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

Goal:  The objective of this study is to enhance predictive capabilities of the model previously developed for polycyclic aromatic hydrocarbons (PAH) formation, for use in source attribution studies and in the development of emission control strategies. 

Rationale:  Polycyclic aromatic compounds are major contributors to air pollution from combustion sources.  These compounds as well as oxy-PAH and soot particles formed from them are all of health concern.  Basic understanding of the factors that govern the detailed chemical composition of the effluents from combustion systems is necessary for the identification of signatures for source attribution and the development of control strategies.  The mechanistic and kinetic model developed in this project provides basic understanding of PAH generation in combustion.  The model is currently being improved and extended to more complicated flow systems. 

Approach:  A predictive model of PAH formation in flames is being developed using elementary reactions to describe the basic flame chemistry and PAH growth up to a mass of 400 amu, and aerosol dynamics to describe all species (both PAH and soot) with masses above 400 amu.  Sectional aerosol equations for soot formation, growth, and oxidation are expressed in a form suitable for concurrent soot aerosol modeling and detailed gas-phase kinetic modeling.  The soot model predicts the effects of soot upon the concentrations of gas-phase species, including PAH of interest.  A technique using ABACUSS, a differential algebraic equation solver, has been developed to isolate sub-mechanisms of the model by incorporating  experimental data.  This technique is used to test the various inputs that make up the over all model.  The interface between the gas-phase kinetics and aerosol dynamics (the soot nucleation step) is a sub-mechanism of the current model to be further developed with the above technique.  The role of PAH as soot growth species is another sub-mechanism addressed by the ABACUSS technique and the application of data sets from one-dimensional flames.  The predictive capability of the model is tested using data obtained previously from the jet-stirred reactor/plug-flow reactor (JSR/PFR) experimental apparatus. 

Status:  Through the use of sensitivity analysis, the PAH/soot model has been drastically reduced in size.  The model now consists of 481 reactions and 107 species and gives essentially identical results to the previous model with 3674 reactions and 174 species.  The reduction in size, combined with the use of a dedicated workstation, have reduced computation times by over an order of magnitude, and the practical simulation of one-dimensional flames is now possible.  Extensive simulations of JSR/PFR system were performed.  The model was used to assess a hypothesized mechanism that rationalizes the data of Marr for the case of naphthalene injection into the PFR.  The data show that naphthalene injected into the PFR quickly drops to concentrations which are, surprisingly, below the baseline case of no injected naphthalene.  The model qualitatively predicts this counterintuitive behavior when soot is treated as a sink to which the PAH add directly, consistent with the PAH being a growth species for soot.  At the same time, the PAH concentrations are affected by the presence of soot. 

A software interface has been written to use CHEMKIN interpreter output to assemble the differential equations describing the model-simulated combustion in the PFR in a format compatible with ABACUSS.  This allows the versatility of using several features of ABACUSS to further develop and test the model.  Foremost is the ability to incorporate experimental concentration profiles into the calculation of the model equations, thereby eliminating unknowns and isolating the error that may be introduced by other portions of the model into a particular sub-mechanism.  ABACUSS also provides the ability to have time dependent rate constants and optimization of model parameters. 

DEMMUCOM, an error analysis software tool, has been used to demonstrate the ability to analyze the error of model concentration predictions based on uncertain rate constants in the PAH/soot model.  It has been found that the error in concentration predictions typically is attributable to the error of ~5 reaction rate constants, with inclusion of the error of more rate constants not changing the error of the output significantly. 

Future Plans:  Future plans include improving the PAH formation model as well as the ability to account for PAH-soot interactions and their effect on PAH concentrations.  In addition, a greater number of 2-10 ring PAH including species containing 5-membered ringes will be included in the model, and new types of PAH reactions will be added to the model. 

Key Personnel 

Graduate Student:  David Kronholm

Goal:  Experiments and model development are performed to understand fundamental reaction processes of oxygenated hydrocarbons (OHC's) such as alcohols and ethers important to gasoline octane blending.  Detailed mechanisms are developed to allow optimization and trend prediction in engine performance and emission reduction. 

Rationale:  Oxygenates, such as dimethyl ether, methanol and ethanol, are scheduled for widespread use as additives and alternative motor fuels.  methyl tertbutyl ether (MTBE), is widely used as an anti-knock component and oxygenate additive in gasolines. Experimental data are needed for model development and validation.  A model based on fundamentals, calibrated by experimental data, will facilitate calculations of trends for future experiment testing and preferred fuel blends to reduce undesirable emissions e.g., HC's, CO, etc. while maintaining or improving engine performance. 
Approach:  Experimental:  Gas mixtures are reacted in a variable pressure, high temperature tubular flow reactor.  Reactor effluent is analyzed for products as a function of temperature, residence time, and fuel equivalence ratio.  Analysis is performed with on-line gas chromatography (GC), flame ionization detection (FID), Fourier Transform Infrared (FTIR) and GC / Mass Spectrometry. 

Modeling:  Reaction mechanisms are built from elementary reactions with rate constants based upon fundamental principles of thermochemical kinetics, statistical mechanics and transition state theory.  Quantum Rice-Ramsperger-Kassel theory for k(E), modified strong collision treatment for fall-off and thermodynamic properties are used in chemical activation and unimolecular reactions.  Ab initio and semiempirical calculations along with applications of group additivity are used to determine properties of transition states. 
Summary:  Oxidation and pyrolysis experiments on mixtures MTBE in methane / oxygen are completed at one to ten atmospheres, fuel equivalence ratios of 0.7 to 1.5.  Variable pressure experiments were initiated on dimethyl ether.  A thermodynamic data base and an initial elementary reaction mechanism has been assembled for MTBE oxidation and evaluated.  Sub-models for oxidation of neopentane, isobutane, and isobutene oxidation are developed and literature data on shown to compare well compared with experimental data. 
An initial reaction mechanism for oxidation of dimethyl ether is also developed and currently being tested by MIT. 
Key Personnel 

Graduate Students:  Ruth Wei and Takahiro Yamad 
Undergraduate Students:  Allan Petrin and Gem Pate 
Others: Rajiv Berr 
CAO Collaborators   Tim Jungkamp, John Seinfeld and Jack Howard

Goals:  Develop a model based on elementary reaction kinetics, pathways and thermodynamic properties which includes microscopic reversibility to understand and describe the photochemical oxidation of aromatic hydrocarbons (benzene, toluene, xylene, phenol, etc.) under atmospheric conditions.  The reaction mechanism will be validated against experimental data in the literature. 

Rationale:  At present there is insufficient kinetic data and no mechanism for modeling atmospheric reactions of aromatic compounds in photochemical, oxidation or other reaction systems.  Development and validation of a mechanism will enable models pertaining to photochemical smog, air-shed transport and oxidation processes, to incorporate aromatic species.  It will also provide an understanding of atmospheric reactions and product formation rates on aromatic moieties. 

Approach:  Reaction mechanisms utilize elementary kinetic parameters coupled with microscopic reversibility and include calculation of steady state levels of active intermediates.  Pressure dependent and chemical activation reaction analysis are included in the reaction kinetic parameter calculations.  Thermodynamic properties and transition state parameters are determined via literature evaluation, group additivity, and ab initio molecular orbital methods.  Fundamental principles of thermochemical kinetics and detailed balance are applied to all reactions.  Models are validated against literature experimental data. 

Status:  Extension of Thermodynamic Property Databas 
Thermodynamic properties: enthalpies, entropies and heat capacities for cyclic hydrocarbons, alkyl peroxides, alkyl trioxides have been validated, improved, or investigated. Significant improvement for key reaction adducts: hydroxyl cyclohexadienyl, hydroxyl cyclhexadiene peroxy radical and these two radical with methyl substitution, are especially important 

Establishment of Toluene Reaction Mechanis 
A detailed reaction mechanism has been built for toluene atmospheric oxidation. The mechanism includes 79 elementary reactions and 30 species. The prediction of kinetic modeling is in good agreement with the experimental results on final product yields (experimental data in parenthesis): benzaldehyde 9.1% (10%), o-cresol 20.1% (20%), p-cresol 4.4% (ca. 4%), m-cresol 0.9% (ca. 1%), nitro-toluene 1.8% (1.8%), glyoxal 14.3% (10 - 15%), methyl glyoxal 14.1% (10 - 15%). 

