National Academies Press: OpenBook

Research Needs Associated with Particulate Emissions at Airports (2008)

Chapter: Chapter 5 - Composition and Physical Properties of Particulate Matter From Aircraft Engines Knowledge and Gaps

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Suggested Citation:"Chapter 5 - Composition and Physical Properties of Particulate Matter From Aircraft Engines Knowledge and Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Research Needs Associated with Particulate Emissions at Airports. Washington, DC: The National Academies Press. doi: 10.17226/14160.
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Suggested Citation:"Chapter 5 - Composition and Physical Properties of Particulate Matter From Aircraft Engines Knowledge and Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Research Needs Associated with Particulate Emissions at Airports. Washington, DC: The National Academies Press. doi: 10.17226/14160.
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Suggested Citation:"Chapter 5 - Composition and Physical Properties of Particulate Matter From Aircraft Engines Knowledge and Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Research Needs Associated with Particulate Emissions at Airports. Washington, DC: The National Academies Press. doi: 10.17226/14160.
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Suggested Citation:"Chapter 5 - Composition and Physical Properties of Particulate Matter From Aircraft Engines Knowledge and Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Research Needs Associated with Particulate Emissions at Airports. Washington, DC: The National Academies Press. doi: 10.17226/14160.
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Page 18

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15 Soot (Nonvolatile PM)— Knowledge Historically, soot from aircraft engines has been monitored indirectly through measurement of smoke number. Smoke numbers are a required measurement during engine certifica- tion testing and have been recorded in the ICAO database since the mid 1970s. Smoke numbers, required to be reported only at the engine power for which it is maximum, are available for all large turbine engines currently employed in the commercial fleet. A smoke number measurement involves drawing a known volume of engine exhaust through a filter. Post expo- sure, the filter is examined optically and its reflectance relative to a calibrated light source is used to calculate a “smoke num- ber.” Clearly a smoke number provides a relative measure of the sootiness of a particular engine as a function of its opera- tion but it imparts no information on the physical and chemi- cal properties of the PM such as size, number, shape, mass, composition, and reactivity. The inadequacy of smoke number and the need for detailed aircraft engine particulate matter (PM) characteriza- tion became apparent in the early 1990s as the atmospheric and environmental scientific communities started to assess the impact of aviation emissions on the atmosphere at cruise altitudes (e.g., NASA’s Atmospheric Effect of Aircraft Program (AEAP)). Since that time groups in the United States and Europe have developed and continue to develop methods for detailed aircraft engine PM characterization using fundamental physical and chemical parameters. Around 2000, in response to a request for information from ICAO, the SAE E31 committee established a special PM sub- committee charged with developing a recommended practice for aircraft engine PM characterization based on fundamen- tal physical and chemical parameters.20 Armed with these new methods for fundamental parame- terization, research programs have been funded to charac- terize the PM emissions with respect to size, number, mass, and composition as a function of engine operating condition for a significant subset of engines currently in service in the commercial fleet. These engines include CFM56-2C1, JT8D- 219, CF6-80A2, CF6-80C2B8F, PW 2037, CFM56-3B1, CFM56-7B22, AE3007A, PW 4158, RB211-535E-4B, and CJ610. The engine class most extensively studied is the CFM56 with a total of 11 engines being examined. For the CFM56 class, measurements have been made at or close to the exhaust nozzle (within 2m), in the near-field plume (~ 10 m, 30 m, and 50 m) and in advected plumes (100 m to 300 m downwind). Smaller datasets exist for the other engines studied to date. Assessment of the results of these studies leads to the following conclusions on PM characteristics and measure- ment methods. PM Characteristics • At the exit plane the exhaust contains nonvolatile or refrac- tory PM, combustion gases, and the precursors for volatile PM evolution (i.e., sulfate and organics). As the plume evolves new particles form through nucleation of volatile organics and sulfates and some of the nucleated particles agglomerate on the nonvolatile PM surfaces. • With respect to the nonvolatile PM from aircraft engines the following characteristics have been established: – Number-based emission index (EIn) falls in the range of 1014–1016 particles/kg fuel burned. – Mass-based emission indices (EIm) fall in the range of 0.01–0.5 g/kg fuel burned. – Particles tend to have approximately spherical geometry and are made up of aggregates of smaller spherical parti- cles, which tend to be smaller and less highly coagulated than the chain aggregates typical of diesel PM. C H A P T E R 5 Composition and Physical Properties of Particulate Matter From Aircraft Engines—Knowledge and Gaps 20 Society of Automotive Engineers Aerospace Information Report 5892 copy- right © 2007 SAE.

