An Unexpected Restructuring of Combustion Soot Aggregates by Subnanometer Coatings of Polycyclic Aromatic Hydrocarbons

We investigated the effect of thin polycyclic aromatic hydrocarbon (PAH) coatings on the structure of soot aggregates. Soot aerosol from an inverted diffusion burner was size classified, thermally denuded, coated with six different PAHs, and then characterized using scanning electron microscopy, light scattering, and mass-mobility measurements. Contrary to our expectation, significant restructuring was observed in the presence of subnanometer layers of pyrene, fluoranthene, and phenanthrene. These PAHs remained in subcooled liquid state in thin films, whereby the liquid layer acted as a lubricant, reducing the force required to initiate the restructuring. Thin layers of PAH of higher melting temperatures (perylene, anthracene, and triphenylene) presumably remained solid because these chemicals induced lesser structural changes. Our results suggest that some of the intrinsic PAH generated during incomplete combustion may induce significant restructuring of soot aggregates, even when present in small quantities, altering the properties and atmospheric impacts of combustion aerosols

Ultrasonic Study of Water Adsorbed in Nanoporous Glasses

Thermodynamic properties of fluids confined in nanopores differ from those observed in the bulk. To investigate the effect of nanoconfinement on water compressibility, we performed water sorption experiments on two nanoporous glass samples while concomitantly measuring the speed of longitudinal and shear ultrasonic waves in these samples. These measurements yield the longitudinal and shear moduli of the water laden nanoporous glass as a function of relative humidity that we utilized in the Gassmann theory to infer the bulk modulus of the confined water. This analysis shows that the bulk modulus (inverse of compressibility) of confined water is noticeably higher than that of the bulk water at the same temperature. Moreover, the modulus exhibits a linear dependence on the Laplace pressure. The results for water, which is a polar fluid, agree with previous experimental and numerical data reported for non-polar fluids. This similarity suggests that irrespective of intermolecular forces, confined fluids are stiffer than bulk fluids. Accounting for fluid stiffening in nanopores may be important for accurate interpretation of wave propagation measurements in fluid-filled nanoporous media, including in petrophysics, catalysis, and other applications, such as in porous materials characterization

Processing of Soot by Controlled Sulphuric Acid and Water Condensation - Mass and Mobility Relationship

The effects of atmospheric processing on soot particle morphology were studied in the laboratory using the Differential Mobility Analyzer-Aerosol Particle Mass Analyzer (DMA-APM) and the DMA-DMA (Tandem DMA) techniques. To simulate atmospheric processing, combustion soot agglomerates were altered by sulphuric acid vapor condensation, relative humidity (RH) cycling, and evaporation of the sulphuric acid and water by heating. Primary investigated properties were particle mobility size and mass. Secondary properties, derived from these, include effective density, fractal dimension, dynamic shape factor, and the mass fraction of condensed material. A transformation of the soot particles to more compact forms occurs as sulphuric acid and water condense onto fresh soot. The particle mass increases and initially the mobility diameter decreases, indicating restructuring of the soot core, likely due to surface tension forces. For a given soot source and condensing liquid, the degree of compaction depends strongly on the mass (or volume) fraction of condensed material. For water and sulphuric acid condensing on combustion soot, a mass increase of 2-3 times is needed for a transformation to spherical particles. In the limit of spherical particles without voids, the effective density then approaches the inherent material density, the fractal dimension approaches 3 and the dynamic shape factor approaches 1. Our results indicate that under typical atmospheric conditions, soot particles will be fully transformed to spherical droplets on a time scale of several hours. It is expected that the morphology changes and addition of soluble material to soot strongly affect the optical and hygroscopic properties of soot

The Impact of Sampling Medium and Environment on Particle Morphology

Sampling on different substrates is commonly used in laboratory and field studies to investigate the morphology and mixing state of aerosol particles. Our focus was on the transformations that can occur to the collected particles during storage, handling, and analysis. Particle samples were prepared by electrostatic deposition of size-classified sodium chloride, sulfuric acid, and coated soot aerosols on different substrates. The samples were inspected by electron microscopy before and after exposure to various environments. For coated soot, the imaging results were compared against mass-mobility measurements of airborne particles that underwent similar treatments. The extent of sample alteration ranged from negligible to major, depending on the environment, substrate, and particle composition. We discussed the implications of our findings for cases where morphology and the mixing state of particles must be preserved, and cases where particle transformations are desirable

Formation of highly hygroscopic soot aerosols upon internal mixing with sulfuric acid vapor

The hygroscopic properties of submicron soot particles during internal mixing with gaseous sulfuric acid have been investigated using a combined tandem differential mobility analyzer (TDMA) and differential mobility analyzer-aerosol particle mass analyzer (DMA-APM) technique. Fresh particles exhibit no change in mobility size and mass at subsaturated conditions, whereas particles exposed to gaseous sulfuric acid (10(9)-10(10) molecule cm(-3), 12 s contact time) experience significant mobility size and mass changes with increasing relative humidity (RH). The DMA-APM measurements reveal that particles of all sizes exposed to H2SO4 vapor gain mass with increasing RH because of absorption of water by sulfuric acid coating. However, on the basis of mobility size measurements using TDMA, upon humidification H2SO4-coated soot agglomerates display distinct hygroscopic growth patterns depending on their initial size and the mass fraction of condensed sulfuric acid. While small particles experience an increase in their mobility sizes, larger particles exhibit a marked shrinkage due to compaction. We suggest that determination of the hygroscopic properties of soot particles using a TDMA alone can be inconclusive. Restructuring of the soot agglomerates and filling of the voids that accompany the condensation of water-soluble materials and subsequent water absorption lead to little or no observable changes in particle mobility size at subsaturated RH even for particles that contain aqueous coatings. Extrapolation of our experimental results to the urban atmosphere indicates that initially hydrophobic soot particles acquire sufficient sulfate coating to become efficient CCN (cloud condensation nuclei) within a time period ranging from a few hours to a few days, dependent on the ambient H2SO4 level. The results imply that internal mixing with sulfuric acid through H2SO4 vapor condensation likely represents a common aging process for a variety of atmospheric aerosols. The variations in the size and hygroscopicity of soot particles during atmospheric processing influence their optical properties, cloud-forming potential, and human health effects

High Sensitivity of Diesel Soot Morphological and Optical Properties to Combustion Temperature in a Shock Tube

Carbonaceous particles produced from combustion of fossil fuels have strong impacts on air quality and climate, yet quantitative relationships between particle characteristics and combustion conditions remain inadequately understood. We have used a shock tube to study the formation and properties of diesel combustion soot, including particle size distributions, effective density, elemental carbon (EC) mass fraction, mass-mobility scaling exponent, hygroscopicity, and light absorption and scattering. These properties are found to be strongly dependent on the combustion temperature and fuel equivalence ratio. Whereas combustion at higher temperatures (∼2000 K) yields fractal particles of a larger size and high EC content (90 wt %), at lower temperatures (∼1400 K) smaller particles of a higher organic content (up to 65 wt %) are produced. Single scattering albedo of soot particles depends largely on their organic content, increasing drastically from 0.3 to 0.8 when the particle EC mass fraction decreases from 0.9 to 0.3. The mass absorption cross-section of diesel soot increases with combustion temperature, being the highest for particles with a higher EC content. Our results reveal that combustion conditions, especially the temperature, may have significant impacts on the direct and indirect climate forcing of atmospheric soot aerosols