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27 6.4 Current Model Limitations through airport policy. Examples of mitigation for PM from each of those sources is as follows: Particle diameter is influenced by throttle setting, but all operating modes produce particles less than 2.5 m in diam- For stationary sources like emergency generators, inciner- eter (PM2.5). Particle chemical composition also varies with ators, power turbines, and oil-fired boilers, particle traps thrust setting (Lobo, Whitefield et al. 2007). can be installed on exhaust stacks to control PM emissions. Accurately estimating throttle setting is important to Ground support equipment PM emissions can be miti- account for changes in engine conditions that influence PM gated by using an ultra-low sulfur diesel fuel. Keeping the emission indices. For certification, throttle setting is specified engines properly maintained and tuned is important for for the LTO cycle describing aircraft operation to a height of minimizing particle emissions as well. Working with ten- approximately 900 m (2,953 ft) by regulation. Over the LTO ants to promote the use of alternative fuels can also be cycle, ICAO specifies generic time in mode and thrust as- beneficial. This might include supplying alternative, lower sumptions for aircraft engine certification with four discrete emission fuels. Working with airlines to install chargers in settings: taxi/idle (26.0 min, 7% throttle), takeoff (0.7 min, the ramp area for electric GSE may encourage greater use 100% throttle), climb (2.2 min, 85% throttle), and approach of zero-emission electric vehicles. (4.0 min, 30% throttle) (ICAO 1993). Installing electrical power and preconditioned air at each The certification prescribed LTO cycle is not necessarily gate can provide the airlines with the power and ventilation representative of actual flight procedures. This affects result- they need without running APU. ing estimates of total PM emissions (Fleuti and Polymeris Many airports have changed from using Jet A or diesel fuel 2004). This question was addressed by APEX1, which looked to propane or other cleaner burning fuels in their fire- at PM emissions at 11 thrust settings: 4, 5.5, 7, 15, 30, 40, 60, fighter training. This change in fuels reduces smoke and 65, 70, 85, and 100% to understand trends at intermediate soot emissions from about 1,000 lbs/1,000 gal for jet fuel to thrusts and below the prescribed idle setting. Particulate about 120 lbs/1,000 gal for propane (FAA and USAF 1997). matter emissions trends below 7% vary by PM component. Controlling ground operations to minimize delays reduces In general, non-volatile PM (black carbon) emissions are rel- aircraft emissions. Establishing airport policies to promote atively small at low thrust settings. However, volatile PM fuel conservation practices among airlines and other tenants components exhibit more complex behavior as precursor can reduce airport emissions. Such a policy might recom- gases condense downstream (Wey et al. 2006). mend single-engine taxiing, de-rated takeoff, enhancing Typically, when modeling airport activity using EDMS, the GSE maintenance, and using ultra-low sulfur diesel in GSE user assumes that once an aircraft has pushed back from the and other vehicles. gate, the APU is turned off and the main engines are used to On the landside, airports do not have any regulatory au- provide power to the aircraft. In reality, however, anticipated thority over passenger vehicles. However, many airports delays prompt pilots to shut off main engines and run the have worked with local taxi companies to encourage, or APU to conserve fuel. Recommended warm-up and cool- even mandate, use of low-emission taxis as a requirement down times are dependent on design parameters for each spe- for serving the airport. For example, all taxicabs permitted cific engine type, and influence a pilot's decision to shut off to pick up passengers at Seattle Tacoma International Air- main engines (ICAO 2000). Although airlines have individual port are required to use compressed natural gas. Through operating procedures, the ultimate decision rests with the fees and licenses, some airports have taken strides to reduce pilot (ICAO 2000). Assuming that the aircraft main engines the frequency of circulating through the airport by hotel, remain operating at 7% thrust throughout the taxi/queue parking, and car rental vans. Similarly, the use of cell phone portion of the LTO is conservative, but may not accurately waiting areas allows vehicles to remain nearby with their represent the actual operation of the aircraft. engines off until passengers are ready to be picked up. Providing ultra-low sulfur diesel or other reduced-sulfur fuel for use in GSE, boilers, emergency generators, etc., can 6.5 Mitigation reduce particulate emissions. A new generation of alterna- Although there are many sources of PM emissions at air- tive fuels, known as synthetic paraffinic kerosene, which in ports, only a few of the sources are under the direct control of their pure form contain no sulfur, show promise in reduc- the airport. Stationary sources, GSE, and some aircraft oper- ing PM emissions from the aforementioned sources and ational characteristics are the most likely to be influenced turbine-powered aircraft (Hileman et al. 2008).