Computational Simulation of Secondary Organic Aerosol (SOA) Formation from Toluene Oxidation
Griffin, Robert J.
Master of Science thesis
Toluene is one of the most prevalent aromatic volatile organic compounds (VOCs) in the atmosphere and has large secondary organic aerosol (SOA) yields compared to many other aromatic VOCs. Recent photo-oxidation studies highlight that toluene oxidation produces more SOA than observed previously, particularly at low levels of nitrogen oxides (NOx). This study focuses on: 1.) the development of a gas-phase chemical mechanism describing toluene oxidation by hydroxyl radicals (OH); 2.) the prediction of SOA formation from toluene oxidation products; and 3.) the impact of NOx level on SOA formation. The oxidation mechanism, which includes multiple pathways after the initial OH attack, has been incorporated into the Caltech Atmospheric Chemistry Mechanism (CACM). Toluene concentrations simulated in chamber experiments by the updated CACM as a function of time are typically within 5% of observed values for most experiments. Predicted ozone and NO2 concentrations are typically within 15% of the experimental values. The gas-phase mechanism indicates the importance of bicyclic peroxy radical reactions in determining the product distribution and thus the likelihood of SOA formation. A gas-aerosol partitioning model is used in conjunction with the gas-phase mechanism to simulate SOA formation. Predicted SOA concentrations are typically within 15% of the experimental values. Under low NOx conditions, simulation shows that more than 98% of SOA mass is contributed by bicyclic products from reactions between bicyclic peroxy radicals and other peroxy radicals. Increasing NOx levels cause bicyclic peroxy radicals to react with NO or nitrate radical, leading to fragmentation products that are less likely to form SOA. SOA yield dropped from 19.26% with zero initial NOx to 13.27% with 100 ppb initial NO because of the change in the amount of toluene consumed. Composition of NOx also has an impact on SOA yield and formation, showing that NO has a greater impact on SOA yield and formation than NO2.
Secondary Organic Aerosol; Formation; Simulation; Toluene; Oxidation