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Biomedical subjects

James P Cowin

Publications and source records attributed to James P Cowin.

5 recordsLinked to original sources
PubMed2006

A new approach to determining gas-particle reaction probabilities and application to the heterogeneous reaction of deliquesced sodium chloride particles with gas-phase hydroxyl radicals.

The reaction kinetics for gaseous hydroxyl radicals (OH) with deliquesced sodium chloride particles (NaCl(aq)) were investigated using a novel experimental approach. The technique utilizes the exposure of substrate-deposited aerosol particles to reactive gases followed by chemical analysis of the particles using computer-controlled scanning electron microscopy with energy-dispersive analysis of X-rays (CCSEM/EDX) capability. Experiments were performed at room temperature and atmospheric pressure with deliquesced NaCl particles in the micron size range at 70-80% RH and with OH concentrations in the range of 1 to 7 x 10(9) cm(-3). The apparent, pseudo first-order rate constant for the reaction was determined from measurements of changes in the chloride concentration of individual particles upon reaction with OH as a function of the particle loading on the substrate. Quantitative treatment of the data using a model that incorporates both diffusion and reaction kinetics yields a lower limit to the net reaction probability of gamma(net) > or = 0.1, with an overall uncertainty of a factor of 2.

Journal Article↗
PubMed2003

Reactions at interfaces as a source of sulfate formation in sea-salt particles.

Understanding the formation of sulfate particles in the troposphere is critical because of their health effects and their direct and indirect effects on radiative forcing, and hence on climate. Laboratory studies of the chemical and physical changes in sodium chloride, the major component of sea-salt particles, show that sodium hydroxide is generated upon reaction of deliquesced sodium chloride particles with gas-phase hydroxide. The increase in alkalinity will lead to an increase in the uptake and oxidation of sulfur dioxide to sulfate in sea-salt particles. This chemistry is missing from current models but is consistent with a number of previously unexplained field study observations.

Journal Article↗
PubMed2003

Nanometer-resolved interfacial fluidity.

Confined liquids can have properties that are poorly predicted from bulk parameters. We resolve with 0.5 nm resolution the nanoscale perturbations that interfaces cause on fluidity, in thin 3-methylpentane (3MP) films. The films of glassy 3MP are much less viscous at the vacuum-liquid interface and much more viscous at the 3MP-metal interface, compared to the bulk of the film. We find that the viscosity at the interfaces continuously returns to the bulk value over about a 3 nm distance. The amorphous 3MP films are constructed using molecular beam epitaxy on a Pt(111) substrate at low temperatures (<30 K). Ions are gently inserted at specific distances from the substrate with a 1 eV hydronium (D(3)O(+)) or Cs(+) ion beam. The voltage across the film, which is directly proportional to the position of the ions within the film, is monitored electrostatically as the film is heated at a rate of 0.2 K/s. Above the bulk glass transition temperature (T(g)) of 3MP (77 K), the ions are expected to begin to move down through the film. However, ion movement is observed at temperatures as low as 50 K near the vacuum interface, well below the bulk T(g). The fitted kinetics predict that at 85 K, the glass is about 6 orders of magnitude less viscous near the free interface compared to that of the bulk.

Journal Article↗
PubMed2002

Quantitative time-resolved monitoring of nitrate formation in sea salt particles using a CCSEM/EDX single particle analysis.

Progress of the nitrate formation in individual sea salt particles was detected as a function of time using aerosol samples collected during the TexAQS 2080 experiment We demonstrate that the time-resolved collection approach coupled with the automated EDX single particle analysis made it possible to follow in detail the time evolution of sea salt particles within a diverse aerosol mixture. Using a custom built Time-Resolved Aerosol Collector (TRAC), particulate samples were taken sequentially on grid-supported 50 nm carbon films with a time resolution of 10 min between two consecutive samples. The samples were analyzed in the laboratory using Computer Controlled Scanning Electron Microscopy with Energy-Dispersed analysis of X-rays (CCSEM/EDX). Between midnight of 08/16/00 and the early morning of 08/17/00, a steady, particularly sea salt rich aerosol was observed at the measurement site, which later showed the effects of atmospheric processing. During the night of 08/17/00 the sea salt particles were almost unprocessed, having elemental composition close to that of seawater. By 12 noon, the evolving atmosphere was able to completely convert them, predominantly to sodium nitrate particles. During the next night this process had nearly stopped and fairly virgin sea salt particles appeared again.

Aerosols↗