Comparison of Available Experimental Data 
We compare recently published data of Koch (1996), of Pagsberg (1996), and of Y. P. Lee (1994), all of which is relevant to the gas phase aromatic chemistry. Koch et al. employed flash photolysis/resonance fluorescence to study the reaction of benzene + OH and benzene-OH + O₂ at tropospheric condition; the [OH] concentration as function of time is monitored. Their data show that the decay of OH in the presence of benzene is bi-exponetial.  This verifiers our model prediction which indicates the dissociation of benzene-OH to benzene + OH occurs and the overall reaction of benzene + OH reaches a pseudo-equilibrium. Compari-sons of our model simulation to the data of Koch et al. are presented in the attached Figure A.  Our model correctly predicts OH profiles in the absence of O₂. When O₂ is present, OH decay are predicted from 0 to 0.2 s and slightly underpredicted thereafter. Koch's data is, however, only in abstract form, and we look to further interpret the data when more detailed information is obtained 

The data of Pagsberg, et al.. are pulse radiolysis experiments (Argon bath) on the OH-initiated oxidation of benzene, and reactions of benzene-OH adduct with NO2.  For the reactions of benzene + OH, phenol is identified as the primary product with relative yield of 25±5%.  They conclude that the reaction: OH + benzene ö> H + phenol, should account for the experimental results, at the rate constant k₅=1.7×10¹¹cm³ mol^-1 s^-1. This rate constant value is significantly higher than the literature data: 1.5×10⁵cm³ mol^-1 s^-1 (Fritz, et al.).  Our QRRK calculation results in k₅=3.4×10⁷ cm³ mol^-1 s^-1, which is between these two values. Another supporting value in Pagsberg's article is the rate constant for benzene-OH + O₂ => products, k=3.0×10¹¹ cm³ mol^-1 s^-1, which is in good agreement with our QRRK calculation: k₇=7.1×10¹¹ cm³ mol^-1 s^-1, and 3 orders of magnitude higher than the literature data: 1.1×10⁸ cm³ mol^-1 s^-1. ³  

The data of Y. P. Lee, et al. is on OH plus benzene using laser-photolysis/laser-induced-fluorescence over 250-500 Torr and 345-385 K.  The rate constants they report for OH + benzene = benzene-OH are: k_f = 1.4×10¹² cm³ mol^-1 s^-1 and k_r=2.9s^-1, compared to our data: k_f = 7.3×10¹¹ cm³ mol^-1 s^-1 and k_r = 0.15 s^-1.  We note the entropy value S°₂₉₈(benzene-OH) they adopted is as low as 75 cal mol^-1 K^-1, compared to our value calculated using the PM3 method: 82.7 cal mol^-1 K^-1 (number of optical isomer assigned as 2)  We further analyze their enthalpies using density functional theory at the B3LYP/6-31G* level to determine the S°₂₉₈(benzene-OH) as 82.2 cal mol^-1 K^-1. We are confident of our entropy data and believe that Lee et al. misinterpreted the entropy of the benzene- OH adduct radical by ca. 7cal mol^-1 K^-1.  This suggests the need to reinterpret their experimental data, and modify their rate constants. 

The comparison of our theoretical work to experimental data indicates: (i) there still exists significant uncertainties and controversies in the experimental data for the benzene + OH + O₂ reaction system; (ii) careful interpretation of experimental data is necessary; (iii) the detailed, elementary reaction model, including the reverse reaction rate constants, are required; (iv) correct thermodynamic data of the primary reaction adduct are necessary. 

Future Plans:  We plan on extending the mechanistic study to oxygenated aromatics such as phenol and cresol, expanding the mechanistic study to alkyl substituted aromatics such at xylenes, ethyl-benzene and styrene, and extending the thermodynamic properties to reaction species of above reaction systems. 

Key Personnel 

Graduate Students:  Takahiro Yamada and Chiung-Ju Chen 
Undergraduate Student:  Satyen S. Amin 

Goals:  The project objectives are to understand what controls the process of post-flame oxidation of unburned hydrocarbons in spark ignition engines.  The questions addressed by the project are: (a) under what conditions (engine operation and relevant temperatures) the production of products of incomplete combustion (particularly important ozone precursors and toxic products) are most likely, (b) the relative roles of mixing and reaction on oxidation during the different phases of post flame oxidation and (c) how well predictions of unburned hydrocarbon product hold against experimental data. 

Rationale:  Hydrocarbon emissions from spark-ignition engines are responsible for a large fraction of ozone precursors in the atmosphere as well as some important regulated toxic hydrocarbons.  The process of oxidation of unburned hydrocarbons in spark ignition engines involves the emergence of hydrocarbons from cold engine walls (crevices, oil layers and deposits) and subsequent diffusion (turbulent and molecular) through the thermal boundary layer into the hot burned gases. 

Experimental evidence (Green, 1995) and calculations (Wu et al., 1991; Lee and Morley, 1994) indicate that oxidation the oxidation process can be represented by a one-dimensional process during the gas expansion phase through bottom dead center, and a more complex large scale vortex mixing processes after bottom dead center.  In order to characterize the hydrocarbon oxidation process, and identify what phases control the formation of ozone precursors and toxic hydrocarbons, one must combine chemical kinetic models to fluid mechanical mixing models.  Given the complexity of the task, it is necessary to simplify the fluid mechanics during each phase of the process (expansion and displacement), and to identify simplifications to the chemical kinetic models that might be useful in more complex fluid mechanical simulations of the gas reaction. 

Approach:  The processes of hydrocarbon emergence and oxidation taking place inside the engine cylinder and exhaust system involve molecular and turbulent diffusion away from the walls into the hot core gases, total or partial reaction and subsequent convective transport out of the cylinder.  The simulations are primarily concerned with the oxidation during the expansion process, during which hydrocarbons emerge from the various cold wall  and oil sources, and are confined to a fairly thin (sub-millimeter) layer adjacent to the wall.  This allows the simulations to be kept one-dimensional, so that detailed chemistry computations are feasible. 

The model is a one-dimensional mixing layer model that includes convective and diffusive transport of species and energy, as well as chemical production and destruction of species, and chemical energy release (thoroughly described in Wu and Hochgreb, 1997a, 1997b).  The initial and boundary conditions are set to match conditions expected around the engine walls.  The evolution of the species released at different times during the expansion process can then be investigated, and the contributions of the different factors (reaction, diffusion and convection) to the overall process can be understood. 

Status:  We have made significant progress in implementing the simulations, and using sensitivity and flux analysis to understand the roles of reaction, diffusion and chemistry under different conditions and with various fuels.  The results are reported in two recent publications (Wu and Hochgreb, 1997a , 1997b). The important results are summarized as follows: 

1. General features of the reactive/ diffusive process:  Simulations show that unburned fuel is transported towards the hot burned gases, where intermediate species are quickly generated.  The cold region near the walls acts as a buffer, which preserves the intermediate species from quick oxidation.  Radicals are generated close to the burned gas during the oxidation process, at much higher concentrations than the original burned gas radical concentrations, indicating that radicals in the burned gas do not have a significant effect on the oxidation level of unburned hydrocarbons, except to initiate the oxidation reactions. 
2.  Roles of diffusion, convection, and reaction:  Flux analysis indicates that the convective term  can be neglected in comparison with diffusive and reactive terms.  Simulation results show that the in- cylinder process is largely controlled by diffusion rates for temperatures above a transition burned gas temperature, and by reaction for temperatures below that level.  These temperatures are of the order of 1700 K for isooctane, 1400 K for propane, 1350 K for ethane and 1300 K for ethene.  Diffusion acts by transporting radicals generated at higher temperatures towards lower temperature regions, and moving intermediate species towards both higher temperature regions for destruction, or towards the colder wall regions, where they accumulate unreacted.  The mechanism of generation (at lower temperatures) and destruction (at higher temperatures) of intermediate species keeps the overall concentrations of intermediate species approximately constant after the intermediate species initially increase. 
3.  Burned gas temperature effects :  The burned gas temperature is the critical factor determining the oxidation level and the switch between diffusion and reaction control.  Fast energy release raises the local temperatures near the walls, simulating the oxidation rate of unburned hydrocarbons in the cases of high initial core temperatures.  In contrast, at low initial core temperatures, radical generation near the burned gas is not significant and the whole conversion process is very slow. 
4.  Turbulent diffusion effects :  Molecular diffusivity dominates in the regions close to the wall (within 0.1 mm) and throughout the reaction zone.  Sensitivity analysis of the effect of diffusion on the oxidation process has been conducted by increasing turbulent diffusivity by a factor of two.  The effect of diffusion strongly depends on the initial burned gas temperature : a large increase in oxidation is possible by increasing diffusion rates at high initial temperatures.  The overall concentrations of non-fuel species is little influenced by the change of turbulent diffusivity, so that the remaining fuel/nonfuel ratio decreases with increasing diffusivity. 
5.  Fuel effects:  The effect of reactant chemistry on the oxidation process has been investigated by comparing the oxidation results between the cases of propane, ethene, ethane, and isooctane. 
 a)  Ethane vs. ethene :  These two compounds have very similar molecular diffusivity, yet the resulting extents of oxidation are different.  Comparisons of the reaction fluxes shows that the oxidation of ethane requires initial conversion into ethene,  which is then either oxidized in the high temperature region or diffuses towards the cold wall.  Ethene undergoes a unidirectional flux from the walls towards the hot gases, facing a shorter oxidation chain towards carbon monoxide.  Since the production of hydrogen atoms and chain branching at high temperatures depends on the last step in the chain of fuel oxidation through formyl radical decomposition (see conclusion VI), the radical production and the overall oxidation rate is higher for ethene than ethane. 
 b)  Propane and Isooctane:  The very long radical chain involved in the oxidation of isooctane, and the large fraction of intermediate products that diffuse towards the cold wall reduce the rate of radical production in the case of isooctane in comparison to the other fuels tested.  The production of low reactivity intermediates (such as isobutene) may also contribute to the slower reaction as compared to propane, but this hypothesis would be better tested when comparing isooctane and normal octane.  Propane oxidation is slower than either ethane and ethene, owing to the same reasons as stated above, related to the reaction chain length, and the diffusion of intermediates towards the cold walls. 

