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4The Clean Air Act requires airports to demonstrate com- pliance with PM emission standards for current operations as well as for expansion and construction projects. Currently airports must meet these requirements using very limited data on PM emissions from aircraft engines and no data on PM emissions from auxiliary power units (APUs). Data on other sources vary in quality and availability, and only limited data are available on ambient PM around airports. Aviation engine PM data are rapidly evolving and with this evolution there is an urgent need to consolidate the work done in the past with the most recent state-of-the-art meas- urements. The scientific communityâs understanding of the nature of aircraft-related PM emissions is hindered since cur- rent data remain incomplete for large fractions of common engines operating in the domestic and global fleets. While there are no data available on APU PM emissions, APUs are essentially small jet engines that consume much less jet fuel and consequently emit much less than aircraft main engines, even in the airport vicinity. Their emissions are believed to be similar in composition to main engine emissions but this is yet to be determined. The need to fill existing data gaps has been identified and initial steps taken in projects recently funded by FAA, NASA, and Transport Canada in their Partnership for AiR Trans- portation Noise and Emissions Reduction (PARTNER) Center of Excellence. Quite a bit of data have been acquired, especially in the last 3 years, on both military enginesâmuch under DODâs Strategic Environmental Research and Devel- opment Program (SERDP) sponsorshipâand on commercial- type wide-body transports and regional jets. Many gaps re- main, however. The current state of available data is described in this report. From this, gaps in the current knowledge base are identified. Understanding the gaps guided the develop- ment of project statements for future research. ACRP Report 6 presents the project results. A survey of 80 airports was conducted, ranging from large hubs to small general aviation airports, inquiring about the significance of PM emissions at that airport. Interviews were conducted with airport operators and researchers who have specific knowledge about PM emissions at airports. The team also conducted a literature review of available information and ongoing research about PM emissions at airports. Based on the findings from the survey, the interviews, the literature review, and the professional knowledge of the team, the researchers prepared an assessment of the current state of knowledge of aviation PM emissions. This final report assesses research needs relative to PM emissions and presents problem statements for future research to meet the most critical needs that would be of significant benefit to air- port operators. Chapter 3 of this report presents a primer on PM emissions from aviation to provide a baseline of information for readers who may be unfamiliar with PM emissions generally and issues faced by the aviation community specifically. Chapter 4 sum- marizes the findings of the PM survey of airports and inter- views with airport operators and PM researchers. Chapter 5 de- scribes current knowledge and gaps regarding PM emissions from aircraft engines. Chapter 6 describes the current state of knowledge concerning other airport emission sources and Chapter 7 summarizes research needs. Chapter 8 includes a prioritized research agenda and problem statements for proj- ects to address airportsâ highest priorities. Chapter 9 includes the literature review and the project bibliography. Appendix A includes a list of airports receiving the survey, a copy of the survey, and a summary of the survey responses. Appendix B includes notes recorded during the interviews. Appendix C presents a summary of hazardous air pollutants for reference. A glossary of key terms is also included. C H A P T E R 2 Background