In response to stringent regulations on fleet-average fuel economy, vehicle manufacturers have increasingly replaced port fuel injection (PFI) engines with gasoline direct injection (GDI) engines. These engines emit substantial quantities of ultrafine particulate matter (PM) and black carbon (BC) which is of concern due to their associated health and climate effects, respectively. This thesis investigated GDI emissions, with a focus on the particle phase, in both laboratory and real-world environments to help understand the air quality impacts of this engine technology. As part of the study, advanced PM measurement techniques were assessed, and a correction protocol for a popular high-time resolution particle sizing instrument needed to accurately measure vehicle exhaust size distributions was developed. A laboratory study to quantify phase-partitioned polycyclic aromatic hydrocarbon (PAH) concentrations was also conducted. Compared to PFI engines, GDI engines emitted elevated concentrations of heavy molecular weight PAHs, including benzo(a)pyrene, a PAH with established associations to negative health outcomes. The GDI engine exhaust also had elevated concentrations of the PAHs pyrene and fluoranthene;​ these PAHs also exhibited the greatest extent of particle-gas partitioning. A study of real-world GDI emissions in an urban environment showed that GDI particle number and BC emissions were in the upper end of the fleet distribution, and that exhaust plumes exhibited dynamic behaviour in the near-road region, with increasing particle number emission factors with increasing distance from the roadway. This behaviour was unique to GDI vehicles, the same effects were not observed for heavy-duty garbage trucks or a PFI-equipped vehicle. Comparing size distributions at different distances from the roadway, rapid particle growth of sub-5 nm soot cores due to condensation of low volatility organic gases, such as pyrene and fluoranthene, was proposed to be the dominant growth mechanism in GDI vehicle exhaust. Comparing laboratory and real-world emission factors, BC emission factors were in good agreement, while real-world particle number emission factors were up to an order of magnitude higher. An estimate of the climate impacts of increased BC relative to fuel savings from GDI also showed that fuel economy gains of up to 12% may be needed to offset the radiative forcing of BC.