6.  Factors determining the oxidation rate:  Reaction path analysis indicates most of hydrocarbons decompose by the attack of radicals (OH, O, and H).  The drastic difference in the oxidation rate of unburned hydrocarbons between the cases of propane and isooctane are mainly due to the difference in the reactant concentrations (especially radicals) in the reaction zone.  Hydrogen atom is the key radical for triggering most of the important chain-branching reactions.  The main pathways of hydrogen atom production are through HCO decomposition and the reaction of carbon monoxide and hydroxyl.  The rate of hydrocarbon conversion (to HCO) determines the rates of radical generation.  The slow rate of conversion of fuel into hydrogen atoms due to long chain length and the diffusion of intermediate hydrocarbons into the cold zone is responsible for the lower concentration of radicals and lower oxidation levels of different fuels for the same initial conditions. 

Future Plans:  Plans include implementing an adaptive gridding for code acceleration, concluding simulations using aromatic and oxygenated fuels.  We shall then compare the simulations and experimental data, and analyze the models and simplify them. 

References 
1. Green, R. G., and Cloutman, L. D., "Planar LIF Observations of Unburned Fuel Escaping the Upper Ring-Land Crevice in an SI Engine," Soc. Autom. Eng. Paper  970823 (1997).  
2. Wu, K. and Hochgreb, S. "The Roles of Chemistry and Diffusion on Hydrocarbon Post-Flame Oxidation", Combust. Sci. Tech., in press, 1997b.  
3. Wu, K. and Hochgreb, S. "Numerical Simulation of Post-Flame Oxidation of Hydrocarbons in Spark Ignition Engines", Soc. Autom. Eng. Paper 970886 (1997a).  

Key Personnel 

Graduate Students:  Kuochun Wu and Ivan Oliveira 

Goals:  The primary objectives of this study are as follows: 

1. To develop an experimental database of PM emissions (mass rate, composition, number and size distribution) from SI engines operating at steady and transient (cold start) conditions as a function of fuel type, operating conditions and fuel delivery technique. 

2. To develop and apply a time-resolved diagnostic for the measurement of instantaneous PM concentrations during transients as a function engine operating conditions, fuel type, and fuel injection characteristics. 

3. To use the developed database to characterize and quantify the important processes that control PM formation and oxidation in the engine. 

4. To develop a physical model for the PM formation and oxidation processes. 

Rationale:  Recent studies have suggested that atmospheric particulate matter (PM) is one of the most important factors correlating with mortality rate increases in urban areas (NRDC, 1996).  A substantial fraction of the total PM is contributed by internal combustion engines, and a fraction of those from spark-ignited engines. 

Only a relatively small number of tests have been done to quantify the total mass of PM emitted by representative SI automobiles and fleets (Rogge et al., 1993; Williams et al., 1989; Pedersen et al., 1980; Greenwood et al., 1996) as well as to examine the chemical composition of PM from SI engines (Alsberg, T. et al., 1985; Hildemann et al., 1991).  This limited research revealed  some of the basic trends in PM emission with respect to engine type and age, steady state engine operating conditions, and fuel/lubricant  composition.  However,  there has been no systematic study to identify the mechanisms of PM formation in SI engines. 

Considering the limited amount of information on the amount, size distribution, chemical composition and pathways leading to PM formation and emissions from SI engines, the current goals are to produce a systematic study of the mechanisms leading to the formation of PM in gasoline SI engines, addressing the following issues: 

1. Total amount, size distribution and chemical composition of PM emitted by a SI engine;
2. Origin of PM (from oil or fuel) and mechanisms of formation; 
3. Effect of fuel type and engine operating conditions; 
4. Effect of transients and cold-starts; 
5. Effect of injection technology (port fuel injection vs. premixed fuel induction); 
7. PM Removal efficiency of contemporary three-way catalysts. 

Approach:  The investigation utilizes experimental techniques already developed for collection and  analysis of PM from SI engines, and applies new techniques necessary for the measurement of PM during transient operation.  The identification of the different mechanisms to PM emissions will be performed by varying engine and fuel parameters so as to test the particular hypothesis, according to the following tasks: 

Task 1. Baseline PM mapping. Determination of engine-out PM amount and origin sampling and analysis, both at selected steady state and cold start conditions. Total PM mass, size distribution, soluble organic fractions (SOF) and chemical composition will be determined, using techniques developed at the MIT Analytical Chemistry Laboratory and at Caltech. A modern four-stroke, four-cylinder engine is used in the study. 

The chemical analysis of the PM is planned for the characterization of the measurements relative to previous measurements in production automobiles, both at steady state conditions as well as during cold start.  The identification of the origin of PM matter can be performed by using chemical analysis (for SOF compounds), elemental analysis (to identify oil-originated metals, or inorganic matter from valve surface pitting). 

Routine emissions measurements of in-cylinder pressure, CO, CO₂, NO, equivalence ratio and total and time-resolved HC will be made for correlations and interpretation of the PM measurements during each experiment.  Examination of the catalytic removal efficiencies will be considered once the baseline measurements are established. 

Task 2. Transient PM measurements. Development and implementation of a time-resolved total PM measurement technique for transient and cold-start emission testing.  Techniques for quantifying PM emissions under steady state operation or over long periods of time are well established, as are techniques to time-resolve the PM concentrations in diesel engines; however, there is a virtual absence of time-resolved measurements of  PM formation at concentrations as low as those expected in an SI engine. 

Laser induced incandescence (LII) will be employed to measure time-resolved PM concentrations far lower than expected in an SI engine (Vander Wal, 1995).  Further, it has been used for sizes down to 100 nm or lower, as well as to measure soot in diesel engines (Dasch, 1984; Dec., 1991).  The LII signal is linearly proportional to soot volume fraction, even at concentrations far below those expected in the engine; consequently, it is expected that the data may be easily interpretable. 

Time-resolved measurements of PM during cold start, as well as during a typical engine acceleration deceleration transient will be made to determine changes in PM emissions.  Correlations of PM emissions during start-up with equivalence ratio excursions and HC emissions will aid in identifying the possible mechanisms for PM formation. 

Task 3.  Identification of possible important PM formation mechanisms. The baseline and transient PM measurements will be used in conjunction with variations in engine parameters to identify the effects of fuel, injection strategy, and operating conditions and thus the possible important mechanisms for PM formation. 

In order to separate the effects of the different potential mechanisms during start-up or transient tests, specific variables (fuel type, injection type and timing, coolant temperature, etc.) will be isolated. 

Status:  Since this project is being cost-shared with the Engine and Fuels Research Consortium, we have been able to get a head start, even though the project only started officially in June, including the following tasks: 

· Implementation and testing of Scanning Mobility Particle Sizer (SMPS) 
· Initial testing and determination of optimum dilution ratio and losses in the dilution tunnel 
· Determination of engine-out particle size distribution, number concentration, and mass concentration from a four-cylinder, spark-ignited engine (Ford Zetec 2.0 liter) under steady-state, and their dependence on: a) air-fuel ratio, b) injection timing, c) speed, d) load 
· Initial chemical analysis of PM collected on filters in the dilution tunnel 
· Initial configuration and equipment selection for LII system 

The most important results are described below: 

1. Determination optimum dilution ratio:  Because the dilution process has a profound effect on the particulate emissions, both on the mass concentration and sizes of particles, experiments were performed in order to determine the optimum dilution characteristics.  Primary concerns were the dilution ratio and the flowrates of both exhaust and dilution air (although they are not independent variables).  Tests were run in two regimes:  a so-called high flowrate (Re » 60,000), where the time for heat transfer with the surroundings is small (residence time within dilution tunnel of about 0.5 seconds), and the degree to which the exhaust cools is dependent upon the amount of dilution air mixed with it; and low flowrate (Re » 9,000), in which the residence time is large (residence time of about 3.5 seconds) and thus the cooling essentially independent of dilution ratio. 

