Center on Airborne Organics
1996 Annual Report
F. RESEARCH PROJECT DESCRIPTIONS
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
Goal: The objective of this project is to enhance predictive capabilities of the previously developed model 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. 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, but the model has only been applied to simplest flow systems. The predictive capability of the model is being improved, so as to extend the use of the model to more complicated systems of practical concern.
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. Predictive capability of the combined model is being improved by inclusion of more 2-6 ring PAH, especially those containing 5-membered rings (some of which are known toxics), use of more accurate values for key rate coefficients and by accounting for the removal of PAH by soot. A directed effort is being made to reduce the computational demands, which at present limit the applicability of the model to simple well-stirred and plug-flow systems.
Status: The FORTRAN program which generates the CHEMKIN-II reaction set for the aerosol portion of the new combined gas-phase/aerosol dynamics model has been updated to allow inputs for: (1) three different cases of the intra-bin distribution function, and (2) all values of the bin boundary factor greater than or equal to two. Predictions using the three different cases of the intra-bin distribution function (all other inputs being equal) show considerable sensitivity of both the predicted particle size distributions and the total soot concentration (about 3 orders of magnitude in total mass, 4 orders of magnitude in total particle number) to the intra-bin distribution function. The results are explained in terms of the extent of deviation from particle number occurring from use of the different distribution function, with the base case intra-bin distribution function (i.e., constant number density with respect to log of molecular weight) coming closest to conserving particle number.
A study of part of the parameter space of the model was performed, in which pairs of collision efficiencies for C2H2-soot and PAH-soot reactions were sought which correctly predicted experimental soot mass concentrations. This study included cases in which only C2H2 added to the soot. The required collision efficiencies (2.2*10-4 to 2.6*10-4) are similar to those found by other workers using the same assumption (of only C2H2 adding to soot.); also, the required collision efficiency increased with increasing equivalence ratio, supporting the notion that other soot growth species such as PAH might be important. For simulation in which both C2H2 and PAH were adding to soot, a set of pairs of collision efficiencies was found which matched the experimental soot mass concentrations. From a rate-of-production analysis, for a prediction with the C2H2-soot collision efficiency equal to 2.2*10-4 and the PAH-soot collision efficiency equal to 0.1, 26.2% of the soot mass growth was found to be due to PAH addition. Comparison of predictions for this case with those for the case where all the soot growth is from C2H2 shows little difference in the predicted critical equivalence ratio for soot formation, or particle size distributions; however, PAH concentrations were two-times smaller for the case with PAH adding to the soot. Also, a method was found for avoiding in the simulations numerical artifacts which appeared as multiple solutions for a given set of conditions.
Pope, C.J. and Howard, J.B., "Simultaneous Particle and Molecule Modeling (SPAMM): An Approach for Combining Sectional Aerosol Equations and Elementary Gas-Phase Reactions," Aerosol Sci .Technol., in press.
Postdoctoral Associates: Christopher Pope and Henning Richter Graduate Student: David Kronholm
Combustion Chamber Deposit Effects on Engine Hydrocarbon Emissions: J. B. Heywood and S. Hochgreb, Massachusetts Institute of Technology
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 NOx emissions.
Rationale: Engine deposits (on intake valve and combustion chamber) increase HC emissions. Some recent data suggest that combustion chamber deposits increase NOx emissions. To meet stringent future emission 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 is subjected to a standardized deposit build-up cycle. An additized fuel (which keeps the intake valves and ports clean and significantly increase the amount of combustion chamber deposits) is used, to isolate the effects of the combustion chamber deposits on the emissions. HC and NOx emission measurements are continuously taken during the deposit accumulation process.
