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42 SECTION 8 Crucial Knowledge Gaps Information gaps identified in this report are described in the impact of severe delays are not captured, nor is plane-to- the following sections. plane variability. Any attempt to quantify the health risk posed by acute effects of HAPs must accurately account for situations in which emissions and concentrations are highest 8.1 Emissions-Related (e.g., long delays, low mixing heights, etc). Knowledge Gaps These information gaps affect the accuracy of emission 8.1.3 Full Characterization of HAP inventories (and any related health risk assessments). Emissions Near Idle Thrust Measurements of HAP emissions indices have been per- 8.1.1 Effect of Temperature on formed at a limited number of power levels near idle (e.g., at Aircraft HAP Emissions Near Idle APEX1,2,3 the common engine thrust settings were 4%, 7%, HAP emission rates can vary by well over a factor of 3 and 15%). A significant amount of interpolation is required within the range of conditions encountered by many airports. to portray the effect of engine power on emission indices. There are no measurements at subfreezing temperatures, even though a significant portion of annual aviation activity 8.1.4 Single-Engine Taxiing occurs during such conditions. As discussed in Section 5.1.3, the prevalence of single- engine taxiing is unclear, though anecdotally is thought to be 8.1.2 Characterization of True Idle Levels rare. If actually prevalent, this could have a large impact on and Times-in-Mode emission inventories. Since the gas-phase HAP emissions are dominated by the idle phase, knowledge of actual idle times is necessary. Addi- 8.1.5 General Aviation tionally, since the emission factor is a strong function of Emissions Characterization throttle setting, knowledge of the real-world power levels used are necessary. Emission indices for true ground idle With the exception of lead emissions, which are well char- (sometimes labeled 4%) can be a factor of 2+ greater than the acterized, emissions from the general aviation category of 7% ICAO certification point. The likely distribution of throt- piston engines are largely unknown. tle settings used as the aircraft progresses from the terminal to the runway (or the taxi-phase of an LTO) is unknown. This 8.1.6 Breadth of Measurements Across time-in-mode problem is probably highly variable and Engine Types dependent on the frequency of flights. This problem does not lend itself to a simple relationship expressed as total emis- The APEX campaigns increased the total number of com- sions per LTO. mercial turbofan engines characterized to more than 10. This Since most emission inventories (i.e., those made under is not necessarily a representative sample of aircraft engines. NEPA requirements) are required to consider average condi- There are scant data on the variation of hydrocarbon emis- tions and not worst-case scenarios, day-to-day variability or sions with engine age and maintenance history.