Many Department of Defense facilities are located in areas designated as nonattainment with respect to the federal fine particulate matter standard, but the impact of emissions from these facilities on local and regional air pollution levels is not well understood. The goal of this project was to gain new fundamental understanding of volatile particulate matter emissions from military aircraft. Specific objectives included: (i) measuring the amount, chemical composition, and other properties of particulate matter in emissions from military aircraft, (ii) quantifying the effects of atmospheric aging on volatile particulate matter emissions, (iii) characterizing the effects of alternative fuels on particulate matter emissions, and (iv) developing a theoretical model to predict the evolution of aircraft particulate matter emissions from the engine exit plane to highly dilute background as a function of photochemical age.
Experiments were performed to characterize the atmospheric evolution of volatile particulate matter emissions from military aircraft engines including CFM56-family and T63 gas-turbine engines. The experiments featured a suite of state-of-the-art instrumentation to comprehensively characterize the gas- and particle-phase emissions. Novel aspects of the experiments included quantification of the volatility and gas-particle partitioning of the volatile particulate matter emissions and characterization of the production of secondary particulate matter in a smog chamber. The data were used to develop parameterizations for use in chemical transport models to predict the contribution of aircraft to ambient fine particulate matter.
The project obtained significant data on gas- and particle-phase emissions from two CFM56 engines and one T63 engine. The composition of the primary particulate matter mass emissions from the CFM56 and T63 engines varied with engine load. For both engines, the particulate matter emissions at low load are dominated by organics (volatile particulate matter) and by elemental carbon (non-volatile particulate matter) at high load. Operating the T63 on Fischer-Tropsch fuel substantially reduced the primary particulate matter emissions.
A significant fraction of the particulate matter emissions is semivolatile at atmospheric conditions. For example, modest heating caused a large fraction of the primary organic aerosol to evaporate. Therefore, a traditional emissions factor cannot be used to represent military aircraft particulate matter emissions in inventories and models. Instead, one must measure the total emissions rate of semivolatile species and the volatility distribution of the emissions.
The smog chamber experiments demonstrated that photo-oxidation creates substantial secondary particulate matter, greatly exceeding (by as much as a factor of 60) the direct particulate emissions after an hour or less of aging at typical summertime conditions. This was observed in every smog chamber experiment except one using exhaust from the T63 engine operating at cruise load on neat Fischer-Tropsch fuel. Therefore secondary particulate matter production must be accounted for in order to assess the contribution of military aircraft emissions to urban and regional air pollution.
The modules developed by this project will lead to more robust assessment of the impacts of both military and civilian aircraft on urban and regional air pollution.