Status: Four deposit build-up tests (100, 50, 25, and 35-hour tests) have already been conducted so far. In the first three tests, the HC emissions stabilized after about 25 hours. The HC emissions increased by an average of 14% (over the different tests and operating conditions) due to deposit build-up. The HC emissions returned to the ìcleanî engine baseline levels after cleaning the combustion chamber. The most recent test, a 35-hour deposit accumulation test proved inconclusive. The main purpose of this test was to quantify the effects of CCDs on HC emissions from a matrix of single-component fuels (isooctane, benzene, toluene, and xylene). The effect of CCDs on the HC emissions from the deposit fuel was not conclusive. There was large scatter in the HC data. Even though there was a general trend of increase in HC emissions with CCD build-up, the HC emissions did not go back to their clean-engine levels as in the previous three tests. This was thought to be due to the variation of the EGR, test to test, at each operating condition. The electric EGR valve ( a change from previous three tests designed to improve repeatability of NOx data) was disconnected at the end of the deposit build-up test. Repeat emission measurements were taken with the deposit fuel and the single-component fuels, with the deposited engine and after the clean-up of the CCDs. The HC data (from all fuels and all operating conditions) showed less scatter and a significant reduction in HC emissions after CCD removal. The experiment showed that the effect of CCDs on HC emissions from these fuels is significant (a 20 to 45% increase in HC emissions between a clean and a dirty engine). Because of the large scatter in the NOx data, no conclusion could be reached on the effect of CCDs on NOx emissions. We believe this scatter is due to the variability in EGR amount from test to test.
Three different mechanisms are being modeled to explain the effect of CCDs on the HC emissions. The first one is the displacement of gas 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, including the effect of viscosity. The deposits are treated as a porous medium with an estimated permeability. Using Darcy's law for flow in a porous medium, the 1-D mass conservation equation in the deposit pores is solved. The amount of fuel stored in the deposits is then estimated. The results of this model are strongly dependent on the porous structure of the deposits. With constant pore geometry, this model is not able to predict the rapid rise in the HC emissions with deposit build-up and its stabilization after 25 hours. The third mechanism consists of ordinary diffusion of fuel molecules in the air (or exhaust gases) trapped in the deposit pores. The fuel molecules diffuse in the deposit pores during the intake, compression, and combustion processes and get released into the combustion gases during the expansion and exhaust processes. The mathematical formulation of the model consists of solving a 1-dimensional unsteady diffusion equation in the deposits pores. The diffusion model results are consistent with the experimental trends in that they predict a rapid increase in the HC emissions with deposit thickness and stabilization of the HC emissions after about 35 hours of deposit accumulation. After including a simple oxidation scheme in the cylinder head and exhaust ports, the diffusion model predictions are in close agreement with the experimental results. In the deposit pores, a combination of these three mechanisms takes place. The first two mechanisms dominate in larger pores, while the diffusion mechanism dominates in smaller pores. The next step in the modeling effort is to identify whether any one of the mechanisms dominates. This will be achieved with the help of mercury porosimetry measurements of the deposit pore size distribution. The mechanism of surface diffusion of fuel molecules on the pore walls, by which fuel molecules get adsorbed on adsorption sites on the pore walls and then start migrating from one site to another, was examined and proved to be insignificant in comparison to the other mechanisms at the deposit temperatures.
Graduate Student: Haissam Haidar
Chemical Kinetic Modeling of Formation of Products of Incomplete Combustion from Spark-ignition Engines: Simone Hochgreb, Massachusetts Institute of Technology
During the first three months of the investigation, a one-dimensional code has been used to investigate the chemistry of oxidation of propane from unburned hydrocarbon layers near cold walls during the post flame expansion. The results show that the core gas temperature has the biggest impact on the oxidation level of unburned hydrocarbons and on the production of incomplete combustion products. Accordingly, at early stages of the expansion process (with core temperatures above 1800 K), oxidation is fast and complete. At later stages, when core temperatures drop below 1600 K, the oxidation rate decreases and the ratio of non-fuel intermediates to the original fuel increases. Finally at the end of expansion, the oxidation rate becomes very slow and a high portion of fuel survives oxidation with smaller production of intermediate hydrocarbons. Simulations show that transport rates and the assumed thickness of the initial thermal boundary layer near the wall have only moderate impact on the oxidation level and fuel/nonfuel ratio.
Graduate Student : Kuochun Wu
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
Goal: Experimental and modeling studies are performed to understand and characterize reactions of oxygenated hydrocarbons (OHC's) such as alcohols and ethers important to gasoline octane blending. A detailed mechanism will be developed to allow optimization and trend prediction by calculation, in engine performance and emission reductions.
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: Gas mixtures are reacted in a uniform, 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.
The reaction mechanisms are based upon fundamental principles of thermochemical kinetics, transition state theory, chemical activation, quantum Rice-Ramsperger-Kassel theory for k(E), modified strong collision treatment for fall-off and thermodynamic properties. Semi empirical and ab initio calculations are used to determine properties of transition states.
