James J. Sloan

Raman spectroscopic characterization of carbonaceous aerosols

We have used Raman spectroscopy to characterize a variety of carbon-containing particulate matter, including samples collected in ambient urban atmospheres. Based on the Raman spectra of known, commercial particles, we derive a simple empirical model that reflects their microchemistry and microphysics. This model gives information on the crystal size and morphology of the graphitic component, which correlates with the known characteristics of the commercial samples. We derive similar information about the graphitic component of the ambient particles, and suggest that this method might be used to characterize ambient particles systematically in the future.

Air Quality in Selected Megacities

Abstract About half of the worlds population now lives in urban areas because of the opportunity for a better quality of life. Many of these urban centers are expanding rapidly, leading to the growth of megacities, which are often defined as metropolitan areas with populations exceeding 10 million inhabitants. These concentrations of people and activity are exerting increasing stress on the natural environment, with impacts at urban, regional and global levels. In recent decades, air pollution has become one of the most important problems of megacities. Initially, the main air pollutants of concern were sulfur compounds, which were generated mostly by burning coal. Today, photochemical smog—induced primarily from traffic, but also from industrial activities, power generation, and solvents—has become the main source of concern for air quality, while sulfur is still a major problem in many cities of the developing world. Air pollution has serious impacts on public health, causes urban and regional haze, and has the potential to contribute significantly to global climate change. Yet, with appropriate planning megacities can efficiently address their air quality problems through measures such as application of new emission control technologies and development of mass transit systems. This review is focused on nine urban centers, chosen as case studies to assess air quality from distinct perspectives: from cities in the industrialized nations to cities in the developing world. This review considers not only megacities, but also urban centers with somewhat smaller populations, for while each city—its problems, resources, and outlook—is unique, the need for a holistic approach to complex environmental problems is the same. There is no single strategy to reduce air pollution in megacities; a mix of policy measures will be needed to improve air quality. Experience shows that strong political will coupled with public dialogue is essential to effectively implement the regulations required to address air quality.

Application of the Hough transform for the automatic determination of soot aggregate morphology.

We report a new method for automated identification and measurement of primary particles within soot aggregates as well as the sizes of the aggregates and discuss its application to high-resolution transmission electron microscope (TEM) images of the aggregates. The image processing algorithm is based on an optimized Hough transform, applied to the external border of the aggregate. This achieves a significant data reduction by decomposing the particle border into fragments, which are assumed to be spheres in the present application, consistent with the known morphology of soot aggregates. Unlike traditional techniques, which are ultimately reliant on manual (human) measurement of a small sample of primary particles from a subset of aggregates, this method gives a direct measurement of the sizes of the aggregates and the size distributions of the primary particles of which they are composed. The current version of the algorithm allows processing of high-resolution TEM images by a conventional laptop computer at a rate of 1-2 ms per aggregate. The results were validated by comparison with manual image processing, and excellent agreement was found.

Characterization of atmospheric aerosols from infrared measurements: simulations, testing, and applications

An inversion method for the characterization of atmospheric condensed phases from infrared (IR) spectra is described. The method is tested with both synthetic IR spectra and the spectra of particles that flow in a cryogenic flow tube. The method is applied to the IR spectra recorded by the Atmospheric Trace Molecule Spectroscopy instrument carried by the Space Shuttle during three missions in 1992, 1993, and 1994. The volume density and particle size distribution for sulfate aerosol are obtained as a function of altitude. The density and size distribution of ice particles in several cirrus clouds are also retrieved. The probable radius of the ice particles in the high-altitude (10-15-km) cirrus clouds is found to be approximately 6-7 microm.

The effect of reagent excitation on the dynamics of the reaction O(1D2)+H2→OH(X 2Π)+H

The effect of H2 translational, rotational, and vibrational excitation on the dynamics of the O(1D2)/H2 reaction are explored in a semiclassical trajectory study involving both of the energetically accessible potential energy surfaces of the system. Landau–Zener probabilities determine surface hopping. At low reagent excitation, the deep H2O potential minimum dominates the dynamics, causing the reagents to reorient towards a H–O–H (insertion) configuration and form the H2O intermediate, irrespective of the initial approach geometry. High vibrational excitation enhances the probability for transitions onto the excited state potential during the interaction. Reactions which sample the excited state potential have fundamentally different dynamics from those which remain on the lower state. For reactions involving H2(v=4), the OH product has a bimodal vibrational distribution, peaking in OH(v’=2) and OH(v’=9). The lower peak is due to reactions which access the excited state potential; the higher peak results...

