Center on Airborne Organics
1996 Annual Report
D. 1996 Highlights
SOURCE AND CONTROL
Modeling Gas-Phase Chemistry and Heterogeneous Reaction of Polycyclic Aromatic Compounds: J.B. Howard and C.J. Pope, Massachusetts Institute of Technology
A model of PAH formation in flames has been developed which allows reactions of PAH with soot particles to be taken into account in studies of combustion-generated PAH. In comparisons of model predictions against experimentally observed behavior, the model is found to predict important characteristics of PAH and soot emissions. For example, the presence of soot is predicted to lower the concentration of PAH under some conditions, evidence of which is observed experimentally. Also, both the abrupt onset of soot formation which is known to occur as the fuel/air ratio is increased and the value known to occur as the fuel/air ratio at which the onset occurs, are well predicted by the model. These results indicate the PAH and soot formation model will be useful in efforts to reduce PAH and soot emissions at the sources.
Combustion Chamber Deposit Effects on Engine Hydrocarbon Emissions: J.B. Heywood and Simone Hochgreb, Massachusetts Institute of Technology
In the past year, a new 35-hr deposit accumulation test was completed. The physical model to explain and quantify the contribution of combustion chamber deposits to the HC emission problem was completed. The model contained four mechanisms for fuel transport into and out of the deposit pores depending on the pore size range: deposit crevice mechanism, Darcy flow, and ordinary and Knudsen diffusion with adsorption. The model predicted that the increase in the HC emissions due to deposits was dominated by the deposit crevice mechanism while the other effects turned out to be small.
Chemical Kinetic Modeling of Formation of Products of Incomplete Combustion from Spark-ignition Engines: Simone Hochgreb, Massachusetts Institute of Technology
An integrated model of the post-flame oxidation of hydrocarbon layers during the expansion phase of combustion in spark-ignited engines was developed. The project demonstrated the role of chemical kinetics on post flame oxidation, and the feasibility of using detailed chemistry models in determining the oxidation of unburned hydrocarbons, including comparisons with experimental data. The results show the influences of chemical structure on the post-flame oxidation of propane and isooctane.
Fundamental Study on High Temperature Chemistry of Oxygenated Hydrocarbons as Alternate Motor Fuels and Additives: Joseph W. Bozzelli, Chemical Engineering and Chemistry, New Jersey Institute of Technology
Experiment and model both predict that addition of methanol in relatively small amounts and up to 50% methanol significantly enhances combustion of methane. A data base on thermodynamic parameters for stable molecules, radicals and transition states relevant to combustion and atmospheric photochemistry of hydrocarbons and oxygenated hydrocarbons has been developed. A method to calculate and represent rate constants for reactions over wide pressure ranges has been developed and implemented for use in the widely employed Chemkin® kinetic integration code. Simultaneous Removal of Soot and NOx from the Exhaust of Diesel Powered Vehicles: H. Shaw and R. Pfeffer, New Jersey Institute of Technology, and John G. Stevens, Montclair State University
An hypothesis was developed that qualitatively explains the observations that in the presence of a catalytic surface containing copper, soot is oxidized and NO reduced. Soot, a solid with an average particle size of 2 microns due to agglomeration, cannot diffuse to catalytically active sites. It appears that sufficient soot (or unburned hydrocarbons contained in the soot) is oxidized at 400 to 600°C by oxygen in a non-catalytic reaction to CO. The CO then catalytically reduces NO to molecular nitrogen while being oxidized to carbon dioxide. An experimental and modeling program is designed to quantitatively describe the process for control of Diesel soot and NOx and to develop catalysts that are resistant to water and sulfur poisoning.
TRANSPORT AND TRANSFORMATION
Elementary Reaction Mechanism and Pathways for Atmospheric Reactions of Aromatics - Benzene and Toluene: Joseph W. Bozzelli and Tsan Lay, New Jersey Institute of Technology
Professor Bozzelli and Dr. Lay have assembled elementary reaction mechanisms of benzene atmospheric oxidation drawing on combined thermodynamic property and kinetic parameters from NJIT, data from Seinfeld at Caltech, e.g., transition state parameters and trioxy thermo data, and data of Edney at RTP on thermodynamic properties of toluene oxidation intermediates. Species profiles from the mechanism are compared to data of the Pagsberg, Y. P. Lee, and Koch/Zetzsch research groups in Sweden, Taiwan and Germany respectively. Results from the elementary mechanism analyses have allowed reduction and in some cases elimination of major discrepancies between pathway interpretations for initial benzene oxidation steps.
Atmospheric Transformation of Volatile Organic Compounds: Gas-Phase Photooxidation and Gas-to-Particle Conversion: J.H. Seinfeld, R.C. Flagan, California Institute of Technology
A significant step was achieved in the understanding of the mechanism of formation of secondary organic aerosols in the atmosphere. On the basis of over 100 outdoor smog chamber experiments on systems ranging from pure aromatic hydrocarbons to evaporated whole gasoline, a remarkably consistent picture has emerged of the aerosol formation process. Semi-volatile compounds are produced either by the initial oxidation of the parent organic or by oxidation of a first-generation oxidation product. These semi-volatile compounds partition themselves between the gas and organic aerosol phases depending on the amount of already-produced organic aerosol. The aerosol-forming potential of whole gasoline vapor was shown to be predicted quantitatively based on the aromatic content of the fuel.
Direct Treatment of Uncertainties in Mathematical Models of the Transport and Fate of Airborne Organics: Gregory J. McRae, Department of Chemical Engineering, Massachusetts Institute of Technology
A comprehensive uncertainty analysis was performed on W.P.L. Carter's SAPRC atmospheric mechanism. Key uncertain inputs identified to contribute to uncertainties in model predictions include the reaction rate of HO + NO2, photolysis rates, and initial conditions. These findings can be used to set priorities for field and laboratory studies.
MONITORING AND SOURCE APPORTIONMENT
Experimental Investigation of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols: Glen Cass, California Institute of Technology
During the late summer of 1996, a major field experiment was conducted in Southern California designed to observe the transformations of single airborne particles as they are acted upon by gas-to-particle conversion processes in the Los Angeles atmosphere. Electronic size distribution monitors, cascade impactors and three novel aerosol time of flight mass spectrometers were deployed at air monitoring stations at Long Beach, Fullerton, and Riverside, CA. Data from the experiment are expected to serve as the basis for testing the performance of air quality models for particle formation and transport in Southern California.
Markers for Emissions from Combustion Sources: Adel F. Sarofim, J.B. VanderSande, Massachusetts Institute of Technology
The use of scanning transmission electron microscopy coupled with energy dispersive x-ray analysis and electron energy loss spectroscopy to characterize the chemical composition of soots has been the main thrust this last year. The comparison of the analyses of diesel mining soot and soot from a jet engine have shown significant chemical differences. Electron energy loss spectroscopy was also used to characterize the extent of oxidation in these soots.
Microengineered Mass Spectrometer for in-situ Measurement of Airborne Contaminants: W. N. Carr and K. R. Farmer, New Jersey Institute of Technology
Gated microtip arrays for use in the prototype microengineered mass spectrometer being developed have been successfully fabricated. Electron emission from these tips has been achieved in route to creating the ion source for the mass spectrometer.