Status: 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. Experiments are beginning on dimethyl ether. A thermodynamic data base and an elementary reaction mechanism has been assembled for MTBE oxidation and is undergoing evaluation. Sub models of neopentane, isobutane, and isobutene oxidation are developed and shown to compare well compared with experimental data. A sub-model for 2 and 3 carbon species is being developed.
The atmospheric chemistry and combustion communities will be able to use the elementary reaction rate constants and models in photochemical smog and combustion modeling. Applications relate to utility of oxygenated hydrocarbons as dedicated fuels, blend components, or additives; and aspects of their environmental impact regarding urban air pollutants such as ozone and peroxyacetyl nitrates (PAN's).
Chiang , H., Glarborg, P. and Bozzelli, J.W., "Effect of SOx and NO on CO Oxidation Under Post Flame Conditions," Intl. J. Chemical Kinetics, 28, 773-790, 1996.
Lay, Tsan, Krasnoperov, L. and Bozzelli, J.W., "Thermodynamic Properties of Gas Phase Alkyl-Chloro Hydroperoxide Compounds and Corresponding Alkyl and Peroxy Radicals, J. Phys. Chem., 100, 8240 - 8249, 1996.
Wei, R. and Bozzelli, J.W., "A Reaction Mechanism for the Early Oxidation of Neopentyl Radical," J. Phys. Chem., submitted November 1996.
Zhong, Xian and Bozzelli, J.W., "Reactions of H, O, OH, and HO2 radicals with Cyclopentadiene: Reactions relative to Combustion and Oxidation of Benzene," Intl. J. Chem. Kinetics, submitted, 1996.
Pitz , W., Koert , D. and Bozzelli, J.W., "Kinetic Modeling: High Pressure Propane Oxidation: Comparison with Experiment," presented at the 25th International Combustion Symposium, 1996.
Graduate Students: Ru Wei, Chiung Ju Chen and Takahiro Yamada Postdoctoral Associate Tsan Lay
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
Goal: The objective of this project is to gain a scientific understanding associated with the catalytic oxidation of soot with the concomitant reduction of NOx to molecular nitrogen. This information is needed to develop equipment for controlling the most problematic pollutants NOx 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 NOx. 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. Conse-quently, 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/NOx reduction is due to a stable intermediate. This intermediate, possibly CO, is produced in the non-catalytic oxidation of soot and reduces NOx 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 NOx. Our research is directed at obtaining the required mechanistic understanding to develop a cata-lyst to reduce Nox while oxidizing soot. Soot, captured under low power operation, will be oxidized in situ by NOx at higher temperatures to produce non hazardous CO2 and N2. Consequently, this system can be self cleaning, if we can stoichiometrically balance soot and NOx. 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-NOx 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 eval-uated. 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 unburned hydro-carbons. 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 oxi-dants. The carbon oxides then react with NOx 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 experi-ments 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.
Postdoctoral Research Associates: Istvan Bagyi and Zhong-Ming Zhao
Graduate Students: Maria Rosa Diacno, Kaiwan Ma (Applied Chemistry), Gui-Hua Qian (Chemical Engineering), William Roy (Environmental Science), Shu Sun (Applied Chemistry), Xiapong-Yong Tang (Applied Chemistry)
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
Goal: 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, 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 a 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 paths are included. 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: We have developed an initial model for benzene - oxidation plus reaction with nitric oxides (Lay and Bozzelli, 1996) and toluene current year work. We compare our model with recently published data from research groups of Koch (1,2), Pagsberg (3) and of Y. P. Lee (4) all of which is relevant to the gas phase chemistry of benzene and toluene - aromatics.
Koch et al., in meeting abstracts, report bi-exponential decay of [OH] in reactions of benzene + OH and benzene-OH + O2 under tropospheric conditions. Our model, without modification, predicts the OH profile of Koch in the absence of O2, in presence of O2 from 0.0 to 0.2 s. It slightly under predicts [OH] after 0.2 s when O2 is present. This verifies our thermodynamic parameters and model which indicate dissociation of benzene-OH to benzene + OH occurs on a sub second time scale. We look to further interpret the data when it is fully published.
