Browsing by Author "Dawson, Matthew L."

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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 sensitivity of secondary organic aerosol in the South Coast Air Basin of California to nitrogen oxides and other chemical parameters
    (Taylor & Francis, 2018) Griffin, Robert J.; Dawson, Matthew L.; Dabdub, Donald
    Sensitivity of secondary organic aerosol (SOA) concentrations in the South Coast Air Basin (SoCAB) of California to nitrogen oxide (NOx) emission is simulated using gas-phase chemistry and gas-particle partitioning modules. These modules are implemented into a three-dimensional air quality model applied for high-pollution summer meteorology and 2008 emissions. To test sensitivity, NOx emissions in all locations and at all times are scaled by factors ranging from 0.1 to 10.0 in separate model runs. The basin-wide average SOA concentration exhibits a ‘turnover’ NOx emission multiplicative factor, above and below which the average SOA concentration decreases. For the entire SoCAB, this critical NOx emission factor is ∼0.3; while the magnitude of SOA concentrations changes with time, this peak value (∼0.2–0.3) appears to be relatively independent of the hour of the simulated day. When considering individual locations within the SoCAB, this peak factor shows a slightly broader range. Projected emissions for 2023 indicate a decrease in basin-average SOA concentration; the response at individual locations, however, can be either positive or negative, indicating the need for location-specific considerations. Ensembles of module simulations based on parameter values selected using efficient sampling techniques (Latin Hypercube method) are used to identify parameters to which SOA predictions are significantly sensitive. Total SOA predictions are most sensitive (in no particular order) to concentrations of O3, unsaturated species formed from the gas-phase oxidation of monoaromatic compounds, and substituted products from long-chain alkane oxidation. Secondary inorganic aerosol species, likely through influencing aerosol liquid water, control at least partially the formation of SOA upwind. In addition, the rate at which unsaturated bicyclic oxidation products of monoaromatic compounds are oxidized by hydroxyl radical impacts significantly SOA prediction. These findings emphasize the need for consideration of long-chain alkanes and monoaromatic species when designing emission control strategies.