of nanoscale aerosols, or nano-aerosols, is a difficult challenge. Here, we report a fiber optical technique that is capable of detecting trace-level nano-aerosols. Our method is based on monitoring the nano-aerosol-induced resonance shift due to the optical Whispering-Gallery-Mode (WGM) in a cylindrical optical fiber resonator. A nearly linear relationship between the WGM resonance shift and the aerosol coverage ratio of silica nanoparticles (40–50 nm dia.) on the fiber resonator was identified in the low coverage regime. Our experimental results imply sensitivity at the level of ～2 nanoparticles per μm² deposited on the fiber resonator, which corresponds to pg-level sensitivity in the total aerosol mass within the effective detection area. The response of this fiber optical sensor is further confirmed by using silica nanoparticles deposited on the fiber surface via electrostatic self-assembly. The fiber optical technique for nanoparticle detection may ultimately lead to an instrument capable of real-time in situ aerosol detection with ultrahigh sensitivity.

Copyright © 2016 American Association for Aerosol Research