James P. Cowin

The transformation of solid atmospheric particles into liquid droplets through heterogeneous chemistry: Laboratory insights into the processing of calcium containing mineral dust aerosol in the troposphere

[1] Individual calcium carbonate particles reacted with gas-phase nitric acid at 293 K have been followed using Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis as a function of time and relative humidity (RH). The rate of calcium carbonate to calcium nitrate conversion is significantly enhanced in the presence of water vapor. The SEM images clearly show that solid CaCO 3 particles are converted to spherical droplets as the reaction proceeds. The process occurs through a two-step mechanism involving the conversion of calcium carbonate into calcium nitrate followed by the deliquescence of the calcium nitrate product. The change in phase of the particles and the significant reactivity of nitric acid and CaCO 3 at low RH are a direct result of the deliquescence of the product at low RH. This is the first laboratory study to show the phase transformation of solid particles into liquid droplets through heterogeneous chemistry.

Immobility of protons in ice from 30 to 190 K

The anomalously fast motion of hydronium ions (H3O+) in water is often attributed to the Grotthuss mechanism, whereby protons tunnel from one water molecule to the next. This tunnelling is relevant to proton motion through water in restricted geometries, such as in ‘proton wires’ in proteins and in stratospheric ice particles. Transport of hydronium ions in ice is thought to be closely related to its transport in water,. But whereas claims have been made that such tunnelling can persist even at 0 K in ice, counter-claims suggest that the activation energy for hydronium motion in ice is non-zero. Here we use ‘soft-landing’ of hydronium ions on the surface of ice to show that the ions do not seem to move at all at temperatures below 190 K. This implies not only that hydronium motion is an activated process, but also that it does not occur at anything like the rate expected from the Grotthuss mechanism. We also observe the motion of an important kind of defect in ices hydrogen-bonded structure (the D defect). Extrapolation of our measurements to 0 K indicates that the defect is still mobile at this temperature, in an electric field of 1.6 × 108 V m−1.

Time-Resolved Aerosol Collector for CCSEM/EDX single-particle analysis

An automated Time-Resolved Aerosol Collector (TRAC) has been developed for sequential sampling of field-collected aerosols for laboratory-based Computer Controlled Scanning Electron Microscopy/Energy Dispersed X-ray (CCSEM/EDX) single-particle analysis. The collector is optimized for the use of grid-supported 50 nm carbon films as deposition substrates. The carbon films have low enough X-ray background to permit EDX analysis down to 0.1-0.2 w m particles, including detection of low-Z elements: C, N, and O. The TRAC provides unattended sampling onto a set of 151 individual grids, at sequential time intervals as short as 1 min. After collection, the samples are sealed and refrigerated pending analysis. The utility of the TRAC-CCSEM/EDX approach is exemplified using the aerosol samples collected during the Texas 2000 Air Quality Study (August 15-September 15, 2000). We are able to follow the time evolution in the relative contribution of nonvolatile particles such as ammonium sulfate, mineral dust, sea salt, and carbonaceous in the aerosol makeup. The results show, among other things, the diurnal cycles in appearance of fine carbonaceous and ammonium sulfate particles and substantial mixing/coating of mineral particles with ammonium sulfates.

Intense surface photoemission: Space charge effects and self-acceleration

Ultraviolet laser irradiation of surfaces, in the course of photoemission or surface photochemical studies, often produce copious electron emission, up to 1000’s of A/cm2. The time‐dependent fields produced by these electrons accelerate some of the electrons up to 5.4 times their initial energies. The steady‐state fields return most of the emitted electrons to the surface. We discuss and illustrate both phenomena with theoretical simulations and experiment, and discuss possible implications.

Imaging insulating oxides: scanning tunneling microscopy of ultrathin MgO films on Mo(001)

Using scanning tunneling microscopy (STM) we have studied the structure of an insulating thin film, MgO grown on Mo(001). Although the bandgap in bulk MgO is 7.8 eV, the films are sufficiently conducting to perform STM. Stable tunneling and imaging can be obtained for MgO films up to 25 A thick. Growth at room temperature produces uniform films with small domains of between 20 and 60 A in diameter. The domains have random shapes with the perimeter of the domains exhibiting no preferred orientation. Films as thick as 8 layers typically have 3 atomic layers exposed. Films grown at temperatures in excess of 1000 K exhibited three-dimensional MgO islands with exposed substrate. Close inspection of these films determined that at voltages as low as 2.5 V and for films as thick as 5 ML the STM directly measures oxide film morphology.