The research group of Pagsberg report on OH-initiated oxidation of benzene, where phenol is identified as the primary product with relative yield of 25.5%. They conclude that the reaction: OH + benzene --> H + phe-nol accounts for the experimental results, at the rate constant k1=1.7E11 cm3 mol-1 s-1. This value is significantly higher than the literature data: 1.5E05. (Fritz et al., 1985) Our QRRK calculation results k1 = 1.2E08 m3 mol-1 s-1, in between the values above. Also reported is the rate constant for benzene-OH + O2 => products (2), k2 = 3.1E11 m3 mol-1 s-1, which is in good agreement with our QRRK calculation: k2 = 7.1E11 cm3 mol-1 s-1, and 3 orders of magnitude higher than the literature data: 1.1E08 m3 mol-1 s-1. Pagsberg's experimental rate constant for benzene-OH + O2 addition is in disagreement with the reaction barrier calculated by Andino et al.
Lin et al. in studies on OH plus benzene over 250-500 Torr and 345-385 K report rate constants for OH + benzene => benzene-OH of: kf = 1.4E12 cm3 mol-1 s-1 and kr = 2.9 s-1, compared to our data: kf = 7.3E11 cm3 mol-1 s-1and kr = 0.15 s-1. Their entropy value S(298) for benzene-OH is however, low at 75 cal mol-1 K-1, compared to our analysis: 82.7 cal mol-1 K-1. We further evaluate this S, using density functional theory, B3LYP/6-31G*, for S(298, benzene-OH) = 82.2 cal mol-1 K-1. We are confident in our value, and believe that Lin et al. misinterpreted the entropy of the benzene- OH adduct radical by more than 7 cal mol-1 K-1. This suggests the need to reinterpret their experimental data, and their rate constants.
1. Koch and Zetzsch, Proc. of the EUTROTRAC 96 Symposium, SPB Academic
Publishing, The Hague, The Netherlands, 1996.
2. Zetzsch, Koch, Bohn, Knispel, Zeiss, Witte, 1996, unpublished results.
3. Bjergbakke, Sillesen, Pagsberg, J. Phys. Chem., 100: 5729, 1996.
4. Lin, Kuo, and Lee, J. Chem. Phys., 101, 2098, 1994.
Bozzelli, J. W. and Lay, T., "Benzene Oxidation in the Troposphere: Reactions with OH and O2 and Detailed Reaction Mechanism," J. Phys. Chem., 100, 6423, 1996.
Bozzelli, J. W. and Lay, T., "Hydrogen Atom Bond Dissociation Groups for Calculation of Thermodynamic Properties of Oxy-Hydrocarbon Radical Species," J. Phys. Chem., 99, 14514 - 27, 1995.
Bozzelli, J. W. and Lay, T., "Thermodynamic Properties of Gas Phase Alkyl-Chloro Hydroperoxide Compounds and Corresponding Alkyl and Peroxy Radicals," J. Phys. Chem. 100, 8240 - 49, 1996.
Lay, T. and Bozzelli, J. W., "Ab Initio Calculations on 3 Strained Peroxy Ring Compounds," Chem. Phys. Letters, submitted, 1996.
Lay, T., Bozzelli, J. W. and Yamada, T., "Thermochemical Properties of 24 Cyclic Oxygenated Hydrocarbons," J. Phys. Chem., submitted, 1996.
Lay, T., Krasnoperov, L., Venanzi, C. and Bozzelli, J. W., "Ab initio Study of Alpha Chlorinated Ethyl Hydroperoxides, Confirmational Analysis, Frequencies, Internal Rotation Barriers, Thermodynamic Properties," J. Phys. Chem., 100, 8240 - 8249, 1996.
Zhong, Xian and Bozzelli, J. W., "Reactions of H, O, OH, and HO2 Radicals with Cyclopentadiene: Reactions Relative to Combustion and Oxidation of Benzene," Intl. J. Chemical Kinetics , submitted.
