Engine Design and Operational Impacts on Particulate Matter Precursor Emissions

Aircraft emissions of trace sulfur and nitrogen oxides contribute to the generation of fine volatile particulate matter (PM). Resultant changes to ambient PM concentrations and radiative properties of the atmosphere may be important sources of aviation-related environmental impacts. This paper addresses engine design and operational impacts on aerosol precursor emissions of $SOx$ and $NOy$ species. Volatile PM formed from these species in the environment surrounding an aircraft is dependent on intraengine oxidation processes occurring both within and downstream of the combustor. This study examines the complex response of trace chemistry to the temporal and spatial evolution of temperature and pressure along this entire intraengine path after combustion through the aft combustor, turbine, and exhaust nozzle. Low-order and higher-fidelity tools are applied to model the interaction of chemical and fluid mechanical processes, identify important parameters, and assess uncertainties. The analysis suggests that intraengine processing is inefficient. For in-service engine types in the large commercial aviation fleet, mean conversion efficiency $(ε)$ is estimated to be 2.8–6.5% for sulfate precursors and 0.3–5.7% for nitrate precursors at the engine exit plane. These ranges reflect technological differences within the fleet, a variation in oxidative activity with operating mode, and modeling uncertainty stemming from variance in rate parameters and initial conditions. Assuming that sulfur-derived volatile PM is most likely, these results suggest emission indices of $0.06–0.13g∕kg$ fuel, assuming particles nucleated as $2H2SO4∙H2O$ for a fuel sulfur content of $500ppm$⁠.