In the high-flowrate regime, the measured concentrations varied with flowrate.  As the dilution ratio increases, the amount of cooling increases, thereby increasing the tendency for HC vapors to condense on existing particles, and increasing the mass of particles (particularly the smaller ones, since they make up a large fraction of the surface area).  However, as the dilution ratio increases, the concentrations of HC vapors, thereby decrease the rate of condensation.  The net effect is an increase in particulate concentrations (corrected for dilution ratio) of up to about a dilution ratio of 15:1, followed by a decrease.  This behavior has been backed up by theoretical models of diesel exhaust dilution by Amann et al. (1980); MacDonald et al. (1980), and Plee and MacDonald  (1980). 

In the low flowrate regime, the dilution ratio does not affect the temperature and hence does not affect the tendency for condensation.  In this case, as the dilution ratio is increased, the particulate concentration decreases until a dilution ratio of approximately 15:1, after which point the condensation appears to be effectively stopped and the concentration remains unaffected by dilution ratio. 

a.  Determination of particle losses:  Tests were performed to determine the magnitude of (a) particle losses as a result of particles sticking to the dilution tunnel walls and (b) particle mass gain as a result of condensation/absorption of HC vapors.  Ideally, tests would have been performed by sampling the diluted exhaust at different points along the dilution tunnel, and comparing the measured concentrations, both on a number and on a mass concentration basis.  In order to perform these tests without interference of water condensation (which occurs in the case of spark-ignition engines), a diesel engine was operated at low load and its exhaust used in the test.  Particles from a diesel are approximately the same size as those from a SI engine, and at low loads, the concentrations can be approximately the same.  The results show that the number of particles decreases by 4%, due to particles losses on the walls.  However, the mass concentration increases by 16%, due to condensation and absorption of vapors. 

b.  Effect of air-fuel ratio on PM production:  The engine air-fuel ratio was varied with intake manifold pressure constant and spark timing held fixed with respect to top center, while HC, PM, NO_x, and CO₂ concentrations were measured. At very lean air-fuel ratios (where HC emissions are high), PM emissions increase substantially on a number basis, but even more dramatically on a mass basis, revealing how strongly the condensation process is affected by vapors.  Additionally, PM emissions change by orders of magnitude when air-fuel ratio is varied, as indicated by the logarithmic scale. 

c.  Effect of liquid fuel:  A dramatic effect on PM production is observed when injection timing is varied with all other engine operating conditions fixed.  The data in Figs. 4-5 suggest that injection of liquid fuel into the cylinder contributes dramatically to PM formation:  although the HC concentration varies by no more than 62% during open valve injection compared to closed valve, the PM emissions vary by orders of magnitude. 

d.  Chemical analysis:  Samples are still undergoing chemical analysis.  Future chemical analyses will test the variation of PM chemical composition with respect to changes in air-fuel ratio, injection timing, and engine operating temperature. 

Future Plans:  We plan on conducting two tests.  The first is the steady state test, in which we will determine the effect of specific engine operating parameters on PM, as in the original schedule proposed.  Additional tests on the effect of spark timing (combustion phasing) on PM emissions will be added.  Tests of catalytic converter efficiency across a matrix of engine speeds and loads will also be tested.  Then we will conduct a transient test.  The LII system will be implemented and bench tested, with calibration made using steady state measurements using the SMPS.  Once calibrated, the system will be used in the engine exhaust during transient measurements. 

References 
1. Amann, C. A., Stivender, D. L., Plee, S. L., MacDonald, J. S., "Some Rudiments of Diesel Particulate Emissions," SAE Paper 800251, 1980. 
2. Alsberg, T., et al., "Chemical and Biological Characterization of Organic Material from Gasoline Exhaust Particles," Environ. Sci. Technol., 19: 43-50, 1985. 
3. Greenwood, S. J., Coxon, J. E., Biddulph, T., Bennett, J., "An Investigation to Determine the Exhaust Particulate Size Distributions for Diesel, Petrol, and Compressed Natural Gas Fueled Vehicles," SAE Paper 961085 , 1996. 
4. Hildemann, L. M., Markowski, G. R., Cass, G. R., "Chemical Composition of Emissions from Urban Sources of Fine Organic Aerosol," Environ. Sci. Technol., 25: 744-759, 1991. 
5. Dasch, C. J., "Continuous-Wave Probe Laser Investigation of Laser Vaporization of Small Soot Particles In a Flame," Applied Optics, 23: 2209-2215, 1984. 
6. Dec, J. E., zur Loye, A. O., Siebers, D. L., "Soot Distribution in a D.I. Diesel Engine Using 2-D Laser-Induced Incandescence Imaging," SAE Paper 910224 , 1991. 
7. MacDonald, J. S., Plee, S. L., DâArcy, J. B., Schreck, R. M., "Experimental Measurements of the Independent Effects of Dilution Ratio and Filter Temperature on Diesel Exhaust Particulate Samples," SAE Paper 800185 , 1980. 
8. Natural Resources Defense Council, Breath-Taking:  Premature Mortality Due to Particulate Air Pollution in 239 American Cities , 1996. 
9. Pedersen, P. S., Ingwersen, J., Nielsen, T., Larsen, E., "Effects of Fuel, Lubricant, and Engine Operating Parameters on the Emission of Polycyclic Aromatic Hydrocarbons," Environ. Sci. Technol., 14: 71-79, 1980. 
10. Plee, S. L., MacDonald, J. S., "Some Mechanisms Affecting the Mass of Diesel Exhaust Particulate Collected Following a Dilution Process," SAE Paper 800186 , 1980. 
11. Rogge, W. F., Hildemann, L. M., Mazurek, M. A., Cass, G. R., "Sources of Fine Organic Aerosol. 2. Noncatalyst and Catalyst-Equipped Automobiles and Heavy-Duty Diesel Trucks," Environ. Sci. Technol., 27: 636-651, 1993. 
12. Vander Wal, R. L., Dietrich, D. L., Zhou, Z., Choi, M. Y., "Development and Applications of Laser-Induced Incandescence," Third International Microgravity Combustion Workshop, Cleveland, Ohio, April 11-13, 1995. 
13. Williams, D. J., Milne, J. W., Roberts, D. B., Kimberlee, M. C., "Particulate Emissions From ÎIn-Useâ Motor Vehicles - I. Spark Ignition Vehicles," Atmospheric Environment, 23: 2639-2645, 1989. 

Personnel 
Graduate Students:  David Kayes and Derek Kim 

Goals:  (1)  To design a carefully-controlled experiment for deposit accumulation and HC emission measurement.  (2)  To assess the effects of combustion chamber deposits on the hydrocarbon emissions from a modern production spark-ignition engine.  (3)  To measure the effect of CCD on HC emissions from single-component fuels.  (4)  To develop and validate a model for the mechanism(s) by which combustion chamber deposits lead to additional HC emissions.  (5)  To study the effects of combustion chamber deposits on NO_x emissions. 

Rationale:  Engine deposits (on intake valve and combustion chamber) increase HC emissions.  Some recent data suggest that combustion chamber deposits also increase NO_x emissions.  To meet stringent future emissions standards, the emissions due to deposits will have to be reduced.  The first step towards that end is to better quantify these emissions and understand the mechanisms involved in their formation. 

Approach:  A four-cylinder, DOHC Saturn engine has been subjected to a standardized deposit build-up cycle.  An additized fuel (which keeps the intake valves and ports clean) was used to isolate the effects of the combustion chamber deposits on emissions.  HC and NO_x emission measurements were taken continuously during the deposit accumulation process.  In parallel a model for the effect of deposits on HC emissions has been developed. 

Status:  The project has now been completed.  Four deposit build-up tests (100, 50, 25, and 35-hour tests) were carried out.  In these tests, the HC emissions stabilized after about 25 hours.  The HC emissions increased by an average of 14% due to deposit build-up.  The HC emissions returned to the clean engine baseline levels after the combustion chamber deposits were removed.  The NO_x emissions, which were expected to increase slightly during these tests, showed substantial scatter and no clear trend was apparent. 

