Browsing by Author "Li, Jingyi"

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    Implementation and Application of SAPRC07 and MCM Mechanisms in the Multi-scale Community Air Quality Model
    (2012-02-14) Li, Jingyi
    A photochemical mechanism is a very important component of an air quality model, which simulates the change of pollutant concentrations due to chemical reactions in the air. The accuracy of model prediction is directly impacted by the photochemical mechanism. In this study, two state-of-the-science photochemical mechanisms, SAPRC07 and Master Chemical Mechanism (MCM) v3.1, were implemented in the Community Multi-scale Air Quality Model (CMAQ) version 4.6 developed by the US EPA to study a high ozone (O3) episode during the 2000 Texas Air Quality Study (TexAQS) from August 16, 2000 to September 7, 2000. Predicted O3 concentrations by S07C are lower than those of S99 with a maximum difference as high as 20 percent. The two mechanisms also show significant differences in the predicted OH, PAN, HCHO and HNO3 concentrations. Although the two mechanisms predict different ozone concentrations, the relative response factors (RRFs) of O3 at rural, urban and industrial sites under emission controls of anthropogenic NOx and VOC by factors 0.6 ? 1.4 predicted by the two mechanisms are very similar. Predicted O3 concentrations by MCM are similar to those of SAPRC07. The MCM predicted total VOC OH reactivity is similar to the SAPRC07 predictions at a suburban site where biogenic emissions dominate the OH reactivity and is slightly lower than the SAPRC07 predictions at an industrial site where anthropogenic emissions dominate. Besides, the predicted 1-hr and 24-hr average concentrations of major O3 precursor VOCs by MCM show under predictions of alkanes and alkenes by a factor of 2-5, 6 for ethane and 8.5 for propane. Major aromatic compounds generally agree better with observations, although benzene is under-predicted by 80 percent. Species specific emission adjustment factors can be derived from these direct comparisons to improve emission inventories in future studies. At the Clinton Drive site, most of the under-predictions occur in the afternoon when industrial facilities are in the immediate upwind direction and the missing industrial emissions are likely evaporative sources whose emission rates are temperature dependent.
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    Modeling Regional Air Quality Using the Near-Explicit Master Chemical Mechanism
    (2014-08-01) Li, Jingyi
    In this study, the Master Chemical Mechanism (MCM), a near-explicit photochemical mechanism, is, for the first time, implemented into the Community Air Quality Model (CMAQ) model (referred to as CMAQ-MCM hereafter), a 3D chemical transport model, to study the spatial and temporal distribution of gaseous and particulate pollutants, and their chemical/physical properties in the eastern U.S. First, CMAQ-MCM with only gas phase chemistry (MCMv3.1) was applied to study primary VOCs in Southeast Texas during a three-week high ozone episode in 2000. The model could reproduce temporal profiles of observed ozone concentrations at major observation stations. However, predicted concentrations of alkanes and alkenes in Southeast Texas are lower than the observation by a factor of 2-5. Missing fugitive and evaporative industrial emissions were determined to be the major cause of the under-prediction. Second, the CMAQ-MCM was updated (to use MCMv3.2) and further developed to link with the inorganic aerosol module in CMAQ to study impacts of stabilized criegee intermediates (SCIs) on sulfate formation in the eastern U.S. Faster reaction rate of SCI with SO2, updated based on a recent experimental measurement (increased from 7?10^(-14) cm^(3) s^(-1) to 3.9?10^(-11) cm^(3) s^(-1)), leads to ~18% increase in surface sulfate concentration. However, the importance of this reaction greatly depends on the competition reaction rate of SCI with water vapor. Moreover, an organic aerosol module that predicts secondary organic aerosol (SOA) formation from equilibrium partitioning of semi-volatile organic compounds (SVOCs) into the organic phase and from reactive uptake of glyoxal, methylglyoxal, and isoprene epoxydiols, was incorporated into the model (termed CMAQ-MCM-SOA) to study SOA formation in this region. The episode averaged total SOA concentration is 2-12 ?g m^(-3), with the highest concentration occurring in the southeastern U.S. Isoprene epoxydiols are the major SOA component if an acidity dependent reactive uptake coefficient is used. Finally, the isoprene mechanism in the MCM was updated to successfully reproduce experimental observed isoprene SOA yield and mass concentrations, and then applied to the regional CMAQ-MCM-SOA model. While total isoprene SOA concentrations were not changed significantly from the base case, more contributions from semi-volatile components and less from surface uptake were predicted.