Ozone loss in soot aerosols

The fractal-like structure of atmospheric soot (e.g., elemental carbon) provides a large surface area available for heterogeneous chemistry in the upper troposphere and lower stratosphere [Blake and Kato, 1995]. One potentially important reaction is ozone decomposition on soot. Although extensively studied in the laboratory, a wide range of reaction probabilities have been observed (γ∼10−3 to γ∼10−7) which have been attributed to differences in reactivity between fresh (i.e., nonoxidized) versus aged (i.e., oxidized) soot [Schurath and Naumann, 1998]. The importance in understanding soot-ozone chemistry is particularly important in light of recent nighttime field measurements [Berkowitz et al., 2000] made over Portland, Oregon. The data revealed episodes of an anticorrelation between ozone mixing ratio and aerosol surface area density. During these episodes a single scattering albedo in the range 0.8–0.9 was measured, indicating an increased absorptive component of the aerosol, perhaps due to elemental carbon. In addition, an increase in the concentration of aerosols contained in the small size range of the fine mode (<0.1–0.15 μm) was observed, suggestive of new aerosol formation. In this article we attempt to explain these field observations. One explanation of the field observations is ozone loss occurring on atmospheric soot aerosol. Here we present laboratory results obtained using a static aerosol reactor that indicate that direct ozone loss on soot aerosol is unlikely under ambient conditions in the troposphere. An alternative and more likely explanation of the field data is based on ozone-mediated organic aerosol production. This could occur by either nighttime nitrate radical oxidation or direct ozone oxidation of hydrocarbons as suggested previously [Starn et al., 1998; Griffin et al., 1999; Kamens et al., 1999; Yu et al., 1999; De Gouw and Lovejoy, 1998].

Sticky Ice Grains Aid Planet Formation: Unusual Properties of Cryogenic Water Ice

There is limited time for the dust in the nebula around a newborn star to form planetesimals: in a few million years or less the stars stellar winds will disperse most of the unagglomerated dust. It has been difficult to explain the efficiency by which dust grains must have agglomerated to form planetesimals in circumstellar disks. A major obstacle is the fragility of aggregates, leading to collisional fragmentation, which makes it difficult for them to grow to, and beyond, meter-sized bodies. The distinct properties of cryogenic (5-100 K) amorphous water ice, which composes or coats the grains in the cooler parts of the nebulae (Jovian distances), may be able to account for the rapid agglomeration. Measurements are presented that show that this ice readily acquires persistent macroscopic electric dipoles, strongly enhancing grain-grain adhesion. In addition, measurements were made showing that vapor-deposited amorphous water ice is also highly mechanically inelastic (≈10% rebound). Together these may explain this efficient net sticking and net growth. Similar properties of higher temperature grains may aid agglomeration in the inner regions of the nebulae.

Making a hybrid microfluidic platform compatible for in situ imaging by vacuum-based techniques

A self-contained microfluidic-based device was designed and fabricated for in situ imaging of aqueous surfaces using vacuum techniques. The device is a hybrid between a microfluidic poly(dimethyl siloxane) block and external accessories, all portable on a small platform (10 × 8 cm2). The key feature is that a small aperture with a diameter of 2-3 μm is opened to the vacuum, which serves as a detection window for in situ imaging of aqueous surfaces. Vacuum compatibility and temperature drop due to water vaporization are the two most important challenges in this invention. Theoretical calculations and fabrication strategies are presented from multiple design aspects. In addition, results from the time-of-flight secondary ion mass spectrometry and scanning electron microscopy of aqueous surfaces are presented.

Soft-landed ions: a route to ionic solution studies

Abstract Solvated ions in condensed phase can be studied with new directness, using a very low energy (≤ 1 eV) mass-selected ion source, to ‘soft-land’ ions on or within surface films. The very low energy allows almost any ion to be studied without impact damage. Results for hydronium ions deposited on water ice are presented, where the lack of hydronium diffusion up to 190 K is evident, and intriguing information on dielectric behavior is measured. Cs + ions moving in n-hexane and 3-methyl pentane are also discussed.

Ion beam source for soft-landing deposition

“Soft-landing” deposition of molecular ions on various surfaces is important in making exotic radicals, modeling electrochemical double layers, and studying aqueous ion interactions. We have built a new mass-selected ion beam source for soft-landing deposition, designed to produce either positive or negative ions, including ions that depend on ion-neutral reactions (e.g., H3O+ and NH4+). The ionizer is a free jet crossed by an electron beam, producing a wide variety of positive and negative ions. The simple, short-length, planar ion deceleration minimizes defocusing and space charge effects. It currently delivers mass-selected ions with energies down to about 1 eV and currents of about 10 nA. The design allows easy maintenance. The performance of the ion beam compares favorably with previous low-energy positive ion systems.