The deposit accumulation process developed has shown that deposits can be built up systematically and reproducibly in engine dynamometer tests.  The HC emissions trends were surprisingly repeatable.  The significant finding was that the HC emissions increased for the first 20 hours of operation and then stabilized, even though deposits continued to build up.  Thus engines will have to be very "clean" to largely eliminate this increase--an important practical issue.  The NO_x emission variability noted above is believed due to variability in the engines EGR system.  Despite efforts to reduce this, no clear trends as deposits build up could be determined. 

A model has been developed to explain the observed increase in HC emissions as deposits build up, and the lack of sensitivity of this increase to fuel compound in the individual hydrocarbon fueled tests.  Critical to the development of this model were studies of the pore size distributions of the cylinder head and piston crown deposits (which had different characteristics). 

Three different mechanisms were examined to explain the effect of CCDs on the HC emissions.  The first is the displacement of fuel-air mixture into and out of the larger deposit pores as the cylinder pressure rises and falls.  The second consists of pressure driven bulk flow into the deposit pores, in the pore size range (1 - 0.1 micrometer ) where viscosity is important.  The deposits are treated as a porous medium with an estimated permeability.  Darcyâs Law for flow in a porous medium forms the basis of this model.  The third mechanism consists of ordinary diffusion of fuel molecules into the air (or exhaust gases) in the deposit pores.  The fuel molecules diffuse into the deposit pores during the intake, compression, and combustion processes and get released into the combustion gases during the expansion and exhaust processes.  During flow in, they are absorbed onto the pore surfaces.  By applying these models to the appropriate pore size range, and weighting the trapped HC by the relative importance of these size ranges, the individual mechanism contributions to the total deposits impact was quantified.  Only the crevice model of the larger (< 1 micrometer ) pores is significant, and the cylinder head deposits contribute many times what the piston deposits contribute.  The model indicates that the pore depth to which fuel penetrates becomes limiting ( ~ 100 micrometer  for the cylinder head) even though the deposit thickness steadily increases beyond that. 

The maximum amount of HC trapped in the deposits is reduced by oxidation and retention in the cylinder.  Allowing approximately for these effects produces estimates of the increase in engine HC emissions comparable to the measured increases. 

Key Personnel 
Graduate Student:  Haissam Haidar 

Goals:  The goal of this project is to gain a scientific understanding associated with the catalytic oxidation of soot with the concomitant reduction of NO_x to molecular nitrogen.  This information is needed to develop equipment for controlling the most problematic pollutants NO_x  and soot from stationary and mobile Diesel engines.  We have an NSF grant intended to demonstrate the use of a rotating fluidized bed reactor (RFBR) containing an attrition resistant high surface area catalytic powder to promote the reaction of soot with NO_x.  The present project has as a goal the development of one or more catalysts that will enhance the beneficial properties of copper exchanged ZSM-5 zeolite while minimizing the poisoning effect of sulfur compounds and water.  Consequently, in addition to catalyst research, we are conducting experimental and theoretical studies on the mechanism and competitive kinetics of soot reactions with the two oxidants, NO and O₂.  We are working with the hypothesis that the observed soot oxidation/NO_x reduction is due to a stable intermediate.  This intermediate, possibly CO, is produced in the non-catalytic oxidation of soot and reduces NO_x on the catalyst.  A mathematical model of the system is being developed to evaluate a method to test the hypothesis. 

Rationale:  This project is designed to find ways to overcome some of the shortcomings of current technology consisting of small passage zeolite catalysts that are used to oxidize Diesel soot, or Pt based oxidation of NO to NO₂ which is then used to oxidize soot collected on a filter.  In the first case, the zeolite acts as a filter for soot capture and plugs when the catalytic surface becomes ineffective due to low temperature operating conditions of the Diesel engine (i.e., city driving).  In the second case, much of the NO₂ is reduced to NO rather than N₂, thus emitting excessive quantities of NO_x.  Our research is directed at obtaining the required mechanistic understanding to develop a catalyst to reduce NO_x while oxidizing soot.  Soot, captured under low power operation, will be oxidized in situ by NO_x at higher temperatures to produce non hazardous CO₂ and N₂.  Consequently, this system can be self cleaning, if we can stoichiometrically balance soot and NO_x.  The use of a RFBR may allow convenient replacement of the filtration solids or catalyst charge over the long life of Diesel engines.  Furthermore, the RFBR rate of rotation couples very well to that of the Diesel engine.  Thus, at low load the bed will rotate relatively slowly and primarily act as a soot filter, and at high load the bed will rotate rapidly and promote the soot-NO_x reaction.  The envisioned RFBR has a relatively small footprint and is expected to fit into the tight design of an automobile.

Approach:  The research consists of both experimental and computer modeling.  The experimental work is being conducted in packed and fluidized quartz reactors.  The quartz reactors are vertical 2.5 cm by 40 cm long and contain a coarse fritted quartz filter to support approximately one cm of catalyst in the center of a three-zone furnace.  Both commercial and experimental catalysts (that we synthesized ourselves) are being evaluated.  The catalysts are manually mixed with carbon black or Diesel soot and reacted with an analyzed gaseous mixtures containing NO, NO₂, O₂, and the balance is He.  The packed reactor is downflow and the fluidized reactor is upflow through the fritted support.  The effluent stream from the reactors flow to one of our on line gas chromatographs.  We are using thermal conductivity detection (TCD) for N₂ and O₂, flame ionization detection (FID) for CO, CO₂ and unreacted hydrocarbons.  The CO and CO₂ are catalytically converted to methane in order to take advantage of the greater sensitivity of FID.  A chemiluminescent NOx analyzer is being used to measure NO and NO₂.  In addition to Cu, modified ZSM-5 that was provided to us by Mobil, we are synthesizing Cu impregnated alumina, ceria, titania, vanadia, and zirconia.  We believe these acidic supports will enhance Cu activity.  Using Cu impregnated alumina, we have seen similar activity, but at 100 C higher temperature, to Cu-ZSM-5.  We still need to conduct lifetime studies to ascertain that the activity is indeed similar. 

The modeling effort consists of studies of a single spherical particle on which carbon (soot) builds up at low temperatures and reacts at elevated temperatures due to the reaction of carbon with the gas phase oxidants.  The carbon oxides then react with NO_x on the catalyst surface.  This particle is considered independent of neighboring particles and represents a fluidized bed.  A set of kinetic rate constants will be obtained from the literature or determined in our laboratory.  Soot is expected to catalytically promote its own oxidation.  The combination of catalytic oxidation and poisoning on a particle with a moving external surface area has never been modeled and will require a new innovative approach. 

Status:  The initial objective of this new effort has been to assemble and calibrate the experimental equipment.  This effort has been accomplished.  Mobil Oil Co. has provided us a samples of 100% Cu-exchanged ZSM-5 with a silica/alumina binder designed to resist attrition in the fluidized beds.  Mobil has also provided us with samples of the binder and ZSM-5 with the silica/alumina binder.  All analytical equipment has been calibrated and has been installed near the quartz reactors.  We are also conducting laboratory experiments using a Thermal Gravimetric Analyzer (TGA) to obtain insight into the oxidation mechanisms.  The quartz flow reactor is also being used to obtain carbon and nitrogen balances.  Three masters theses have been completed on the model and the small scale laboratory research. 

Future Plans:  We plan on publishing two papers.  The first is on Fluid Dynamics of a Rotating Fluidized Bed Reactor, and the second is on Soot/NO_x Catalytic Mechanism.  We will then conduct research with Fe-ZSM-5, which is resistant to water poisoning, and compare catalysts used in this study at high space velocity (viz., Al₂O₃, Cu-Al₂O₃, SiO₂/Al₂O₃, ZSN-5/Al₂O₃, Cu-ZSM-5/SiO₂/Al₂O₃, Fe-ZSM-5, Granular Activated Carbon (GAC) and Cu-GAC). 

Personnel 
Co-PI: John G. Stevens
Postdoctoral Associates: Istvan Bagyi and Zhong-Ming Zhao 
Graduate Students: Gui-Hua, Wei Song, Maria Rosa Diakno, Shu Sun, Xiapong-Yong Tang, Kaiwan Ma, Shan Xiao and Ian W. Buedick 

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

Goal:  To identify and monitor in the laboratory various intermediate species formed in the photooxidation of aromatic compounds, in order to better establish the mechanism of this atmospheric degradation process;  and to measure directly rate constants for various elementary reaction steps of atmospheric importance involving such intermediate species as reactants. 