– Size distributions are typically lognormal with number- based geometric mean diameters in the range 20 nm to 80 nm. – Nonvolatile PM is largely made up of (elemental) carbon. – Nonvolatile PM dominates the PM mass distributions at high engine powers. – From near field plume and advected plume measure- ments, the physical properties of the nonvolatile PM do not change but provide surfaces upon which volatile material condense and volatile PM in the plume can agglomerate. Measurement Methods • Reliable and accurate diagnostic tools for PM size and number have been developed. • A methodology for nonvolatile PM characterization is in the advanced stages of development. These conclusions fairly represent the current state of knowl- edge for the nonvolatile PM component of aircraft engine PM and the state of the art for measurement methods. Knowledge Gaps Using the foregoing summary of the state of knowledge for nonvolatile PM generated by aircraft engines, the following gaps in our knowledge and understanding become apparent. PM Characteristics • The engine PM emissions database is incomplete. In particular it lacks data for current advanced technology en- gines such as the GE 90, PW GP7000, and RR Trent 900. • There is little knowledge on the impact of engine-to-engine variability on nonvolatile PM emissions for engines of the same type. • There is little or no knowledge of impact of engine age and maintenance on nonvolatile PM emissions for engines of the same type. • The nature of nonvolatile PM density and structure as a function of engine operating condition and particle size is not known. • There is only limited knowledge of the dependence of non- volatile PM emissions on fuel composition, especially alternate fuels. • There is a lack of knowledge of the health impacts of non- volatile PM, particularly as a function PM size, number, and composition although some recent European studies may provide some information. Measurement Methods • There are open questions with line loss and sampling meth- ods although these will be answered in part in the reports of the NASA and SERDP-sponsored methodology develop- ment studies in 2006–2007. • Real-time calibrations for line loss and instrument perfor- mance are essential but currently surrogate PM calibration sources have to be used since no reliable combustion aerosol calibration source exists. • Currently there are no reliable or practical direct mass measurement tools. Long run times are required when using current filtration techniques, which are impractical and cost prohibitive for aircraft emission sampling. • Existing direct mass measurements are time-consuming and are subject to sampling artifacts and interferences. There is limited connection between fast time response instruments for size and number and a direct mass meas- urement appropriate for measuring aircraft emissions. Applications • There are no data available to develop correlations be- tween emissions data acquired under standard testing conditions and emissions predictions for aircraft under actual operations. • Standard testing conditions provide no information on the nature of transients, especially the impact of changes in ambient temperature, pressure, relative humidity, and the engine operating conditions during actual operations. The Delta ATL and OAK advected plume studies may pro- vide insights into the influence of ambient conditions on the production of nonvolatile PM emissions. • While the FOA (First Order Approximation) provides a means of estimating PM emissions based on the best data available, it remains an approximation. Developing a comprehensive database of PM emission indices would provide a reliable and accurate source for modeling PM emissions. Volatile PM—Knowledge Volatile particles are defined to be those that are formed from condensable gases after the exhaust has been cooled to temperatures below engine exit conditions (e.g., sulfuric acid particles).21 Thus they do not exist at the engine exit plane as particles and the associated mass is only present as gas-phase particle precursors. Their formation and evolution can 16 21 Society of Automotive Engineers Aerospace Information Report 5892 copyright © 2006 SAE.

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

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

Next: Chapter 6 - Particulate Matter From Other Airport Sources »
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TRB’s Airport Cooperative Research Program (ACRP) Report 6: Research Needs Associated with Particulate Emissions at Airports examines the state of industry research on aviation-related particulate matter emissions and explores knowledge gaps that existing research has not yet bridged.

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