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2000 Progress Report: Partitioning of Semivolatile Organic Compounds in Organic and Inorganic Aerosols: A Unified Approach

EPA Grant Number: R826771
Title: Partitioning of Semivolatile Organic Compounds in Organic and Inorganic Aerosols: A Unified Approach
Investigators: Kamens, Richard M. , Chandramouli, Bharadwaj , Jang, Myoseon , Jaoui, Mohammed , Lee, Sangdon
Current Investigators: Kamens, Richard M. , Chandramouli, Bharadwaj , Jang, Myoseon
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1998 through September 30, 2001
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $562,536
RFA: Air Pollution Chemistry and Physics (1998)
Research Category: Engineering and Environmental Chemistry

Description:

Objective:

The objective of this work is to provide a unified model to predict the equilibrium gas-particle partitioning (G/P) of semi-volatile organic compounds (SVOCs) on both organic and inorganic aerosols. To do this it is necessary to: (1) implement new models for both SVOC gas-liquid (absorptive) and gas-solid (adsorptive) behavior; (2) provide an experimental data base to test these models; and (3) demonstrate that these models can be used and applied to a variety of different ambient situations. This work will provide a theoretical framework and user-friendly procedure for estimating the gas-particle (G/P) phase distribution of almost any semi volatile organic, whether it is a new environmental chemical for which only chemical structural information exists, a carcinogenic compound, or an environmental estrogen or an environmental endocrine disrupter.

Progress Summary:

  • We performed partitioning experiments with aerosols from meat cooking and from gasoline engines to expand our knowledge of partitioning on different kinds of aerosols. Aerosols from meat cooking can comprise a substantial portion (> 20) of the urban organic aerosol mixture and can be important in determining the partitioning characteristics of a SVOC. Regular hamburger meat containing 21 percent fat was grilled on a charcoal grill and the resulting particles were injected directly into our 190-m3 Teflon chamber. A "cocktail" of SVOCs including PAHs, substituted PAHs, alkanes, phenols and acids was then introduced into the chamber and gas and particle samples were taken to measure the partitioning coefficient KP. A model composition drawn from meat aerosol composition data was used to calculate the activity coefficients (g) and KP of the SVOCs. A comparison of observed and predicted values is shown in Figure 1. We can see that the model does a good job of predicting KPs. The outliers are phenols and carboxaldehydes and the results are probably due to sampling artifacts, which lead to unreliable data. More work is needed to account for water uptake by the aerosol and to find reliable markers for use in aerosol mixture experiments.

    Similar experiments were conducted using emissions from un-catalyzed and catalyzed gasoline engines. These emissions were injected into the two 25-m3 chambers and SVOC partitioning was studied using the same set of chemicals as the meat experiment. The composition changes between the un-catalyzed and catalyzed particle emissions and this resulted in order of magnitude changes in the activity coefficients and KPs of certain SVOCs like alkanes. More work is needed to account for water uptake and characterize this aerosol.

  • We further expanded the a-Pinene aerosol kinetic prediction model and can now successfully predict daytime particle formation in the presence of NOx under various sunlight conditions and concentration regimes. A kinetic mechanism was used to describe the gas and aerosol phase reactions of a-pinene in the presence of sunlight, ozone (O3) and oxides of nitrogen (NOx). Reaction products and aerosol formation from the kinetic model were compared to outdoor smog chamber experiments conducted under natural sunlight in the presence of NOx, and in the dark in the presence of O3. A Scanning Mobility Particle Sizer system (3936, TSA) and a Condensation Particle Counter (3025A, TSA) were used to track secondary organic aerosol formation, and a denuder/filter pack sampling system was used for collecting simultaneous gas and particle phase for analysis and total filter mass (TSP) information. The gas-particle partitioning of semivolatile organics generated in the gas phase was treated as an equilibrium process between particle absorption and desorption. Figure 2 shows model fits of observed particle mass, and fits of photochemical oxidant concentrations for one of these runs.

    During the course of this work, more than 16 products were identified and quantified using an array of GC-MS and LC-MS methods coupled with derivatization methods. Measurements show that 10-hydroxypinonic acid, 10-hydroxypinonaldehyde, 2-oxopinonic acid, and 10-oxopinonic acid are observed in the early stage in the aerosol phase and play an important role in the processes of nucleation. On average, measured gas and particle phase products accounted for ~40 to 60 percent of the reacted a-pinene carbon. Model predictions suggest that organic nitrates accounts for another 25-35 percent of the reacted carbon, and most of this is in the gas phase. Organic nitrate and PAN products were not measured in this study and will constitute an important part of future work. Measured particle phase products accounted for 60 to 100 percent of the particle filter mass. Measurements show that pinic acid is one of the primary aerosol phase products at the end of the reaction.

  • We investigated the formation and partitioning of oxidation products of a toluene-propylene-NOx-sunlight system. The formation of secondary aerosol from substituted benzenes in the atmosphere is well documented, but existing particle phase data is very sketchy. Products in both the gas and aerosol phases were collected with a filter-filter denuder sampling system, separated by gas chromatography and detected with an electron-impact and chemical-impaction trap mass spectrometry. In addition, secondary aerosols were collected directly on an ungreased zinc selenide (ZnSe) FTIR disk (25 mm in diameter) by impaction. These samples were extracted and also analyzed directly using Fourier Transform Infrared (FTIR) spectrometry to obtain additional functional group information for products in the aerosol phase. Compound analyzed from the FTIR disks represent aerosol-associated products and are not prone to the possible sampling artifacts often ascribed to filter sampling systems.