Rationale:  The atmospheric oxidation of aromatic compounds plays an important role in the generation of pollutants in urban atmospheres.  Reaction rate constants are well established only for the initial oxidation step, which is predominantly reaction with the hydroxyl (OH) radical; in contrast, the rates for most of the subsequent steps have not been directly characterized in the laboratory.  Reliable measurements of such rate constants will increase current understanding of the mechanism of formation of ozone and other pollutants in urban atmospheres, and will enable improved model predictions of photochemical smog production. 

Approach:  The formation of reaction intermediates will be investigated by means of a steady state turbulent flow reactor operating in the 100 - 760 Torr total pressure range, and  fitted with a chemical ionization mass spectrometer (CIMS) for the detection of reactants and products.  The intermediates of interest are generated in the flow tube by mixing the parent aromatic compound, present at small concentrations in an inert carrier gas, with a radical such as OH generated with a microwave discharge. The CIMS detector consists of a chamber operated at pressures below those of the flow tube ¾ typically around 5 - 20 Torr, in which a portion of the flow tube effluents are mixed with a reactant ion such as SF₆^- or  O₂⁺ , generated with a corona discharge or a radioactive polonium source.  The product ions are extracted into a differentially pumped vacuum chamber containing a quadrupole mass analyzer; the ions are collimated and focused by means of electrostatic lenses towards the entrance of the mass analyzer. 

Status:  A positive reactant ion source operating at several Torr total pressure and employing a corona discharge has been developed to produce ions which include He⁺, O₂⁺, N₂⁺ and H₂⁺.  Toluene has been monitored as C₇H₉⁺ as a reaction product with these reactant ions; using O₂⁺; the sensitivity has been determined to be of the order of 10¹⁰ molecule cm^-3.  Methyl-hydroxycyclohexadienyl radicals (the toluene--OH adduct) have been generated in the flow tube by mixing toluene with OH radicals produced by a microwave discharge of trace amounts of water vapor.  These adduct radicals have been successfully monitored with the CIMS detector: the mass spectrometer yields a signal with a mass corresponding to the adduct (C₇H₉OH⁺), a signal which is detected only when both toluene and OH are present in the flow tube. 

Future Plans:  A flow tube - CIMS instrumentation will be developed, the reaction rates of the toluene-OH adduct radical with O₂ and with NO₂ will be investigated by measuring the decay of the CIMS signal as a function of reaction time, and detection of additional reaction intermediates in the atmospheric photooxidation of toluene will be investigated, and reaction rates involving such intermediates will be measured. 

Key Personnel 
Graduate Students:  Keith Broekhuizen 
Research Staff:  Luisa T. Molina and Rafael Navarro-Gonzales 

Goal:  The objective of this project is to gain a better fundamental understanding of the atmospheric oxidation of volatile organic compounds (VOCs) important in urban and regional air quality.  specific aims are to determine the gas-phase mechanisms of reaction of important VOCs with the hydroxyl radical, the atmosphere's most ubiquitous oxidizing species, and to elucidate the mechanisms of formation of organic aerosols from the atmospheric oxidation of VOCs. 

Rationale: Gas-to-particle conversion is a ubiquitous process in the atmosphere, determining the size and composition of particles from the polluted urban atmosphere to the remote marine boundary layer.  Understanding the detailed chemistry and physics of atmospheric gas-to-particle conversion will allow us to predict the effects of primary gaseous and particulate emissions changes on airborne particulate matter, in the urban and regional setting, and the effects of sulfur and other species on the generation of cloud condensation nuclei in the remote atmosphere.  A principal goal of the research program is the development of comprehensive air quality models based on the most complete description of atmospheric chemistry and physics.  These models are forerunners of those that will eventually be used in the regulatory process.  This research is aimed at developing the organic portion of advanced gas-aerosol models, and to advance the current state of understanding of molecular processes.  The component of the proposed research on gas-phase photooxidation chemistry has the goals of adding to the body of kinetic and mechanistic data for atmospheric organics, with particular emphasis on those VOCs that are potential aerosol precursors. 

Approach:  The integrated research program to determine the mechanisms of photooxidation and secondary organic aerosol formation in the atmosphere for a number of important anthropogenic and biogenic hydrocarbons is carried out in the Caltech outdoor reactor and via ab initio molecular simulation.  Experiments in the indoor reactor are used to probe chemical mechanisms.  The large outdoor smog chamber is employed to study the integrated gas-phase and gas-to-particle conversion dynamics. 

Status:  A. Ab Initio Studies of Atmospheric Reaction Mechanisms 

Atmospheric reactions of the butoxy radical (C₄H₁₀0) has been the subject of numerous investigations in recent years.  A key intermediate in the photooxidation of n-butane in the troposphere, the 1-butoxy radical is the simplest alkoxy radical that can undergo isomerization via 1,5-H shift to create a d-hydroxyalkyl radical forming numerous subsequent products.  1-Butoxy and 2-butoxy are thus representative of a large class of alkoxy radicals that are produced by the photooxidation of >C4 aliphatic hydrocarbons in the troposphere and whose fate determines the ultimate end products of these reactions. 

The atmospheric oxidation mechanism of n-butane has been investigated by means of density functional theory and ab initio calculations.  Calculation of energies of reactants, transition states, and stable intermediates predicts the detailed pathways leading to experimentally observed products of n-butane oxidation.  Also serving as a  model system for the oxidation of larger alkanes, quantitative information is obtained for elementary reaction steps that heretofore have been subject to speculation.  Complete basis set model chemistries CBS-4 and CBS-q were used with B3LYP/6-31G(d,p) optimized geometries to calculate energies of over 70 stable species and transition states.  Energies based on density functional theory were obtained at the B3LYP/6-311+G-(3df,2p)//B3LYP/6-31G(d,p) level of theory.  The principal pathway following formation of the 1-butyl radical from hydroxyl (OH) attack on n-butane is found to be 1,5-H shift of the 1-butoxy radical.  After conversion to the d-hydroxy-1-butoxy radical, another 1,5-H shift is expected to be the primary route to 4-hydroxy-1-butanal.  4-Hydroperoxy-1-butanal can be formed after 1,6-H shift in chemically activated 4-hydroxy-1-butylperoxy radicals.  Whereas b-scission in 1-butoxy is an endothermic process, fragmentation of 2-butoxy into C2H5 and CH3CHO is predicted to be the major degradation pathway of the secondary butyl radicals. 

B. Secondary Organic Aerosol Formation 

A series of experiments was performed in an outdoor, photochemical smog chamber in order to investigate the aerosol forming potentials of biogenic hydrocarbons.  One class of these compounds, monoterpenes (C₁₀H₁₆), can be classified according to one of three structure types:  bicyclic monoolefins, cyclic diolefins, and acyclic triolefins.  A number of compounds of each type were investigated.  In addition, two sesquiterpenes (C₁₅H₂₄) and the two oxygenated monoterpenes were examined.  Data were used to explore the relationship between the noted structural differences in these biogenic compounds and their secondary organic aerosol yields.  A series of experiments was also run in a dark system in the presence of ozone or nitrate radicals in order to identify the primary oxidant responsible for secondary organic aerosol formation. 

Future Plans:  Plans include continuing ab initio studies of atmospheric reaction mechanisms, interacting with the group of Joseph Bozzelli where appropriate.  Also, outdoor smog chambers studies of organic aerosol formation from biogenic hydrocarbons and data analysis with respect to molecular characteristics of the hydrocarbons will be completed, and application of three-dimensional modeling of gas/aerosol behavior in South Coast Air Basin will continue. 

Key Personnel 
Postdoctoral Fellow:  Tim Jungkamp
Graduate Students:  Jay Odum, Robert Griffin, and David Cocker 
CAO Collaborators:  Joe Bozzelli and Larry Lay (NJIT) 

Goal:  The goal of this research is to develop a new mathematical and computational framework for the systematic sensitivity and uncertainty analysis of the complex transport and transformation processes that control the concentration dynamics of airborne organics.  A particular focus is the implementation of numerical procedures that are much more computationally efficient than even the best Monte Carlo sampling strategies.  Once the tools have been developed, the approach is to carry out a detailed investigation of the photochemical oxidation mechanics for aromatic compounds. 

Rationale:  One of the consequences of using models to describe the formation and transport of photochemical air pollution is that some approximations are involved.  In addition, there are also measurement errors in the data used to develop inputs and kinetic parameters for the reaction mechanisms.  The key issue is not that uncertainties are involved, they will always be present, but to identify which of the inputs contributes most to the uncertainty in predictions.  The present set of tools available to the research community are simply computationally intractable for the complex reaction schemes needed to describe the photochemistry of airborne organics. 

