Investigations of the Diurnal Cycle and Mixing State of Oxalic Acid in Individual Particles in Asian Aerosol Outflow

The mixing state of oxalic acid was measured in Asian outflow during ACE-Asia by direct shipboard measurements using an ATOFMS single-particle mass spectrometer. Oxalic and malonic acids were found to be predominantly internally mixed with mineral dust and aged sea salt particles. A persistent diurnal cycle of oxalic acid in mineral dust occurred for over 25 days in marine, polluted marine, and dust storm air masses. The preferential enrichment of diacids in mineral dust over carbonaceous particles and their diurnal behavior indicate a photochemical source of the diacids. Oxalate was only detected simultaneously with elevated aged dust particle counts. This suggests that the diurnal production of diacids most likely results from episodic atmospheric processing of the polluted dust aerosol. We propose a mechanism to explain these observations in which the photochemical oxidation of volatile organic compounds is followed by partitioning of the diacids and precursors to the alkaline Asian dust, with subsequent heterogeneous and aqueous oxidation. Our data indicate that the particulate diacids were produced over just a few hours close to the source; no significant production or destruction appears to have occurred during long-range transport to the ship. No evidence of extensive cloud processing of the sampled aerosol was found. This mixing state of diacids has important implications for the solubility and cloud nucleation properties of the dominant fraction of water-soluble organics and the bioavailability of iron in dust

In Situ pH Measurements of Individual Levitated Microdroplets Using Aerosol Optical Tweezers

The pH of microscale reaction environments controls numerous physicochemical processes, requiring a real-time pH microprobe. We present highly accurate real-time pH measurements of microdroplets using aerosol optical tweezers (AOT) and analysis of the whispering gallery modes (WGMs) contained in the cavity-enhanced Raman spectra. Uncertainties ranging from ±0.03 to 0.06 in pH for picoliter droplets are obtained through averaging Raman frames acquired at 0.5 Hz over 3.3 min. The high accuracy in pH determination is achieved by combining two independent measurements uniquely provided by the AOT approach: the anion concentration ratio from the spontaneous Raman spectra, and the total solute concentration from the refractive index retrieved from WGM analysis of the stimulated cavity-enhanced Raman spectra. pH can be determined over a range of −0.36 to 0.76 using the aqueous sodium bisulfate system. This technique enables direct measurements of pH-dependent chemical and physical changes experienced by individual microparticles and exploration of the role of pH in the chemical behavior of confined microenvironments

Morphology of Organic Carbon Coatings on Biomass-Burning Particles and Their Role in Reactive Gas Uptake

Inorganic salts are a significant component of biomass-burning aerosol (BBA) and have inconsistently been observed to undergo chemical reactions with strong acids and other reactants during atmospheric aging, altering particle hygroscopicity and further reactivity while also liberating reactive halides such as ClNO2(g) and HCl­(g) and recycling or removing nitrogen oxides. The condensation of organic carbon to BBA coemitted by wildfires and other biomass combustion processes can affect aerosol particle reactivity with trace gases. These organic coatings along with deliquescence of chloride salts requiring high relative humidities >80% were recently proposed to explain the low observed reaction probability of N2O5(g) with BBA. We performed a series of single-particle analyses to characterize the morphology and composition of laboratory-generated BBA from authentic fuels using transmission and scanning electron microscopies (T/SEM) to test this hypothesis. Stable organic coatings that appear thicker or more oxidized than the particle bulk (likely tar balls) were observed to form on some spherical BBA particles but only when photooxidation was not applied. Inorganic salt components were inconsistently observed to react during simulated photooxidative atmospheric aging, sometimes undergoing chloride displacement reactions with strong acid vapors to produce sulfate and nitrate salts. Particles were also observed where chloride-salt regions were not completely depleted by reaction with strong acids. Organic carbon particle coatings plus the physical phase of chloride salts and deliquescence limitations appear to play a significant role in determining in which particles and fuel types these chloride displacement reactions can occur and the extent of these reactions with acidic vapors

Response of the Reaction Probability of N₂O₅ with Authentic Biomass-Burning Aerosol to High Relative Humidity

