Secondary Organic Aerosol Formation from Gasoline Direct Injection Vehicles: The Impacts of Exhaust After-Treatment and Fuel Content
A majority of atmospheric aerosol in urban areas is attributed to transportation emissions, and particularly, gasoline vehicle emissions. The particulate matter (PM) emitted directly from the tailpipe consists of primary organic aerosol (POA) and black carbon. In addition to the particulate emissions, common gaseous pollutants include, nitrogen oxides (NOx), and non-methane organic gases (NMOG), which can photochemically react in the atmosphere to produce secondary organic aerosol (SOA).
SOA formation from gasoline vehicles has received considerable attention in recent years, with the majority of studies focusing on older technology port-fuel injection (PFI) engines. There is limited information on the SOA formation from current technology gasoline direct injection (GDI) engines, despite the abundance of information on the primary emissions from these engines. GDI technology is considered a major pathway to reduce greenhouse gas emissions. However, GDI engines have been measured to emit increased PM mass when compared to similar PFI engines. To counteract the increased PM emissions, manufacturers and regulators may need to utilize various emission control strategies. The goal of this work was to investigate the effects of select emission control strategies on the secondary aerosol potential of newer technology GDI vehicles.
First, the effects of a catalyzed gasoline particulate filter (GPF) were explored. The addition of a GPF has shown drastic reduction in tailpipe PM mass for GDI vehicles, however its effect on secondary emissions were unknown. This study aided to understand if a catalyzed GPF is effective in the removal of secondary aerosol precursors, thus increasing the significance of the after-treatment technology. Next, the impacts of high ethanol fuel blends, and varying driving conditions on secondary aerosol were assessed. This study analyzed the variations in composition and morphology of the emissions from vehicles operated on 10% to 78% ethanol fuels (% volume). In addition to the effects of vehicle exhaust from high ethanol fuel blends, this study investigated how a controlled surrogate environment can affects the reaction potential of vehicle exhaust. The effects of aromatic and ethanol content were further explored with lower ethanol blends (0%-20% by volume). These fuels were more similar to current commercial fuel blends and focused on the effects of smaller variations in ethanol and aromatic content on the physicochemical properties of the secondary aerosol. Finally, connections between the different vehicle certification standards, fuels, driving cycles, and reaction conditions were explored to form relationships between measured tailpipe emission concentrations and the resulting SOA formation potential from vehicles.