Approach:  In this research a new approach termed the Deterministically Equivalent Modeling Method (DEMM) has been developed.  Uncertain parameters are treated as random variables that are in turn approximated using orthogonal basis function expansions in the probability space.  For example, if the uncertain inputs are independent and Gaussian-distributed, the expansion is based on standard Hermite polynomials.  A complete description of the method is contained in Tatang (1994).  The utility of the DEMM -- Collocation approach in conducting uncertainty analyses of large, "black box" type models was demonstrated last year using the Statewide Air Pollution Research Center (SAPRC) mechanism which has over a hundred uncertain parameters. 

Status:  Set out below are accomplishments of the project work in 1996 - 97 (termination): 

ð  The effects of uncertain isoprene reactions were investigated by comparing the current Regional Acid Deposition Model (RADM) and a mechanism supplement with explicit reactions of isoprene and its products.  Explicit treatment of isoprene reactions was important for the characterization of the effects of uncertain parameters on the predictions.  In addition to the total amount of isoprene, uncertainties in the product coefficients of secondary carbonyls compounds, methacrolein and methyl vinyl ketone, also contributed to uncertainties in the predictions of ozone and other photochemically-reactive compounds. 

ð  The DEMM -- collocation approach was applied to three atmospheric mechanisms with different design assumptions.  A comparison of the uncertain predictions of the SAPRC, RADM, and Carbon Bond IV (CB4) mechanisms showed that the predictions are indistinguishable given the effects of current parametric uncertainties.  Any observations could be reproduced by the mechanisms using parameters that were within their ranges of uncertainties.  Therefore, a research priority should be to reduce parametric uncertainties of the key variance-contributing parameters in these mechanisms. 

ð  The uncertainties in the predictions of the three-dimensional CIT Airshed model due to uncertainties in the chemical processes were investigated.  The uncertainties in the predicted ozone concentrations due to uncertain chemical parameters were about 25% of the nominal value.  This uncertainty did not explain the discrepancies between model predictions and field observations. Other sources of model and data uncertainties dominate the predictions and / or observations. 

ð  The same set of uncertain parameters contributed to uncertain predictions of all three mechanisms investigated and the CIT Airshed model.  The variance-contributing inputs included: photolysis rates of NO₂ and HCHO, the initial condition / sources of NO_x and the reaction rates of HO + NO₂ and PAN reactions. 

Uncertainty analysis is a valuable tool to understand and differentiate models.  DEMM was applied in a series of uncertainty analyses to identify chemical parameters that contribute to uncertain model predictions.  Priorities for field and laboratory studies can be set accordingly.  This type of analysis can be extended to a wide range of chemical and environmental problems. 

Key Personnel 
Graduate Student:  Betty Pun 

Goal:  The purpose of this research project is to conduct a field experimental program in which the evolution of the size distribution and chemical composition of the urban aerosol complex is observed using methods that focus on the evolution of the individual aerosol particles.  The goal is to directly observe how primary particles emitted from the many air pollution sources in a major city are modified by gas-to-particle conversion processes in the atmosphere. 

Rationale:  The US Environmental Protection Agency recently has established a new ambient air quality standard that limits fine particle concentrations in the atmosphere.  In order to devise emissions control strategies that can be relied upon to achieve specified improvements in fine particle air quality, it is necessary to understand both the relative contributions that important primary particle emissions sources make to ambient particle levels and also how those particles are modified over time in the atmosphere by gas-to-particle conversion processes. 

Recently, two developments have occurred that make it possible to examine the transformation and fate of single particles in the atmosphere.  First, aerosol time-of-flight mass spectrometers (ATOFMS) have been designed and are being tested that can determine both the size and the chemical composition of individual aerosol particles at the rate of hundreds of thousands of particles per day.  Both organic and inorganic composition can be examined.  By sampling with ATOFMS systems at multiple sites located along single air parcel trajectories, it should be possible to observe in detail the nature of particle transformations as particles age in the atmosphere due to atmospheric chemical reactions.  Second, in a separate development a Lagrangian particle air quality model has been developed at Caltech that tracks the evolution of aerosol particles in the atmosphere in a way that it is possible to account for the particle-to-particle differences in chemical composition for particles of the same size that is evident in the ATOFMS data.  Versions of that model are under development that retain information on the original sources from which the various primary seed particles were emitted (which is particularity useful for preliminary emission control strategy evaluation).  Experiments are proposed here that will provide atmospheric data on single particle chemical composition in the Southern California atmosphere that in the short term can be used to observe and describe the transformations of individual particles by chemical reaction in the atmosphere and that in the future could be used to test air quality models against data on single particle composition in the atmosphere. 

Completion of the research proposed here on the evolution of the Southern California aerosol as it is transported across the Los Angeles urban area will meet several needs.  First these experiments will serve to determine how the single particle data base generated by time-of-flight mass spectrometers can be aggregated to recreate the bulk aerosol size distribution and chemical composition as measured by cascade impactors and electronic size distribution analyzers.  Second, the results will describe the evolution of the Southern California aerosol as it is transported and transformed in the atmosphere.  Since Rubidoux near the proposed trajectory end points at Riverside probably has the highest fine particle concentrations in the nation, detailed information on how that aerosol is created is expected to advance our understanding of how such severe air quality problems can be controlled.  Third, the experiments will provide a model verification data set for later testing the predictions of Lagrangian particle aerosol processes air quality models that are needed for use as design tools during the control strategy testing phase of the state implementation planning process for airborne particulate matter. 

Approach:  Experiments will be conducted in which the background marine aerosol first is characterized based on measurements made at Santa Catalina Island which is located upwind of the Los Angeles area in the summer.  Then Lagrangian air parcels will be sampled as they are transported across the urban Los Angeles area to Riverside, CA, in the presence of direct emissions from urban pollution sources and as the aerosol is modified by gas-to-particle conversion processes.  Both organic and inorganic aerosol species will be sampled simultaneously, (1) by time-of-flight mass spectrometers that view single particle size and composition, (2) by cascade impactors from which particle chemical composition can be measured as a function of particle size, (3) by filter-based samplers and (4) by electronic instruments that measure particle size distributions directly and continuously. 

Status:  During September and October of 1996, a field experimental program was conducted in the Los Angeles area in which the evolution of the size distribution and chemical composition of the urban aerosol complex was observed using methods that focus on the evolution of the individual aerosol particles.  Experiments were conducted in which the background marine aerosol was first characterized as it flows across the Pacific coastline in Southern California.  Lagrangian air parcels were sampled as they were transported across the urban Los Angeles area from Long Beach to Fullerton to Riverside, CA, in the presence of direct emissions from urban  pollution sources and as the aerosol is modified by gas-to-particle conversion processes.  Both organic and inorganic aerosol species were sampled simultaneously using the methods discussed above. 

There were three principal objectives of the 1996 field experiments. These include (1) calibration of the ATOFMS units under field operating conditions in order that they can be used to represent the atmospheric particle size and composition distribution accurately, (2) description of the evolution of the atmospheric particle mixture as it is transformed by passage across Southern California, and (3) organization of a data base that could be used at a later date to test the predictions of atmospheric models for particle formation and transport.

In response to the first objective, these experiments were designed to determine how the single particle data base generated by time-of-flight mass spectrometers could be aggregated to recreate the bulk aerosol size distribution and chemical composition as measured by cascade impactors and electronic size distribution analyzers.  That objective has been met.  Through comparison of particle size distribution data collected by MOUDI impactors, electrical aerosol analyzers, and optical particle counters to the particle counts as a function of particle size measured by the aerosol time of flight mass spectrometers (ATOFMS) it has been possible to determine the counting efficiency of the ATOFMSs.  As shown in Figure 1, the counting efficiency of  an ATOFMS declines rapidly as particle size is reduced, but the counting efficiency performance is stable and reproducible during the experiments.  Next, calibration curves that relate the ATOFMSs peak heights for individual chemical species to the absolute quantity of the chemical species in the atmosphere have been constructed.  The particle counting efficiency correction curves described above first are applied to the ATOFMS output, then the appropriate ion counts that represent the important chemical species in the particles are compared to the concentrations of these species as a function of particle size measured from the MOUDI impactor substrates.  Examples of the ATOFMS calibration curves for nitrates and ammonium ion as a function of particle size are shown in Figures 2 and 3.  Analogous calibration curves for organics and for metals are under development at present.  By scaling the original particle counts taken by the ATOFMSs according to the calibration curves, it has been possible to convert the ATOFMS from a qualitative to a quantitative instrument for measurement of particle size and composition at the single particle level.  This is important because the calibration experiments are very labor intensive and can be performed only periodically, while the ATOFMSs are able to take data continuously for weeks on end.  Through the application of the calibration curves (which effectively replicate the particles in proportion to the degree to which they are undercounted), the ATOFMS has been converted into a continuous air monitoring instrument at the single particle level whose output can be interpreted in terms of absolute concentrations as a function of particle size. 

