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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
Introduction
Methods
Results
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.
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)2SO4 seed were 7 ppmC, 3.8 ppmC, 0.7 ppm and 36 mg/m3 respectively. The mechanism
provides an acceptable description of the time dependence of NO, NO2 and O3. 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. 3. Comparison of observed (solid line) and
simulated (dashed line) time dependence of NO,
NO2 and O3.
Overall Approach
Conclusions
Kinetic Mechanism
Development
Simulation of
Experiment
Results
Smog Chamber
Experiment
O3
SOA
Product
Analysis
Oxygenated
Products
Degradation
Fig. 2. Illustration of gas and particle phase reactions
in Toluene system.
Fig. 1. Scheme of overall approach
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)
Fig. 3. Major products in gas and particle phases
The underestimation of NO2 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.
Fig. 4. Comparison of observed (solid line) and
simulated (dashed line) time dependence of
toluene, propylene and total particle mass.
Literature Cited
1Calvert,
J. G. et al., The Mechanism of
Atmospheric Oxidation of Aromatic
Hydrocarbons. Oxford 2002.
2Jang, M. et al., Heterogeneous Atmospheric
Aerosol Production by Acid-Catalyzed ParticlePhase Reactions. Science 2002, 289, 814.
3Kalberer, et al., Identification of Polymers as
Major Components of Atmospheric Organic
Aerosol, Science 2004, 303, 1659
Further Information
Acknowledgments
Please contact [email protected] or
[email protected] .
This work was funded by USEPA-STAR program.
We thank Harvey Jeffries for the Allomorphic
Mechanism Software.