Practical limitations of aerosol separation by a tandem differential mobility analyzer–aerosol particle mass analyzer

<p>A cavity ring-down spectrometer and condensation particle counter were used to investigate the limitations in the separation of singly and multiply charged aerosol particles by a tandem differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM). The impact of particle polydispersity and morphology was investigated using three materials: nearly monodisperse polystyrene latex nanospheres (PSL); polydisperse, nearly spherical ammonium sulfate (AS), and polydisperse lacey fractal soot agglomerates. PSL and AS particles were easily resolved as a function of charge. For soot, the presence of multiply charged particles severely affects the isolation of the singly charged particles. In cases where the DMA–APM was unable to fully resolve the singly charged particles of interest, the peak mass deviated by up to 13% leading to errors in the mass specific extinction cross section of over 100%. For measurements of nonspherical particles, nonsymmetrical distributions of concentration as a function of mass were a sign of the presence of multiply charged particles. Under these conditions, the effects of multiply charged particles can be reduced by using a second charge neutralizer after the DMA and prior to the APM. Dilution of the aerosol stream serves to decrease the total number concentration of particles and does not remove the contributions of multiply charged particles.</p

Measurement of Gas and Aerosol Phase Absorption Spectra across the Visible and Near-IR Using Supercontinuum Photoacoustic Spectroscopy

We demonstrate a method to measure the absorption spectra of gas and aerosol species across the visible and near-IR (500 to 840 nm) using a photoacoustic (PA) spectrometer and a pulsed supercontinuum laser source. Measurements of gas phase absorption spectra were demonstrated using H₂O_(g) as a function of relative humidity (RH). The measured absorption intensities and peak shapes were able to be quantified and compared to spectra calculated using the 2012 High Resolution Transmission (HITRAN2012) database. Size and mass selected nigrosin aerosol was used to measure absorption spectra across the visible and near-IR. Spectra were measured as a function of aerosol size/mass and show good agreement to Mie theory calculations. Lastly, we measured the broadband absorption spectrum of flame generated soot aerosol at 5% and 70% RH. For the high RH case, we are able to quantifiably separate the soot and water absorption contributions. For soot, we observe an enhancement in the mass specific absorption cross section ranging from 1.5 at 500 nm (<i>p</i> < 0.01) to 1.2 at 840 nm (<i>p</i> < 0.2) and a concomitant increase in the absorption Ångström exponent from 1.2 ± 0.4 (5% RH) to 1.6 ± 0.3 (70% RH)

Reduction of Suspended Graphene Oxide Single Sheet Nanopaper: The Effect of Crumpling

The recent development of aerosol generated crumpled graphene oxide nanopaper provides a test bed for studying the reactivity of physically deformed graphene-based materials. In this work, we measured the thermal reduction of aerosolized single sheet, crumpled graphene oxide (GO) nanopaper. Online aerosol mass analysis was used to monitor the mass evolution of individual crumpled GO nanopaper during reduction. For the first time, the extent of sp² bonding within the material was characterized using photoacoustic spectroscopy. The chemical composition of reduced GO nanopaper was evaluated using X-ray photoemission spectroscopy (XPS). Thermal reduction kinetics was determined using both the loss of mass and the change in optical absorption, each measured as a function of temperature. The activation energies were different for the two techniques, suggesting mass loss and light absorption probe different processes of GO reduction. We measured a constant effective density at high reduction temperatures suggesting that portions of reduced GO unzip from the sheet similar to what has been observed in oxidized graphene

Restructuring of Graphene Oxide Sheets into Monodisperse Nanospheres

We demonstrate a method to restructure graphene oxide sheets into monodisperse solid 17 nm nanospheres by tuning the solution ionic strength. This method enables the preparation of both two-dimensional self-assemblies comprising three-dimensional GO nanospheres and three-dimensional super assemblies of GO clusters via dispersal into an aerosol. The GO super assemblies are more thermally stable than single crumpled sheets. Finally, we demonstrate that GO nanospheres and their assemblies can be thermally processed to form reduced GO with high aromatic character while still maintaining their spherical conformation

Structural Contributions to Charge Transport across Ni-Octanedithiol Multilayer Junctions

We report the fabrication and characterization of multilayer thin films incorporating 1,8-octanedithiols and Ni atoms. Low-temperature charge transport measurements exhibit inelastic co-tunneling and resonant tunneling features that correspond energetically to vibrational excitations of the molecular multilayer. Several junctions exhibit changes in conductance features characteristic of charge defect-gating. Transport through our junctions is shown to be dominated by the intrinsic properties of the multilayer

Crumpled Nanopaper from Graphene Oxide

Graphene oxide (GO) in aqueous solution was aerosolized and rapidly dried to produce crumpled nanopaper-like sheets. Online size selection and aerosol mass analysis was used to determine the fractal dimension (<i>D</i>) of crumpled GO nanosheets as 2.54 ± 0.04. That is identical to macroscale materials, such as crumpled balls of paper and foil. Thermal reduction of crumpled GO nanosheets did not change <i>D</i>, even after loss of nearly 25% of the nanosheets mass. We demonstrate that <i>D</i> is able to be tuned by altering solvent conditions. A 10% acetone mixture increased <i>D</i> to 2.68 ± 0.02. Calculations of the confinement force show that crumpling of GO nanosheets is driven by the capillary force associated with rapid solvent loss

