The Framework of Modeling SOA Formation from Toluene Oxidation Di Hu and Richard Kamens Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill This work was funded by USEPA-STAR program. We thank Harvey Jeffries for the Allomorphic Mechanism Software. Aromatic hydrocarbons contribute significantly to concentrations of volatile organic compounds (VOC) in the urban atmosphere. It is well established that the photo-oxidation of aromatic hydrocarbons in the presence of nitrogen oxides (NOx) play an important role in ozone and secondary organic aerosol (SOA) formation. 1 There are several existing mechanisms that contain aromatic reactions. However, none of them incorporates the newly discovered particle phase heterogeneous reactions that lead to large amounts of SOA formation. 2,3 A “new generation” aromatic chemical mechanism is needed that can capture the essence of the fundamental chemistry that leads to SOA formation and simultaneously is consistent with the known gas phase chemistry. Toluene, which is the most abundant of the aromatics, was chosen to be the study compound for kinetic model development. To develop the kinetic mechanism, it is first necessary to characterize the gas phase reactions that generate semi-volatile oxygenated products. Results from our chamber study and available kinetic and product data from the literature have been incorporated into the mechanism. Gas and particle phase reactions are linked together by equilibrium organic gas-particle partitioning. Gas phase highly oxygenate compounds will first partition onto existing particles, and then undergo a series of reaction in the particle phase. (Fig. 2) As particle phase reactions continue, more gas phase products will partition to the particle phase, and the particles will increase in size. Some of the major products in gas and particle phases are shown in Fig. 3. The overall goal of this study is to integrate gas and particle phase reactions as a unified, multi-phase, chemical reaction mechanism, which will ultimately permit the prediction of SOA formation in the toluene system. This approach involves: 1. kinetic mechanism development that incorporate detailed gas and particle phase reactions, 2. outdoor smog chamber toluene/NOx experiments to monitor ozone, products, and SOA formation and, 3. an investigation of the degradation of major oxygenated products. These results are to be used to develop and test a new generation toluene mechanism. (Fig 1) The underestimation of NO 2 after its peak suggests that more organic nitrate compounds need to be included in the mechanism. The under-prediction of toluene reveals that there may be an OH radical source missing from the mechanism. There are many uncertainties in particle phase chemistry, such as particle phase reaction rates and amount of polymer formed on the particle phase. A series of chamber experiment are under way to investigate the degradation of the key oxygenated products, such as their particle formation potential, their quantum yields and photo-degradation products. 1 Calvert, J. G. et al., The Mechanism of Atmospheric Oxidation of Aromatic Hydrocarbons. Oxford Jang, M. et al., Heterogeneous Atmospheric Aerosol Production by Acid-Catalyzed Particle- Phase Reactions. Science 2002, 289, Kalberer, et al., Identification of Polymers as Major Components of Atmospheric Organic Aerosol, Science 2004, 303, 1659 Fig. 3. Major products in gas and particle phases Fig. 3. Comparison of observed (solid line) and simulated (dashed line) time dependence of NO, NO 2 and O 3. Please contact or Fig. 4. Comparison of observed (solid line) and simulated (dashed line) time dependence of toluene, propylene and total particle mass. Fig. 2. Illustration of gas and particle phase reactions in Toluene system. Experimental and simulated concentration-time profiles for a toluene/propylene/NOx experiment on 08/20/2004 are shown in the Fig.4 and 5. The initial concentrations of toluene, propylene, NO and (NH4) 2 SO 4 seed were 7 ppmC, 3.8 ppmC, 0.7 ppm and 36  g/m 3 respectively. The mechanism provides an acceptable description of the time dependence of NO, NO 2 and O 3. The model predicts a reasonable toluene decay for the first 2 hours, but it was about 30% too low at the end of experiment. For total particle mass, the model simulation is substantially higher than measured after particle mass reaches its maximum. Fig. 1. Scheme of overall approach IntroductionMethods Overall Approach Results Conclusions Literature Cited Further InformationAcknowledgments Kinetic Mechanism Development Smog Chamber Experiment O3 SOA Product Analysis Oxygenated Products Degradation Simulation of Experiment Results