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16 Size distributions are typically lognormal with number- Measurement Methods based geometric mean diameters in the range 20 nm to There are open questions with line loss and sampling meth- 80 nm. Nonvolatile PM is largely made up of (elemental) ods although these will be answered in part in the reports of carbon. the NASA and SERDP-sponsored methodology develop- Nonvolatile PM dominates the PM mass distributions ment studies in 20062007. Real-time calibrations for line loss and instrument perfor- at high engine powers. From near field plume and advected plume measure- mance are essential but currently surrogate PM calibration ments, the physical properties of the nonvolatile PM do sources have to be used since no reliable combustion aerosol not change but provide surfaces upon which volatile calibration source exists. Currently there are no reliable or practical direct mass material condense and volatile PM in the plume can agglomerate. measurement tools. Long run times are required when using current filtration techniques, which are impractical and cost prohibitive for aircraft emission sampling. Measurement Methods Existing direct mass measurements are time-consuming Reliable and accurate diagnostic tools for PM size and and are subject to sampling artifacts and interferences. number have been developed. There is limited connection between fast time response A methodology for nonvolatile PM characterization is in instruments for size and number and a direct mass meas- the advanced stages of development. urement appropriate for measuring aircraft emissions. These conclusions fairly represent the current state of knowl- Applications edge for the nonvolatile PM component of aircraft engine PM There are no data available to develop correlations be- and the state of the art for measurement methods. tween emissions data acquired under standard testing conditions and emissions predictions for aircraft under Knowledge Gaps actual operations. Standard testing conditions provide no information on the Using the foregoing summary of the state of knowledge for nature of transients, especially the impact of changes in nonvolatile PM generated by aircraft engines, the following ambient temperature, pressure, relative humidity, and gaps in our knowledge and understanding become apparent. the engine operating conditions during actual operations. The Delta ATL and OAK advected plume studies may pro- PM Characteristics vide insights into the influence of ambient conditions on the production of nonvolatile PM emissions. The engine PM emissions database is incomplete. In While the FOA (First Order Approximation) provides a particular it lacks data for current advanced technology en- means of estimating PM emissions based on the best data gines such as the GE 90, PW GP7000, and RR Trent 900. available, it remains an approximation. Developing a There is little knowledge on the impact of engine-to-engine comprehensive database of PM emission indices would variability on nonvolatile PM emissions for engines of the provide a reliable and accurate source for modeling PM same type. emissions. There is little or no knowledge of impact of engine age and maintenance on nonvolatile PM emissions for engines of the same type. Volatile PM--Knowledge The nature of nonvolatile PM density and structure as a Volatile particles are defined to be those that are formed function of engine operating condition and particle size is from condensable gases after the exhaust has been cooled to not known. temperatures below engine exit conditions (e.g., sulfuric acid There is only limited knowledge of the dependence of non- particles).21 Thus they do not exist at the engine exit plane as volatile PM emissions on fuel composition, especially particles and the associated mass is only present as gas-phase alternate fuels. particle precursors. Their formation and evolution can 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 21 Society of Automotive Engineers Aerospace Information Report 5892 copyright may provide some information. 2006 SAE.