The second objective of the 1996 experiments was to describe the evolution of the Southern California aerosol as it is transported and transformed in the atmosphere.  This work on descriptive analysis of the data collected is underway at present.  The experiments were designed such that a single air parcel (hopefully) could be followed as it moved progressively over the Long Beach, Fullerton, and Riverside air monitoring sites.  As seen in Figure 4, that experimental design succeeded, and we now have raw data on essentially the same air mass as it passes over each site in turn.  Those data are presently being examined in order to describe the particle transformations that are observed empirically as the air masses examined are aged over time in the presence of effluents from the urban emissions sources.  Descriptive analysis of the experimental data should be completed by June of 1998. 

The third objective of the experiments was to provide a model verification data set that could be used in the future to test the predictions of aerosol processes air quality models that seek to predict the compositional differences between particles in the atmosphere at nearly the single particle level.  By June of 1998, that data base will have been constructed following full application of the calibration data to all of the ATOFMS data, and following the merger of the roughly 5000 chemical species concentration measurements made via manual samplers into a consolidated array of the data. 

Future Plans:  The remaining research to be conducted involves data analysis and preparation of journal article manuscripts that describe: the experimental design and particle evolution as seen by bulk chemical measurements made along the air parcel trajectories, the calibration of the particle counting efficiency of the aerosol time of flight mass spectrometers, the calibration of the chemical species sensitivity of the aerosol time of flight mass spectrometers, and a description of the chemical evolution over time of the Los Angeles area particulate air pollution as seen by the aerosol time of flight mass spectrometers. 

Key Personnel 
Post Doctoral Scholar:  Jonathan Allen 
Graduate Students:  Lara Hughes and Michael Kleeman 
Undergraduate Student:  Robert Johnson 

Goal:  The goal of this project is to develop a prototype microengineered mass spectrometer for in-situ measurement of airborne contaminants.  This mass spectrometer uses standard silicon processing techniques to scale down the dimensions of an existing mass spectrometer to make it portable.  It can be manufactured economically and has the potential to be integrated with more complex structures including transistor integrated circuits. 

Rationale:  One of the stated objectives of the Center for Airborne Organics is to provide tools to reliably connect the identities and concentrations of airborne organic compounds with major emission sources by "developing and using improved techniques to sample and analyze emissions from sources and material in the air."  This project directly addresses that objective.  By developing a low-cost, portable mass spectrometer we expect ultimately to enable on-site testing at unprecedented levels.  An important capability of the sensor is optimization for particular ions that can be identified as tracers for specific pollution source types.  The information gained from both the increased and the more focused testing can be expected to impact all aspects of the Center's research, providing data and feedback for the synergistic work of the Center's other investigators.  In addition, the prototype is expected to serve as leverage for significant external research and development funding, particularly in the drive toward hand-portable instruments. 

Approach:  The proposed device is fabricated using two silicon substrates, each approximately 1cm by 1cm.  One of the substrates contains a novel microtip field emitter cathode electron source and ion extraction electrodes, the other provides ion collector, extraction and focusing electrodes.  Gas molecules near the cathode are ionized by electrons generated by field emission.  An electrostatic field and a uniform magnetic field of 3000 Gauss are used to establish separate trajectories for each ionized mass isotope.  Each mass is collected at a different location on the ion collector plane and measured as a current through individual detector electrodes.  Voltage supply levels between 0 and 50 V, and the 3000 Gauss magnetic field permit collection of ions up to 222 amu (radon).  The minimum detectable density level for a given isotope is proportional to the active ionizing volume, and is limited by the input noise level of the ion collection detector circuits.  From calculations, we expect to be sensitive to partial pressures below 10-11 Torr. 

Status:  We are in the final stages of completion of the second year of this two year project and expect to deliver a final project report in the spring of 1998.  To test the feasibility of a miniature mass spectrometer, analyses of electron and ion trajectories for a number of designs have been carried out using the SIMION simulation program.  A design has been selected based on its ability to provide focusing for optimum sensitivity and mass selectivity.  Fabrication details for this optimized structure have been developed, photolithography masks have been made and the first round of processing has been carried out in NJIT's cleanroom.  The most challenging fabrication task has been the production of the novel field emitter structures, which has required several iterations.  To date, we have successfully built the microtip devices, and tested them under high vacuum conditions.  Under applied biases ranging from 60 to 170 Volts, the tips have displayed Fowler-Nordheim emission characteristics, an important milestone in creating the ion source.  Despite this success at the end of the first year, a number problems were encountered with the initial tip arrays, including significant leakage between the tips and the gate, lack of sufficient tip sharpness and low tip lifetime.  During the second year we have concentrated on addressing these issues by changing the fabrication process from a one mask to a three mask sequence, and by changing one critical metallization step from omni-directional sputtering to uni-directional e-beam deposition.  Our latest devices have 5 orders of magnitude less leakage than the first samples and significantly improved tip sharpness.  We have mounted these devices in a UHV vacuum system and measured relatively stable F-N emission.  More importantly, we have also measured ionization of residual gas molecules.  Currently, we are in the process of mounting the devices in a permanent magnetic field to demonstrate mass selectivity. 

Key Personnel 
Post Doctoral Scholar:  Qi Xing Sun 
Graduate Student:  Chao Sun 

Goal:  Soot particles generated and emitted by combustion processes have a microstructure and trace element composition which can be observed with high resolution electron microscopes.  Since the microstructure and chemical composition are functions of fuel type, they can be useful markers of the origin of soots.  The goal in this project is to develop methods for quantifying the soot structure and elemental composition in order to determine their potential use as signatures for the source of particulate carbon in ambient air. 

Rationale:  Combustors are a major source of airborne organics which contribute, directly or indirectly, to health hazards and to visibility degradation.  Fuel formation and combustor design are expected to be steadily changing, and therefore it is important to have reliable means for identifying the true sources of the organic contaminants in ambient air so that effective corrective engineering and policy strategies can be formulated and implemented in a timely manner.  To this end, the objective of the research is to identify, for combustion sources, chemical and structural characteristics of soots that are sufficiently unique and stable to be reliable signatures for the fuels and combustors responsible for their emission.

Approach:  Work involves: (i) development of methods for obtaining quantitative measures of the soot structure and composition, (ii) determination of the relation of soot microstructure and composition to combustion conditions and fuel type through the use of well defined experiments, and (iii) development of a library of soot structures for use in emissions source attribution studies.  All research is performed in the Department of Materials Science and Engineering facilities at MIT and in the Center for Materials Science and Engineering at MIT. 

Status:  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 trace elements present in soots are the result of engine wear, fuel additives, and/or combustion processes.  A comparison of  the analyses of several soots have shown significant chemical differences (Figure 1).  Figure 2 shows a jet soot aggregate and three elemental analyses (EDX), viewed as spectra.  The jet soot data revealed a greater number of elemental contaminants than the diesel soots.  Figure 3 is an enlarged spectrum from the jet soot.  The diesel soots showed a greater tendency toward chemical impurities with higher engine load.  Quantification of peak height from spectra was accomplished, after background and absorption corrections, and by means of standard analysis and conversion of peak ratios to concentration ratios.  The richness of these spectra and our ability to quantify results represents an opportunity to accomplish source identification in a novel, powerful way.  Electron energy loss spectroscopy is another way of analyzing light elements in thin specimens.  EELS allows the energy losses in the transmitted beam, which corresponds to inner-shell ionization in the atoms characteristic of the sample, to be detected.  This analysis technique is particularly useful for elements of low atomic number, Z=3-11.  It has been shown that in soot, the energy loss of the p electrons increases with the amount of oxidation at high temperatures.  Both the jet soot and the diesel soots studied showed EELS spectra characteristic of amorphous material. 

Future Plans:  The primary thrust of our research in the next few years will be to greatly increase out data base, thus allowing identification and classification of unknown soots into specific categories.  All of our data will eventually comprise the MIT Soot Catalogue.  Further refinement of STEM analysis methods including EDX and EELS will also be emphasized. 

Key Personnel
Postdoctoral Associates:  Arpád Pálotás and David C. Bell
Research Personnel:  Lenore C. Rainey