Source contributions to organic aerosol in the eastern United States

Organic aerosols (OA) and elemental carbon (EC) are important components of atmospheric particulate matter (PM), potentially posing health hazards and contributing to global climate change. Secondary organic aerosol (SOA) is formed when condensable products from the oxidation of volatile organic compounds (VOCs) in the gas phase partition into the aerosol phase. Implementation of effective control strategies for organic PM2.5 (organic particles with diameters less than 2.5 mum) requires the quantification of the contribution of each source to the ambient OA and EC concentrations.;Significant discrepancies exist between the source-resolved model and the CMB model predictions for several of the sources. There is strong evidence that the organic PM emissions from natural gas combustion are overestimated. Other similarities and discrepancies between the source-resolved model and the CMB model for primary OA and EC are discussed along with problems in the current emission inventory for certain sources.;Next, the importance of isoprene as a source of SOA is determined using PMCAMx to predict the isoprene SOA concentration across the eastern US. Isoprene, the most abundant non-methane hydrocarbon emitted into the troposphere, has generally not been considered a major source of SOA due to the relatively high volatility of its oxidation products. The variability of the measured SOA yields in the available smog chamber studies is captured by combining the base case scenario with upper and lower bound estimates of the measurements. For the base case simulation, the predicted annual average isoprene SOA concentration in the southeast is 0.09 mug m-3 (bounds 0.04-0.23 mug m-3). PMCAMx predictions are compared to available measurements of some isoprene SOA components in North Carolina and New York State. These modeling results suggest that on an annual basis isoprene oxidation is a small but non-negligible organic aerosol source in the eastern U.S. Its contribution is relatively more important during the summer and in the southeast U.S.;Afterward, an updated SOA module based on laboratory results from recent smog chamber experiments is implemented in PMCAMx. A new modeling framework is implemented based on the SOA volatility basis-set approach instead of the two-product approach used in existing models. The role of chemical aging is investigated by adding to the base case parameterization gas-phase reactions oxidizing the semi-volatile SOA compounds to lower volatility products. The predicted OA concentrations are compared to the available ambient measurements from the EPA Speciation Trends Network (STN) and the Interagency Monitoring of Protected Visual Environments (IMPROVE). The base case OA simulation slightly overpredicts the ambient measured OA concentrations at the rural IMPROVE sites with a bias of 0.33 mug m-3. The base case underpredicts the measured OA at urban STN sites but is performing significantly better than the two-product module in PMCAMx compared to observations. The contribution of biogenic and anthropogenic SOA to the predicted organic aerosol concentrations and the potential role of chemical aging are discussed.;The overall goal of this work is to determine which sources contribute the most to the organic aerosol concentrations across the eastern US. First, a source-resolved model is developed to predict the contribution of eight different sources to primary organic aerosol concentrations. Primary organic aerosol (OA) and elemental carbon (EC) concentrations are tracked for eight different sources: gasoline vehicles, non-road diesel vehicles, on-road diesel vehicles, biomass burning, wood burning, natural gas combustion, road dust, and all other sources. The results of the source-resolved model are compared to the results of chemical mass balance (CMB) models for Pittsburgh and multiple urban/rural sites from the Southeastern Aerosol Research and Characterization (SEARCH) network.;Finally, the secondary organic aerosol module is updated to incorporate NOx-dependent SOA yields. Under low-NOx conditions, a distribution of products form with lower volatilities from the reaction of the RO2 radicals with other peroxy radicals resulting in higher SOA yields. At high NOx conditions, the SOA yields are lower because aldehydes, ketones, and nitrates dominate the product distribution. Based on recent laboratory smog chamber experiments, high-NOx SOA parameterizations were created using the volatility basis-set approach. Compared to the ambient OA measurements from IMPROVE and STN, the NOx-dependent SOA model underpredicts the daily OA concentrations with a bias of -0.13 mug m 3 and -1.62 mug m-3, respectively. As the NOx emissions are reduced by 25%, the domain average SOA concentration does not significantly change while the predicted average SOA concentrations increase in northern US cities by 2.8%, but decrease in the rural southeast US by 5.2%.