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OCR for page 17
17 happen either in the plume as the exhaust mixes and cools Due to the dependence on ambient conditions, volatile PM with the ambient air or in a probe and sampling system. measurement methodology development is still in its While the volatile contribution to the particles does not initial stages. occur in concert with the combustion process, the volatile The compositional characterization of volatile particles is particles that are emitted into the atmosphere may affect still not complete. In particular, the speciation of the local air quality. EPA rules require airports to evaluate both organic contributions has not been definitive since the or- nonvolatile and volatile PM emissions. For this reason it is ganic make up is apparently quite complex. essential that the aviation community develop a better un- derstanding of and capability for quantifying the volatile PM Knowledge Gaps emissions from aircraft engines. Good tools have been developed for quantifying the number, size, and composi- Using the foregoing summary of the state of knowledge for tion of these volatile particles, using the same tools as for volatile PM generated by aircraft engines, the following gaps nonvolatile particles for number and size. Techniques for in our knowledge and understanding become apparent. quantifying the mass of volatile particles are not well devel- oped and further work is needed on them. Current understanding is incomplete concerning volatile PM evolution in the plume (or sampling system) and its de- pendence on atmospheric conditions such as temperature, Volatile PM Characteristics relative humidity, and background pollution levels. Sulfate and organic precursor gases both contribute to No model currently exists that adequately describes the full volatile PM mass. evolution of volatile PM as it forms and grows in the Sources of the organic component may include contribu- exhaust plume. tions from both partially combusted fuel (products of Laboratory-based tools for simulating the complex evolu- incomplete combustion) and engine lubricants. tion of volatile PM have not yet been developed, although Three modes of particles are typically measured that have EPA has been working on understanding this process for a volatile component: some time. newly formed PM (totally volatile particle formed in the To provide proper inputs to local- and regional-scale air exhaust plume), quality models, there is a need to adequately represent the coated nonvolatile PM, and thermodynamic and photochemical state of the volatile coated ambient particles (ambient particles entrained in PM that is emitted into the atmosphere. the plume that take on a coating from condensable exhaust There is currently only a minimal understanding of or- gases). ganic speciation of the volatile PM component relative to The volatile PM characteristics are dependent on fuel carcinogens and other toxic compounds. composition, most dramatically evident in the sulfate con- In particular, the contribution of lubrication oil to tribution being dependent on fuel sulfur levels. volatile PM is poorly understood, especially as it relates Volatile PM dominate the total number of particles at down- to variations in engine technology and operational stream locations where the exhaust has cooled to ambient procedures. temperatures. There is at present limited knowledge of the dependences The volatile component evolves as plume expands and the of volatile PM emission properties on fuel composition, resulting particle properties depend on ambient condi- including how the use of alternate fuels may impact volatile tions such as temperature, relative humidity, and back- contributions. ground pollutant levels. This dependence of volatile PM Although health impacts are a significant driver for the properties on ambient conditions presents complications measurement of volatile particles, we lack knowledge of for measurement using conventional nonvolatile PM health impacts of volatile PM as a function of size, number, measurement methods. and composition. There is extensive literature on the health effects of PM; however, there is little specificity on the small particles common to aircraft engine emissions. Measurement Methods EPA has found that smaller particles are of greater concern Good tools have been developed for quantifying the than larger particles and has adjusted its regulatory number, size, and composition of these volatile particles, structure over time to focus more intensively on smaller using the same tools as for nonvolatile particles for num- particles. Also, health effects based on particle composition ber and size. are not well understood.

OCR for page 17
18 Measurement Methods acquired under standard testing conditions and emissions predictions for aircraft under actual operations. The methodology development for volatile PM is still in Research is currently underway to connect the local air the initial stages. quality model, the Emissions and Dispersion Modeling The specific gaps identified for nonvolatile particles also System (EDMS), with the regional Community Multi- apply to volatile particles. scale Air Quality (CMAQ) tool. Further research into methods for modeling the volatile elements of PM in both Applications local and regional-scale air quality models is needed, As for nonvolatile particles, correlations must be devel- however. oped that make a connection between emissions data