N2O5 and ClNO2, important oxidant reservoirs, were recently demonstrated to be produced in simulated nocturnal aging of biomass-burning smoke. However, the heterogeneous kinetics of N2O5(g) reactive uptake, γ­(N2O5), and ClNO2(g) product yields, φ­(ClNO2), are still under investigation. Our previous experiments on biomass-burning aerosol (BBA) revealed unexpectedly low and consistent N2O5 reaction probabilities despite often large chloride aerosol mass fractions. This could be explained by the inaccessibility of N2O5 to chloride due to the lack of chloride salt deliquescence or inhibition from organic coatings. In this work, an entrained aerosol flow tube system was deployed to examine the reaction probability of dinitrogen pentoxide and the nitryl chloride yield at 86% relative humidity (RH) for four types of BBA sampled from combustion emissions. At 86% RH, γ­(N2O5) ranged from 3.4 × 10–3 on longleaf pine needle BBA to 16 × 10–3 on black needlerush BBA with a 100–300% increase in γ­(N2O5) for high-chloride fuels and little change in low-chloride fuels compared to previous determinations of γ­(N2O5) at <75% RH. These trends demonstrate how aqueous chloride phases drive N2O5 reactive uptake and that organic coatings do not limit γ­(N2O5) in high-chloride fuels at high RH. φ­(ClNO2) was substantial in experiments with high-chloride BBA, where φ­(ClNO2) approached 100% at 86% RH. We conclude that the complex chemical composition and morphology of BBA along with the solid phase state of chloride salts in BBA at RH < ∼80% limit the ability for N2O5 to heterogeneously react with BBA and produce ClNO2(g)

Development and characterization of a “store and create” microfluidic device to determine the heterogeneous freezing properties of ice nucleating particles

Understanding heterogeneous ice nucleation induced by ice nucleating particles (INPs) is hindered by analytical challenges in accurately determining the freezing temperature spectrum, abundance, and physicochemical properties of INPs. Here we evaluate the performance of a microfluidic device that employs a “store and create” approach to measure the ice nucleation properties of approximately 600 uniformly sized nanoliter water droplets. These droplets are immersed in surfactant-free environmentally sustainable squalene oil and do not contact the polymer walls of the microfluidic device. The device interfaced with a cold plate temperature controller has a greatly reduced background freezing temperature spectrum for filtered water droplets compared to conventional microliter droplet-on-substrate freezing methods. Droplets containing particles of interest are readily generated on-chip from a suspension of particles in water. Background freezing for 6 nL water droplets exhibits a median freezing temperature of −33.7 ± 0.4 °C, close to the theoretical freezing temperature of −34.5 °C. The immersion freezing temperature spectra obtained from Snomax bacterial and illite mineral particles compares well with literature data, and the freezing contribution from either type of particle can be separated from a mixed suspension. Our approach generates a highly uniform droplet size distribution, causes no clogging of the microfluidic device, and is capable of reproducible droplet refreezes. The high-resolution freezing spectra obtained from large droplet number arrays enables the use of the derivative INP temperature spectrum analysis to quantitatively distinguish between different classes of INPs. The lower and consistent filtered water background freezing temperature enables measurements of almost the entire immersion freezing temperature regime from −33 to 0 °C, and quantification of weaker but often abundant INPs such as those found in biomass-burning smoke aerosol. Copyright © 2019 American Association for Aerosol Research</p

Survey of medical staff performing intubation/extubation with enclosure on mannequins (n = 39).

Survey of medical staff performing intubation/extubation with enclosure on mannequins (n = 39).</p

Design and dimensions of the enclosure: A schematic of the enclosure showing the three cutout openings.

One on the cephalad (provider) side to provide access for the healthcare worker’s hands, a second on the caudal (patient) side to allow room for the patient’s torso, and a third on the lateral side to enable ventilator and other tubing access. The provider access cutout is large to maximize mobility.</p

Summary of statistical result for cover comparison.

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Comparison of total particle concentration generated from real and simulated coughs.

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Complete survey results of medical professionals that had used the enclosure for simulated intubation/extubation procedures on mannequins at two different hospitals.

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