Center on Airborne Organics
1997 Annual Report

1997 HIGHLIGHTS
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SOURCES AND CONTROL 

Combustion Chemistry of Polycyclic Aromatic Compounds: Jack B. Howard, Massachusetts Institute of Technology 

A model for predicting PAH and soot concentrations in flames has been appreciably reduced in size (10-fold smaller computation time) without significantly affecting its predictive capability. The model can be used in practical flame simulations. Model predictions have confirmed that PAH reactions with soot can significantly reduce PAH concentrations. We have written an interface to new software which allows the modeling calculations to incorporate, and to benefit from, experimental species concentration profiles that are more accurate than could be predicted by the model. This feature can greatly improve  the predictive capability for concentrations of other important species for which ab initio modeling parameters are not adequately known. 

Fundamental Study on High Temperature Chemistry of Oxygenated Hydrocarbons as Alternate Motor Fuels and Additives: Joseph W. Bozelli, Tsan Lay, New Jersey Institute of Technology 

The reaction of the dimethyl ether radical CH3OCH. with oxygen, O2, is shown to be chain propagating through reactions that are all exothermic. There is no kinetic barrier. Reaction paths are verified by experimental data and ab initio calculations (MP2/CBSq and G2). Reactions paths easily explain the explosive nature of ethers relative to hydrocarbons. A variable pressure mechanism and Chemkin Code are developed for MTBE and for dimethyl ether combustion. 

Kinetics, Reactions Path Analysis and Elementary Reactions Mechanism Development for Atmospheric Photochemical Oxidation of Aromatics and Oxygenated Aromatics: Joseph W. Bozelli and Tsan Lay, New Jersey Institute of Technology 

We have established a detailed reaction mechanism and the associate thermodynamic database for toluene atmospheric oxidation. The mechanism includes 79 elementary reactions and 30 species. Predictions of our kinetic modeling are in good agreement with the experimental results. We will extend our studies to other alkyl substituted and oxygenated aromatics, such as ethyl benzene, xylene, phenol, and cresol. 

Chemical Kinetic Modeling of Products of Incomplete Combustion in Spark-Ignition Engines: Simone Hochgreb, Massachusetts Institute of Technology 

We determined the roles of chemistry and diffusion on post-flame hydrocarbon oxidation in spark-ignition engines, and the temperature regimes under which chemistry or diffusion are controlling. We explained the origin of formation and survival of products of incomplete combustion in post-flame oxidation. We also determined the reasons for vastly different in-cylinder oxidation rates between paraffinic fuels, and have shown that chemical rates have at least as important a role as volatility and solubility in determining emission rates. 

Investigation of the Formation of Particulate Matter in Spark-Ignition Engines: Simone Hochgreb, Arthur L. Lafleur, Massachusetts Institute of Technology 

We determined the loss of particulate matter from within a dilution tunnel as a function of flowrate (and Reynolds number), and established an optimum dilution ratio. We also determined steady state particulate matter emissions from a 4-stroke spark-ignition engine, and characterized effects of equivalence ration and liquid fuel on these emissions. 

Combustion Chamber Deposit Effects on Engine Hydrocarbon Emissions: John Heywood and Simone Hochgreb, Massachusetts Institute of Technology 

This work has shown that deposit build-up may cause an increase in HC emissions of about 15 percent, depending on fuel, engine and operating condition details. However, this increase occurs rapidly, in about twenty-four hours of engine operation, so it will be difficult in practice to reduce this HC emissions source substantially. During the project, the importance of deposit morphology became clear. Owing to their porosity and thickness, cylinder head deposits dominate HC emissions. Further research is needed on how the morphology of combustion chamber deposits depend upon specific processes  responsible for their formation and removal. 

Simultaneous Removal of Soot and NOx from the Exhaust of Diesel-Powered Vehicles: Henry Shaw, Robert Pfeffer, New Jersey Institute of Technology 

We have verified that CO is a possible intermediate reducing agent gas needed to react with NO at 400 to 600oC. Sufficient CO is formed even at temperatures as low as 300oC from soot or other solid carbon sources to effectively reduce 1000ppm NO to N2. The CO is in turn oxidized to CO2. CO is as effective as light hydrocarbon gases in reducing NO. We are starting to model the system to gain greater insight on the NOx reduction mechanism. 

TRANSPORT AND TRANSFORMATION 

Laboratory Studies of Intermediate Steps in the Atmospheric Oxidation of Organic Compounds: Mario J. Molina, Massachusetts Institute of Technology 

The flow tube - chemical ionization mass spectrometer apparatus has been assembled and tested. The toluene-OH adduct (i.e., the methyl-hydroxycyclohexadienyl radical) has been successfully generated and detected with this apparatus using O2+ as a reactant ion. 

Atmospheric Transformation of Volatile Organic Compounds: Gas-Phase Photo-oxidation and Gas-to-Particle Conversion: John H. Seinfeld, Richard C. Flagan, California Institute of Technology 

We have obtained a comprehensive data set on the atmospheric aerosol-forcing potential of biogenic hydrocarbons. Results of prior smog-chamber studies on the secondary organic aerosol formation have been integrated into a three-dimensional gas/aerosol model. Ab initio reaction mechanism studies reveal behavior of alkoxy radicals. 

Direct Treatment of Uncertainties in Mathematical Models of the Transport and Fate of Airborne Organics: Gregory J. McRae, Massachusetts Institute of Technology 

Uncertainty analysis can be a valuable tool to guide resource allocation in measurement and model development programs. In this study, the predictions from three different mechanisms were found to be indistinguishable given current levels of parametric uncertainties. Because all the chemical mechanisms investigated, as well as the three-dimensional CIT Airshed model, are sensitive to the same set of parameters, reducing parametric uncertainties should be a key research priority for model improvement purposes. 
 
 

MONITORING AND SOURCE ATTRIBUTION 

Experimental Investigation of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols: Glen R. Cass, California Institute of Technology 

A field experiment has been conducted in which the evolution in the chemical composition of air pollutant particles has been observed as air parcels pass over Los Angeles urban area. Gas-to-particle conversion processes have been observed at the single-particle level using aerosol time-of-flight mass spectrometers calibrated to read in terms of absolute particle number concentrations and chemical species concentration. 

Micro-engineered Mass Spectrometer for in-situ Measurement of Airborne Contaminants: William N. Carr, Kenneth R. Farmer, New Jersey Institute of Technology 

Important accomplishments include the successful fabrication of improved gated microtip array, and the subsequent achievement of relatively stable electron emission from these tips. More importantly, we have now been able to observe an ion current from residual molecules in our UHV system. This is the last major milestone en route to creating the ion source part of the mass spectrometer. Mass separation in a magnetic field is currently being attempted. 

Markers for Emissions from Combustion Sources: John Vander Sande, Massachusetts Institute of Technology 

This 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 of this project during the last year. A comparison of diesel soots from two separate test engines has shown similarities and differences. Electron energy loss spectroscopy was also used to characterize the extent of oxidation in these soots. 
 

 
 
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