Graduate Student: Takahiro Yamada
Atmospheric Transformation of Volatile Organic Compounds: Gas-Phase Photooxidation and Gas-to-Particle Conversion: J. H. Seinfeld, R. C. Flagan, California Institute of Technology
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 both indoor and outdoor reactors. 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: During this past year we conducted a large number of experiments to assess the secondary aerosol-forming potential of aromatic compounds. Seventeen individual aromatics were studied. Then, in an effort to evaluate the hypothesis that the aerosol-forming potential of whole gasoline vapor can be determined based on the contributions from its aromatic content, we conducted a series of outdoor smog chamber experiments with 12 Auto/Oil Air Quality Improvement Research Program fuels. The results show conclusively that this is the case. Finally, on the theoretical front, we are continuing ab initio analysis of atmospheric reaction mechanisms. During the coming year we hope to increase our level of interaction with the group of Joe Bozzelli in this regard.
Mathematical Models of the Transport and Fate of Airborne Organics: Gregory J. McRae, Department of Chemical Engineering, Massachusetts Institute of Technology
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 mechanisms for airborne organics.
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).
Status: Set out below are several highlights of the project work in 1995 - 96:
This project demonstrates the utility of DEMM in conducting uncertainty analyses for complex models with many uncertain parameters. Such an approach can be extended to a wide range of chemical and environmental engineering problems. DEMM has been applied successfully to SAPRC, a complex photochemical mechanism for organic compounds. The next phase of the research is to carry out a detailed uncertainty analysis of isoprene reactions in the atmosphere, and to compare the uncertainties due to the approximations and assumptions of different photochemical mechanisms, such as RADM and CB-4.
Graduate Student: Betty K. Pun
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
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 determine how primary particles emitted from the many air pollution sources in the city are modified by gas-to-particle conversion processes in the atmosphere.
Rationale: 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 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. Through comparison of ambient particle composition to the composition of particles emitted from major emission source types (which is being measured in a separate study), it is hoped that new tools and insights will be developed that can clearly identify those particles in the atmosphere that evolve from the emissions from specific source types.
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: Aerosol observatories were established at Santa Catalina Island, Long Beach, Fullerton and Riverside during September and October of 1996 in the hope that at least some air parcels can be observed as they pass over these sites in succession. At each of the three on-land air monitoring stations, aerosol instruments were sited for detailed measurement of aerosol size and chemical composition. Continuous aerosol size distribution measurements were made in sizes below 2 micrometers particle diameter via combinations of electrical aerosol analyzers and laser optical particle counters. Single particle size and chemical composition also was monitored continuously via time-of-flight mass spectrometers located at each site. An existing time of flight mass spectrometer system located at Professor Kimberly Prather's laboratories at UC Riverside was used to anchor the Riverside end of the air parcel trajectories. Two portable mass spectrometers were placed at Cal State Fullerton and at Cal State Long Beach. A pair of MOUDI cascade impactors was placed at each of these three sites, one running foil impaction substrates for organics and elemental carbon determination and one running teflon substrates for multi-elemental analysis by neutron activation analysis and ion chromatography. A pair of MOUDI impactors also was run at Santa Catalina Island on the day prior to the start of the experiment to measure the marine back-ground aerosol size and composition distri-bution. Filter-based samplers were used to collect coarse particle (dp>2 micrometers) and fine particle (dp<2 micrometers) samples for full chemical analysis. The MOUDI im-pactors and filter samplers were operated over 4-hr sampling periods in order to collect sufficient material to support detailed chemi-cal analysis by bulk chemical analysis meth-ods. The impactors and filter samplers were employed from 6-10 a.m. at Long Beach, 10 a.m.-2 p.m. at Fullerton and 2-6 p.m. at Riverside in order to capture samples taken as the sea breeze front passes across the air basin. Ozone and NO2 concentrations were measured as well. Nitric acid was measured by the denuder difference method employing nylon filters. Ammonia was measured using a stacked filter unit involving collection of gaseous NH3 on oxalic acid impregnated back-up filters. Vapor-phase organics were measured by capture in internally electro-polished stainless steel canisters followed by gas chromatography with flame ionization de-tection. NO, NO2, SO2 and ozone data also are being acquired from governmental air monitoring stations in the air basin.
Five days of observation were undertaken. Field operations went remarkably well con-sidering the complexity of the experiment and the limited time following receipt of project funds. At present, the laboratory analysis of filters and impactor samples for mass and ionic species has been completed at Caltech, and analyses for trace elements and organic carbon plus elemental carbon are underway at co-operating laboratories. Data reduction is just beginning, with the first task to use the Caltech particle counter data and impactor data to determine the particle counting effi-ciency and chemical species response factors for the time-of-flight mass spectrometers.
