Modeling of secondary organic aerosol in Houston in summer
| Project Period: | 04/01/2008 - 08/31/2008 |
| Total Budget: | $150,000 |
| Sub-Contractors: | UNH/UH
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Three-dimensional air quality models have traditionally underestimated organic aerosol (OA) levels. Therefore, it is difficult to use such models for regulatory purposes with respect to OA. It is generally believed that this overall underprediction results from an underprediction of secondary OA (SOA) formation. This project will improve closure between simulated and observed levels of OA in the chosen model domain of the Houston area.
A mechanistic approach for simulation of SOA will be adopted using existing computer codes (the Caltech Atmospheric Chemistry Mechanism (CACM) and the Model to Predict the Multi-phase Partitioning of Organics (MPMPO)). CACM predicts numerically the formation of multi-functional products that are formed from the atmospheric oxidation of a suite of volatile organic compounds (VOCs), both biogenic and anthropogenic in origin. Based on estimated properties of these products, MPMPO distributes these compounds between the gas and condensed phases thermodynamically. CACM and MPMPO will be updated to include relevant new experimental data and processes that are thought to be a significant fraction of the missing SOA in simulations. First, oxidation of isoprene leading to SOA via traditional gas-particle partitioning will be included in the model. Second, heterogeneous/particle-phase reactions will be included in two ways: inclusion of aerosol uptake of small dicarbonyls (glyoxal, methyl glyoxal, etc.) formed in oxidation of VOCs (including isoprene and aromatic compounds) and particle aging (conversion of semi-volatile aerosol material into non-volatile material within the condensed phase). While other processes likely contribute to SOA formation and are not included in the model (sesquiterpene oxidation, in-cloud processing, changes in nitrogen oxide concentrations, etc.), it is hypothesized that the two pathways discussed constitute the majority of the missing SOA. The updated modules will be applied to simulate the Houston area using the Community Multi-Scale Air Quality (CMAQ) model. The data availability from the Texas Air Quality Study II (TexAQSII) provides a tremendous opportunity for comparison of model output to very recent state-of-the-art observations.
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