Browsing by Author "Xu, Jialu"

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    Development of aroCACM/MPMPO 1.0: a model to simulate secondary organic aerosol from aromatic precursors in regional models
    (Copernicus Publications, 2016) Dawson, Matthew L.; Xu, Jialu; Griffin, Robert J.; Dabdub, Donald
    The atmospheric oxidation of aromatic compounds is an important source of secondary organic aerosol (SOA) in urban areas. The oxidation of aromatics depends strongly on the levels of nitrogen oxides (NOx). However, details of the mechanisms by which oxidation occurs have only recently been elucidated. Xu et al. (2015) developed an updated version of the gas-phase Caltech Atmospheric Chemistry Mechanism (CACM) designed to simulate toluene and m-xylene oxidation in chamber experiments over a range of NOx conditions. The output from such a mechanism can be used in thermodynamic predictions of gas–particle partitioning leading to SOA. The current work reports the development of a model for SOA formation that combines the gas-phase mechanism of Xu et al. (2015) with an updated lumped SOA-partitioning scheme (Model to Predict the Multi-phase Partitioning of Organics, MPMPO) that allows partitioning to multiple aerosol phases and that is designed for use in larger-scale three-dimensional models. The resulting model is termed aroCACM/MPMPO 1.0. The model is integrated into the University of California, Irvine – California Institute of Technology (UCI-CIT) Airshed Model, which simulates the South Coast Air Basin (SoCAB) of California. Simulations using 2012 emissions indicate that “low-NOx” pathways to SOA formation from aromatic oxidation play an important role, even in regions that typically exhibit high-NOx concentrations.
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    Simulated impact of NOx on SOA formation from oxidation of toluene and m-xylene
    (2014-04-22) Xu, Jialu; Griffin, Robert J.; Cohan, Daniel S.; Embree, Mark
    Despite the crucial role of aromatic-derived secondary organic aerosol (SOA) in deteriorating air quality, its formation mechanism is not well understood, and the dependence of aromatic SOA formation on nitrogen oxides (NOx) is not captured fully by most SOA formation models. In this study, NOx-dependent mechanisms of toluene and m-xylene SOA formation are developed using the gas-phase Caltech Atmospheric Chemistry Mechanism (CACM) coupled to a gas/aerosol partitioning model. The updated models were optimized by comparison to eighteen chamber experiments performed under both high- and low-NOx conditions at the University of California – Riverside. Correction factors for vapor pressures imply uncharacterized association chemistry, likely in the aerosol phase. The newly developed model can predict strong decreases of m-xylene SOA yield with increasing NOx. Simulated SOA speciation implies the importance of ring-opening products in governing SOA formation (up to ~40-60% for both aromatics). Speciation distributions under varied NOx levels imply that competition between hydroperoxide radical and NO for reaction with a bicyclic peroxide radical may not be the only factor influencing SOA formation. The reaction of aromatic peroxy radicals with NO competing with self-cyclization also affects NOx-dependence of SOA formation. Comparison of SOA formation yield and composition between toluene and m-xylene suggests aldehyde/ketone chemistry from a ring-opening route and a phenolic route play important roles in governing SOA formation, with their relative importance likely varying due to the number of methyl groups on the aromatic ring. Sensitivity studies of the NOx/Aromatic/SOA system with a range of NOx and hydrocarbon concentrations are carried out for toluene, m-xylene, and a mix of toluene and m-xylene. The impact of a non-aromatic hydrocarbon not expected to form SOA (propene) also is investigated. The effect of temperature variations on model predictions of the NOx effect on SOA formation is also explored. The updated box model will be implemented into a three-dimensional model to evaluate NOx effect on aromatic SOA formation on a larger scale.