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22 At low thrust levels, the number-based emission index The Delta Atlanta-Hartsfield Study yielded the following was substantially greater downstream (~30 m [98 ft]) conclusions from the extractive sampling measurements: than at the exhaust nozzle, indicating that significant gas- to-particle conversion occurred as the plume cooled and For the JT8D engines, number mean particle diameters in- aged. creased with engine thrust. The number mean diameter of the total particles increased The number-based emission index was highest at takeoff, linearly with thrust. exhibited a smaller peak at idle, and revealed a minimum at thrust levels corresponding to approach. From the gas-phase species measurements performed during The mass-based emission index behaves similarly to the APEX, the following conclusions were drawn: number-based emission index and is higher at idle, exhibits a minimum at approach, and then rapidly increases to a The emission indices for NOX, CO, and HC agreed with maximum at takeoff. values archived in the ICAO Aircraft Engine Emissions The JT8D number and mass emissions trends are consis- DataBank. tent with behavior of the CFM56-2C1 engine studied in the The NO2 fraction of NOX varied from ~0.7 at idle to ~0.09 at APEX campaign. take off. The two JT8D engines in this study have greater black carbon Although substantial at idle, the HC emission index de- emission indices than any other engine tested in the APEX creased with increasing thrust and was below the minimum studies. detection limit (roughly 0.01 to 0.05 g/kg-fuel depending The CF6 and PW2037 have a greater number-based emis- on the species) above 15% rated thrust. sion index than the JT8D. At a given thrust condition and The HC emission index depended strongly on ambient for a given fuel, the JT8D emits fewer but larger particles conditions such as temperature. A 20C decrease in ambient while engines designed to reduce smoke number certifica- temperature increased the emission index of HC species by tion measurements (those more recently developed) emit a factor of 10. more numerous quantities of smaller particles. The emission index of SO2 was greater for the high sulfur For thrust conditions near idle, the amount of volatile fuel (1600 ppm sulfur) than for either the high aromatic or organic PM emitted by JT8D-219 engines decreases rapidly base fuels (400 ppm sulfur). with increasing thrust, consistent with the thrust dependence Unburned hydrocarbons are emitted as a variety of com- of the emission index of UHC. This observation is consis- pounds, including ethylene, formaldehyde, acetaldehyde, tent with nucleation/condensation of the least volatile and benzene. Emissions of the various HC species rise and UHC to form organic PM. fall with one another, regardless of engine type or thrust setting. Even when the absolute magnitudes increase by a The LIDAR analysis gave similar qualitative trends for all of factor of 10 or more (as is the case for older engine tech- the engines studied, but the reliability of the system employed in nology or for operation at low thrust condition or low this study was judged to be low for quantitative measurements. ambient temperature), the ratio of one HC species to the Upon completion of the dedicated engine testing, the next remains constant. extractive sampling systems were positioned downwind of Slight differences in formaldehyde emission index were active runways. Exhaust plumes transported from source air- observed between the various fuel types. craft by the prevailing winds were continuously sampled and the source aircraft tail numbers logged. The tail numbers provided a unique method for correlating the plumes with spe- 5.2 Delta Atlanta-Hartsfield Study cific aircraft and, hence, specific engines. In excess of 500 taxi The second of the APEX series of studies was carried out and takeoff events were sampled during a three-day period. with the support of Delta Airlines at Hartsfield-Jackson The following general conclusions from the wind-blown Atlanta International Airport in September 2004. Mobile plume analyses can be drawn: laboratories operated by Missouri University of Science and Technology (Missouri S&T), Aerodyne Research, Inc. (ARI), The combination of the PM and gas analysis of the trans- and NOAA were deployed to conduct both dedicated engine ported plume provides unique identification of the engine tests and runway studies (see Chapter 3 for details). The full operating condition generating the plume (i.e., idle, spool- LTO cycle for MD-88 and JT8D engines was studied during up, maximum thrust, etc.). the Delta Atlanta-Hartsfield Study. Only thrust settings less Much more volatile material converts to the particle phase than 60% full rated thrust were examined for larger engines during plume transport across the runway than is observed such as CF6 and PW2037. during dedicated engine tests.