Graduate Student: Lara Hughes
Christoforou, C.S., Salmon, L.G., Hannigan, M.P., Solomon, P.A. and Cass, G.R., "Trends in Fine Particle Concentration and Chemical Composition in Southern California," submitted to J. Air & Waste Management Assoc., 1996.
Hannigan, M.P., Cass, G.R., Lafleur, A.L., Busby, W.F. Jr., and Thilly, W.G., "Seasonal and Spatial Variation of the Bacterial Mutagenicity of Fine Organic Aerosol in Southern California," Environ. Health Perspectives, 104, 428-436, 1996.
Markers for Emissions from Combustion Sources: Adel F. Sarofim, J. B. VanderSande, Massachusetts Institute of Technology
Goal: Soot particles generated and emitted by combustion processes have a microstructure and trace element composition which can be observed with high resolution electron micro-scopes. Since the microstructure and chemi-cal 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 deter-mine 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 visi-bility 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 for-mulated 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 com-position, (ii) determination of the relation of soot microstructure and composition to com-bustion conditions and fuel type through the use of well defined experiments, and (iii) de-velopment of a library of soot structures for use in emissions source attribution studies.
Status: In the past report we discussed the use of high resolution microscopy and image analysis software (SEMPER6P©) to quantify the structural features of several types of soots and graphites. Those structural features that could be used to identify oxidation history and extent of thermal treatment include interlayer spacing of carbon fringes, and the coverage, circularity, length, and angle of these fringes. The order of the structures increased with oxidation while interlayer spacing decreased and lattice length increased. The ability to pinpoint small differences in structure among various carbonaceous samples was also shown. This method provided reliable and statistically significant information on the internal structure of soots. 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. The comparison of the analyses of diesel mining soot and soot from a jet engine have shown significant chemical differences. Results of their elemental analysis (EDX), viewed as spectra, showed the jet soot to have a greater number of elemental contaminants than the diesel soots. 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.
Graduate Student: Arpád Bence Palotás
Palotás, Á.B., Rainey, L.C., Feldermann, C.J., Sarofim, A.F., and VanderSande, J.B., "Soot Morphology: An Application of Image Analysis in High-Resolution Transmission Electron Microscopy," Microscopy Res. and Technique, 33, 266-278, 1996
Palotás, Árpád Bence, "Quantitative Measures of Carbon Microstructure," for Master of Science in Chemical Engineering, Massachusetts Institute of Technology, September 1995.
Palotás, Á.B., Rainey, L.C., Sarofim, A.F., VanderSande, J.B., and Ciambelli, Paolo, "Effect of Oxidation on the Microstructure of Carbon Blacks," Energy & Fuels 10, 254-259, 1996.
Rainey, L. C., Palotás, Á. B., Bolsaitis, P., VanderSande, J. B., and Sarofim, A. F., "Application of High-Resolution Electron Microscopy for the Characterization and Source Assignment of Diesel Particulates," Appl. Occupational and Environ. Hygiene, 11(7), 777-781, 1996.
Microengineered Mass Spectrometer for in-situ Measurement of Airborne Contaminants: W. N. Carr and K. R. Farmer, New Jersey Institute of Technology
Goal: The goal of this project is to develop a prototype microengineered mass spectrometer for in-situ measurement of airborne contam-inants. 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 com-pounds 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 de-veloping a low-cost, portable mass spec-trometer we expect ultimately to enable on-site testing at unprecedented levels. An important capability of the sensor is opti-mization 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 1 cm by 1 cm. 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: The first full year of this two-year project has been completed. 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 chal-lenging fabrication task has been the production of the novel field emitter struc-tures, 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 charac-teristics, an important milestone in creating the ion source. During the second year we expect first to create ions using the device and ultimately to construct and test the full prototype structure, demonstrating mass selectivity.
Postdoctoral Associate: Qi Xing Sun Graduate Student: Chao Sun (Electrical Eng)
Sun, C., "Microengineered Mass Spec-trometer with Novel Electron Source," Poster, 43rd National Symp. of the Amer. Vacuum Society, Philadelphia, PA, October 16, 1996.