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

This study describes the characterization of a H2O-dispersible, highly-absorbing carbonaceous nanomaterial that mimics the morphological and spectroscopic properties of aged black carbon aerosol (BC). When atomized from aqueous suspension, the material forms particles with a collapsed morphology resembling aged soot or BC. The material is >90 percent elemental carbon and has a mass absorption coefficient (MAC) and spectral dependence consistent with BC values published in the literature. The MAC at a wavelength of 532 nm decreased monotonically from 8.5 to 5.8 m2 g−1 for aerosol with mobility diameters between 150 nm to 500 nm. The single scatter albedo (SSA) at wavelengths of 405 nm and 660 nm was a function of both wavelength and mobility diameter and increased from 0.1 to 0.4 with mobility diameters between 150 nm to 400 nm. The Ångström absorption exponent (AAE) between λ = 405 nm and 780 nm decreased monotonically from 1.4 to 0.6 for aerosol with mobility diameters between 150 nm to 400 nm. We demonstrate that this material can be used for fast, efficient calibration of aerosol photoacoustic spectrometers and for evaluation of spectroscopic-based measurements of aerosol mass concentration using in-situ photoacoustic spectroscopy (PAS) and filter-based light attenuation measurements for up 50 µg m−3, enabling inter-method and inter-laboratory instrument comparison. Copyright © 2019 American Association for Aerosol Research</p

Absorption Spectra of Martian Dust Simulants

Understanding the Martian climate requires a detailed characterization of the optical properties of Martian dust as it is a ubiquitous component of the atmosphere. The continued improvement of Martian atmosphere observations motivates measurements of terrestrial simulants under controlled conditions to support field studies and computational modeling. This investigation demonstrates an in situ method to entrain and directly measure the absorption spectrum of six Martian dust simulants using photoacoustic spectroscopy. Measured size distributions and absorption cross sections (Cabs) are used in Mie theory calculations to retrieve the effective imaginary component of the refractive index (keff) of each sample while constraining the real component to n = 1.5, the refractive index of silica. The weakly absorbing simulants have keff values ranging between 0.002 and 0.03 at a wavelength of 500 nm that are correlated with the mass fraction of iron oxides

Common Single-Use Consumer Plastic Products Release Trillions of Sub-100 nm Nanoparticles per Liter into Water during Normal Use

This study demonstrates that commonly used consumer products such as single-use food grade nylon bags and hot beverage cups lined with low-density polyethylene release nanometer-sized plastic particles at number densities >1012 L–1 when exposed to water. The number of particles released was a function of the initial water temperature (high temperature vs ambient) for each of the tested materials. Mean particle diameters were between 30 and 80 nm with few particles >200 nm. The number of particles released into hot water from food grade nylon was 7 times higher when compared to single-use beverage cups. On a particle number density basis, particles released into water from a single 300 mL hot beverage cup equate to one particle for every seven cells in the human body in a size range available for cellular uptake

Measured Wavelength-Dependent Absorption Enhancement of Internally Mixed Black Carbon with Absorbing and Nonabsorbing Materials

Optical absorption spectra of laboratory generated aerosols consisting of black carbon (BC) internally mixed with nonabsorbing materials (ammonium sulfate, AS, and sodium chloride, NaCl) and BC with a weakly absorbing brown carbon surrogate derived from humic acid (HA) were measured across the visible to near-IR (550 to 840 nm). Spectra were measured in situ using a photoacoustic spectrometer and step-scanning a supercontinuum laser source with a tunable wavelength and bandwidth filter. BC had a mass-specific absorption cross section (MAC) of 7.89 ± 0.25 m² g^–1 at λ = 550 nm and an absorption Ångström exponent (AAE) of 1.03 ± 0.09 (2σ). For internally mixed BC, the ratio of BC mass to the total mass of the mixture was chosen as 0.13 to mimic particles observed in the terrestrial atmosphere. The manner in which BC mixed with each material was determined from transmission electron microscopy (TEM). AS/BC and HA/BC particles were fully internally mixed, and the BC was both internally and externally mixed for NaCl/BC particles. The AS/BC, NaCl/BC, and HA/BC particles had AAEs of 1.43 ± 0.05, 1.34 ± 0.06, and 1.91 ± 0.05, respectively. The observed absorption enhancement of mixed BC relative to the pure BC was wavelength dependent for AS/BC and decreased from 1.5 at λ = 550 nm with increasing wavelength while the NaCl/BC enhancement was essentially wavelength independent. For HA/BC, the enhancement ranged from 2 to 3 and was strongly wavelength dependent. Removal of the HA absorption contribution to enhancement revealed that the enhancement was ≈1.5 and independent of wavelength