A fleet of various fixed- and rotary-wing aircraft is an integral part of the modern U.S. Armed Forces. The growing demand for jet air service for war fighting and transport of cargo and soldiers is likely to generate increased emissions, which can cause deterioration of local and regional air quality and visibility. Aircraft fuel and emissions contain mixtures of gas and particulate pollutants that are known to be harmful to human health and the environment. The total amount of particulate emissions for all aircraft in the U.S. is about three million kilograms per year, but an accurate measure of military aircraft emissions has not been established. Some of these emissions are classified as air toxics by the Environmental Protection Agency (EPA) and required to be reported. However, the EPA lacks a standard methodology for measuring aircraft emissions. Conventional measurement technologies for gases and particulate matter have not been evaluated for military aircraft emissions in field conditions.
The objectives of this project were (1) to develop a comprehensive emissions measurement program by employing both conventional and advanced measurement techniques, (2) to develop emission factors for military aircraft of fixed- and rotary-wing configurations, and (3) to investigate the spatial and temporal evolutions of pollutants in the exhaust plumes.
The combined use of commercial and research-grade measurement techniques produced reliable, high-quality, aircraft emissions data for the U.S. military. In contrast to emissions measured in an engine test cell, the aircraft emission factors derived from field measurements were representative of aircraft that are currently in service or are expected to be in service for future decades. The measurements were conducted in military airfields at pre-selected distances from the engine exhaust exits. The data enabled the examination of plume dynamics and established a direct source-receptor relationship of the emissions sources. The sampling methodology and monitoring techniques included (1) a tunable diode laser absorption spectroscopy (TDLAS), an ultraviolet differential optical adsorption spectroscopy (UV DOAS), an open-path Fourier transform infrared spectroscopy (OP-FTIR) for online remote-sensing of carbon monoxide, carbon dioxide, nitrogen oxides, sulfur dioxide, and air toxics; (2) a scanning mobility particle spectrometer (SMPS), an aerodynamic particle sizer (APS), several differential mobility analysis (DMA) based systems, a nanometer aerosol size analyzer (nASA), a micro-orifice uniform deposition impactor (MOUDI), and a frequency-modulated coherent microburst laser induced differential absorption radar (LIDAR) for online in-situ and remote-sensing of aerosol particle mass concentration, number density, size distribution, and chemical speciation; (3) an aerosol beam focused laser-induced plasma spectrometer (ABFLIPS) and inductively coupled plasma mass spectroscopy (ICP-MS) for off-line measurement of toxic metals and organometallic compounds; and (4) site-specific standard surface meteorology and auxiliary engine performance data.
The project yielded valuable insights in terms of deployment of conventional and research-grade measurement techniques to military aircraft emissions. The emission indices or factors derived as the “source terms” could be used directly for air quality modeling in the design of air pollution control strategies for airports and surrounding areas and compliance to mobile source emissions and the ambient air quality standards. The data obtained in these studies validated the usefulness of sampling and measurement methodology for nonvolatile particulate aircraft emissions and highlighted the needs for further research on volatile particulate matter and semi-volatile species in the exhaust. The particulate measurements for the fixed-wing and rotating-wing engines clearly indicated that the military aircraft emitted particles are ultrafine (defined as aerodynamic diameter less than 100 nanometers). Although EPA does have a PM2.5 standard, there is currently no EPA standard or sampling methodology for engine-exhaust particulate matter in this size category. A large volume of literature data however indicates ultrafine particles could potentially cause serious adverse health effects due to their high lung-penetrating ability and large surface area to host a variety of highly toxic / carcinogenic molecules produced in combustion. Remote-sensing measurement methodology tested and developed in this research such as open-path FTIR, UVDOS, TDLAS, LIDAR and FPS instrument yielded new insight on the transient behavior of emissions that would otherwise be difficult to obtain. Real-time variation and chemical conversion of engine-emitted species in plume were observed at various distances across the plume. Emission factors of selected gases derived from the array of open-path techniques do not generally agree with those derived from on-line in-situ measurements. The discrepancy appears to be approximately 75% at low engine power conditions and < 20% at cruise and high power condition. The experience and data provided invaluable information for the development of new sampling and measurement technology for species responsible for the formation of secondary particulate matter in the atmosphere, and suggested the importance to investigate volatile component of the engine exhaust in addition to the non-volatile species that have been addressed in this project.
This project provided valuable information for future development of an effective emissions monitoring program for fixed- and rotary-wing military aircraft. High-quality and comprehensive emissions data were obtained in field conditions that help to significantly reduce the uncertainties associated with existing emission estimates. Conventional and research-grade emission measurement techniques were evaluated and the data will guide future improvement of these techniques. The end products of this project include (1) state-of-the-art measurement techniques and instruments developed and tested for military aircraft emissions measurement in field conditions, and (2) high quality aircraft emissions factor data sets were collected, which can fill the gaps in the EPA Mobile Source Air Toxics (MSAT) program and for Toxic Release Inventory (TRI) reporting, as well as assist in future decision making and design of cost-effective air pollution control strategies.