The effect of reagent translational excitation on the dynamics of the reaction H+Cl2→HCl(v’,J’)+Cl

We use fast time‐resolved Fourier transform spectroscopy and low pressure infrared chemiluminescence techniques to determine the product energy distribution in the title reaction. We create the reagent hydrogen atoms with 2.3 eV of translational excitation by photofragmentation of H2S at 193 nm, and observe the time evolution of the infrared chemiluminescence from the product HCl(v’,J’) under single gas kinetic collision conditions. The initial vibrational distribution, determined from the first observation after creation of the H atoms is P(v’=1:2:3:4:5:6:7:8)=1.0:0.84:0.74:0.59:0.34:0.24:0.17:0.13. The initial HCl rotational distribution in each vibrational level is broad, showing no identifiable maximum. The fraction of the total available energy entering HCl vibration and rotation, respectively, are 0.19 and 0.10. The time evolution of the observed vibrational and rotational distributions gives information about the changes in the reaction dynamics consequent on reduction of the reagent translational ...

Application of a Microscale Emission Factor Model for Particulate Matter to Calculate Vehicle–Generated Contributions to Fine Particulate Emissions

Abstract This paper discusses the evaluation and application of a new generation of particulate matter (PM) emission factor model (MicroFacPM). MicroFacPM that was evaluated in Tuscarora Mountain Tunnel, Pennsylvania Turnpike, PA shows good agreement between measured and modeled emissions. MicroFacPM application is presented to the vehicle traffic on the main approach road to the Ambassador Bridge, which is one of the most important international border entry points in North America, connecting Detroit, MI, with Windsor, Ontario, Canada. An increase in border security has forced heavy–duty diesel vehicles to line up for several kilometers through the city of Windsor causing concern about elevated concentrations of ambient PM. MicroFacPM has been developed to model vehicle–generated PM (fine [PM2.5] and coarse ≤10 μm [PM10]) from the on–road vehicle fleet, which in this case includes traffic at very low speeds (10 km/h). The Windsor case study gives vehicle generated PM2.5 sources and their breakdown by vehicle age and class. It shows that the primary sources of vehicle–generated PM2.5 emissions are the late–model heavy–duty diesel vehicles. We also applied CALINE4 and AERMOD in conjunction with MicroFacPM, using Canadian traffic and climate conditions, to describe the vehicle–generated PM2.5 dispersion near this roadway during the month of May in 2003.

Compact standalone data acquisition system for submicrosecond time‐resolved Fourier transform spectroscopy

We report the development of a data acquisition system which can be used with any commercial Michelson interferometer to carry out time‐resolved Fourier transform spectroscopy with a time resolution of one microsecond. The sensitivity and spectral resolution of the experiment are limited only by the interferometer. The instrument occupies one slot on a conventional VME enclosure (6U height). It includes all hardware and firmware to control the experiment which generates the spectral transients and to collect the time‐resolved interferograms. We are presently transferring this design to a single 6U height printed circuit board (with a mezzanine), in order to make it available as a standalone system for use with any VME platform.

Development and characterization of a laminar aerosol flow tube

We have developed a new laminar aerosol flow tube (AFT) to study transformations such as ice nucleation, deliquescence, and efflorescence in model atmospheric aerosols. The apparatus consists of four sections which can be independently cooled to reproduce temperature profiles relevant to the troposphere and stratosphere. An automatic control system maintains the average axial temperature along each section between 100 and 300K, within ±0.1K. Changes in aerosol composition, phase, and size distribution are monitored at the tube exit using infrared spectroscopy (AFT-IR). We used computational fluid dynamics simulations to investigate flow velocity and temperature distributions within the flow tube. Based on these computations, the final design was formulated to eliminate turbulent mixing zones and buoyancy-driven convection cells. The latter can occur even under conditions where the Reynolds number indicates laminar flow. In either case, recirculation causes aerosol residence times and temperature histories...

Dissolution of Solid NaCl Nanoparticles Embedded in Supersaturated Water Vapor Probed by Molecular Dynamic Simulations

The dissolution process for small, on the order of 1000 atoms, crystalline NaCl particles with defects embedded in highly supersaturated water vapor was studied by the molecular dynamics (MD) simulation method. We found that a breakdown of the crystal lattice does not occur unless (1) the thickness of water layer covering the surface of salt particles exceeds several molecular layers and (2) there are a considerable number of defects in the crystal. The collapse of the crystal lattice starts when the amount of water taken up by a salt particle reaches about half ( approximately 50%) of the amount of salt in this particle. The number of defects required to initiate subsequent dissolution of the NaCl crystal on the time scale accessible by our simulations ( approximately 40 ns) is in the range of 10 to 12%. We also report the estimates for the time required to form supersaturated aqueous solutions of NaCl from originally crystalline particles as a function of the number of defects in the crystal.