Secondary Organic Aerosol Formation from Emissions from Combustion Sources

Atmospheric aerosols have a significant impact on human health, climate, and air quality. Fine atmospheric particulate matter (PM) is composed of a complex mixture of organic and inorganic materials. The traditional definition of organic aerosol (OA) being comprised of directly emitted PM (primary OA or POA) and formed from atmospheric oxidation of volatile precursors (secondary organic aerosol or SOA) has been challenged with recent research. This dissertation examines the atmospheric evolution of OA from combustion sources with three main objectives. The first objective is to identify the relative importance of primary and secondary PM emissions from combustion sources, the second is to explore the role of non-traditional SOA precursors, and the third objective is to investigate the role of precursor structure and fuel composition on secondary PM formation.;To address the first objective, field experiments were performed with exhaust from gas-turbine engines. Photo-oxidation created substantial secondary PM, greatly exceeding the direct PM emissions at each engine load after an hour or less of photo-oxidation. Large amounts of lower-volatility organic vapors were measured in the exhaust; they represent a significant pool of SOA precursors that are not included in traditional SOA models. These results underscore the importance of accounting for atmospheric processing when assessing the influence of aircraft emissions on ambient PM levels.;To address the second objective of this thesis, separate experiments were performed with emission surrogates (diesel fuel and motor oil) and single component systems (n-pentacosane) to explore the role of non-traditional SOA precursors. These materials are thought to be important components of actual combustion emissions. Oxidation of motor oil and n-pentacosane, which are semivolatile organic compounds (SVOCs), creates substantial SOA, but this SOA is largely offset by evaporation of POA. The net effect is a cycling or pumping of SVOCs between the gas and particle phases, which creates more oxygenated OA but little new OA mass. The coupling of partitioning and chemistry blurs the traditional definition of OA in these experiments.;The third objective was addressed with smog chamber experiments to quantify the SOA formation potential of different diesel fuel formulations. The aromatic content of diesel fuel was shown to be an important factor in SOA formation, however it was not the only factor. SOA modeling demonstrated that other classes of compounds (alkanes, olefins, oxygenates) might contribute significantly. The use of synthetic fuel was shown to greatly reduce SOA formation compared to petroleum-based jet fuel in a gas-turbine engine. These results have important implications for the impact of alternative fuels on secondary PM emission from gas-turbine engines, as well as broader implications for other combustion engines.;The findings presented in this thesis have important implications for controlling PM emissions from combustion sources and developing effective regulatory strategies, especially in areas where combustion sources are dominant contributors to air pollution.