    The photooxidation reaction of toluene in the gas phase leads to substituted aromatics, multifunctional non-aromatic ring retaining and ring opening products. In this work, many ring cleaved multifunctional oxy-products retaining a carboxylic acid, carbonyl, and a hydroxyl group were newly identified, along with ring retaining hydroxyl products. These highly oxidized products produced from gas phase photooxidation reactions are semivolatile, can migrate from the gas to the particle phase, and contribute to some of the major components in the particle phase. The carboxylic acids, carbonyls and hydroxyl groups of reaction products may then be involved in other process in the particle phase.

    The major aromatic products found in the particle phase under the high NOx conditions of the above experiment included 2-methyl-4-nitrophenols, 4-methyl-2-nitrophenol, and methyldinitrophenol isomers. The principal non-aromatic ring retaining compounds were, 2-methyl-1,4-benzoquinone and methylcyclohexene tricarbonyls. The major ring opening products in the particle phase were, 2-buten-4-al-oic acid, methyl-4-oxo-2-pentenoic acid, 2-hydroxy-3-penten-1,5-dial, 2-hydroxy-5-oxo-3-hexen-5-al, 5-hydroxy-4,6-dioxo-2-heptenal and their structural isomers.

    The partitioning behavior of major secondary aerosol products was also explored by comparing experimentally observed Kp values of products to predicted ones. Compounds which had aldehydic and OH functional groups exhibited much higher experimental partitioning coefficients (more in the particle phase) than would be predicted from vapor pressure and activity coefficient relationships. This study suggests that chemical reactions in the particle phase may dramatically influence observed partitioning behavior. This was particularly true for compound ring cleavage products with alcoholic functional groups. Possible particle reactions include hemiacetal or acetal formation as a "zipping" reaction resulting in a drastic lowering of vapor pressure (3-5 orders of magnitude). These lower vapor pressures permit hemiacetals to exist to a greater extent in the particle phase than their parent compounds. Lastly, and most importantly, hemiacetals or acetals can be possibly "unzipped" to their original aldehydes and alcohols via backward reactions involving hydrolysis or heat during sample workup and analysis. Hence, these types of compounds can go undetected using conventional methods and have only been recently observed on secondary aerosols in Paul Ziemann's group by direct "cold" particle collection and subsequent direct thermal desorption into the ion source of a mass spectrometer.

    • Figure 1

    Figure 1: Comparison of observed vs. predicted KP for meat aerosol.

    Figure 2

    Figure 2: Prediction of aerosol formation from the gas phase reaction of 0.95 ppmV a-pinene in the presence of NOx and sunlight. Thin lines are model predictions.

    Future Activities:

    The following activities will be performed in the upcoming year:

    1. We will work to predict partitioning of SOCs on mixtures of organic aerosols. Two or more aerosols will be injected into the chamber and their concentrations tracked using suitably selected tracers and using their unique distribution.

    2. We will develop and implement user-friendly programs to estimate KPs on different particle types. These programs will include a database of compound properties like boiling point, vapor pressure, and group contributions for the most important SVOCs to facilitate ease of use and also the ability to take in user inputs for new compounds/aerosol compositions and estimate vapor pressures and KPs.

    3. We will refine our biogenic secondary aerosol prediction model and test it on different biogenics like b-Pinene and Limonene. We will also work at predicting formation from a mixture of these compounds. We will also refine our analysis techniques to work at higher sensitivity and lower concentrations.

    4. We will expand our FTIR techniques for determining the composition of secondary aerosol from aromatic precursors.


    Journal Articles on this Report: 3 Displayed | Download in RIS Format

    Other project views: All 8 publications 8 publications in selected types All 8 journal articles

    Type Citation Project Document Sources
    Journal Article Jaoui M, Kamens RM. Mass balance of gaseous and particulate products analysis from a-pinene + NOx in the presence of natural sunlight. Journal of Geophysical Research - Atmospheres 2001;106(D12):12,541-12,558. R826771 (2000)
    R826771 (Final)
    		 not available
    Journal Article Jaoui M, Kamens RM. Characterization of secondary aerosols from the photooxidation of toluene in the presence of NOx and propylene. Journal of Geophysical Research. R826771 (2000)
    		 not available
    Journal Article Kamens RM, Jaoui M. Modeling aerosol formation from alpha-pinene plus NOx in the presence of natural sunlight using gas-phase kinetics and gas-particle partitioning theory. Environmental Science & Technology 2001;35(7):1394-1405. R826771 (2000)
    R826771 (Final)
    R828176 (2002)
    R828176 (Final)
    		 not available
    Supplemental Keywords:

    gas-particle partitioning, secondary aerosol formation, vapor pressure, solvation, energy relationships, compound polarizability. , Toxics, Air, Scientific Discipline, RFA, Engineering, Chemistry, & Physics, Ecological Risk Assessment, air toxics, particulate matter, Environmental Chemistry, National Recommended Water Quality, VOCs, aerosol partitioning, aerosols, chemical mixtures, vapor phase, exposure and effects, semivolatile organic compounds, aerosol particles, combustion, monitoring, particulates, PM 2.5, air modeling, PAH, linear solution energy realtionships, gas/particle partitioning

    Progress and Final Reports:
    1999 Progress Report
    Original Abstract
    Final Report

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    The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.

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