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25 3.1 Conclusions The PM EIs from several in-service APU engines were determined to be very similar to EIs for larger aircraft jet engines, which have been characterized extensively in prior studies. Relatively few APUs have had their emissions studied, and those that have represent a very limited set of data from older model APUs. Data from this project has confirmed that APU PM EIs from in-service engines are much lower than those reported in those prior studies. EIs, comparable to those used in advanced emissions models, were computed for three APU types, which are representative of the current technol- ogy most commonly found in the U.S. commercial airline fleet. The PM EIs were computed at the three operational power settings. Measurements made near landing runways indicate PM emissions of particles smaller than 10 µm are from tire wear. Indeed, emissions measurements show that particles are emitted into the atmosphere when aircraft land. These are characterized by a puff of smoke associated with the tires contacting the runway. More than 100 aircraft landings were monitored, and measurements collected from 23 aircraft landings were used to characterize tire emissions. Estimates of the maximum amount of PM mass released during these landings suggest that less than 1% of total PM emitted during an LTO comes from the aircraft tires. Notably, little or no PM is emitted when aircraft land on a wet runway. Several emissions plumes were sampled during the brak- ing phase of landing aircraft. No distinct emissions signatures from aircraft brakes were discernable and it is infeasible to compute an EI for aircraft brakes. However, the data analyzed suggests that contribution of brake PM to overall PM inven- tory at this particular airport is negligible. As a significant contribution to the current state of the art in airport emissions assessment, EIs were determined for APUs and tires. These indices can be incorporated into cur- rent emissions models used for airport air quality studies. 3.2 Suggested Research Three types of emissions sources at airports were consid- ered in this study. The research team offers recommendations for each type in turn. 3.2.1 APUs Several representative APU engines were measured in the present study, which demonstrated that current in-service APU EIs are not significantly different from those of main engines. The EIs obtained can be used to characterize the fleetâs APU operations, in combination with operational times-in- mode and fuel flow data. However, additional research could be conducted to include more APU manufacturers and mod- els in a database that would provide APU emissions data for emissions inventory analysis. Such a database could be structured similarly to the ICAO databank for main engines and could provide a similar ser- vice to the airport community. 3.2.2 Tires The measurements performed in this study provided estimates for the emissions from tires that occur when the tires contact the runway pavement upon landing. These are estimated to be a small fraction of a typical LTO PM emis- sions from the main engines of the aircraft. Thus, if all PM is considered equally, the mass from the tires should be con- sidered negligible. One avenue for future research could be to determine if there is anything particularly harmful in the tire emissions that should be of concern from a health perspective. As far as the tire emissionsâ number and mass is concerned, these results have shown very clearly that tire emissions are very small compared with engine emissions. So, unless there is something that is particularly toxic or in other ways harmful to the environment, the contribution C H A P T E R 3 Conclusions and Suggested Research
26 from tire emissions is essentially negligible given current and projected future engine emissions. This conclusion is consis- tent with Bennett et al. 2011. A second avenue of research for tire emissions could be to better understand the tire material that is deposited on the runway during landings. Material could be collected from touchdown areas of landing run- ways and analyzed for mass and composition. Some of this material may end up in runway runoff and get transported to the airport grounds and perhaps into groundwater. Further, airports clean their runways, and the amount of tire wear material could be tracked and its destination determined. This is not an air pollution issue but possibly a groundwater and waste disposal issue. 3.2.3 Brakes With regard to brake emissions, the research team cannot quantify an average emissions level. However, after the dedi- cated field study, the team can conclude that brake emissions are less observable than tire emissions. Because tire emissions are so small compared with engine emissions, these emissions are also negligible compared with engine emissions. If a quanti- fied number is required, the team suggests that future tests be performed at a brake testing facility, where measurements very close to a braking system could be carried out. If such a num- ber is not required, the team does not advocate further work in this area. Translating measurements at a brake-testing facil- ity to emissions during aircraft operation may be difficult and uncertain, but such a test could conceivably be used to try to put some more quantitative bounds on brake emissions levels. However, the tests performed under this study strongly indi- cate that sensitive equipment cannot detect brake emissions, even when very low levels of tire emissions were measured and quantified. This conclusion is not inconsistent with Bennett et al. (2011), where brake dust was found adhering to the air- craft wheel cowling and underbody. Apparently, little is sus- pended in the air at levels even commensurate with the low level of tire emissions.
